Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years.
This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades.
A well-resolved, robustly supported consensus tree (MPTs = 360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as “rauisuchians,” a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic “rauisuchians” and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of “rauisuchians,” and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms (“Sphenosuchia”) were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line.
A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.
INTRODUCTION
Archosauria consists of two extant clades, crocodylians and birds (Gauthier and Padian, 1985; Gauthier, 1986; Benton and Clark, 1988; Benton, 1990a; Sereno and Arcucci, 1990; Sereno, 1991a; Parrish, 1993; Juul, 1994; Cao et al., 2000). However, these clades represent two body-form extremes in a long, complex, evolutionary history dating to the Triassic (Benton, 1990a; Sereno, 1991a; Gower and Sennikov, 2000; Nesbitt, 2003). In the Triassic, non-archosaurian archosauriforms such as Proterosuchus, Erythrosuchus, and Euparkeria represented a new diapsid body plan not present in the Paleozoic. Immediately after the Triassic divergence of the avian and crocodilian lineages, the crocodylian lineage split into several clades that dominated in diversity, numbers, and body forms (Brusatte et al., 2008). During the Triassic, archosauriforms were present on nearly all continents and mastered terrestrial (e.g., dinosaur, “rauisuchian,” aetosaur, crocodylomorph), aquatic terrestrial (phytosaur, Vancleavea), and aerial (pterosaur) habitats. Furthermore, several clades became herbivorous independently (aetosaurs, ornithischians, sauropodomorphs), whereas most archosaurs remained carnivorous. Following the Triassic, only two lineages remained, the Crocodylomorpha and the Dinosauria.
Previous Work
The evolution of studies of basal archosaur relationships has been on the forefront of the transition from “precladistic” methods to modern cladistic practices largely because of the work of Gauthier (1984, 1986) and Gauthier and Padian (1985). Prior to the 1980s, most of the taxa in what we now know as Archosauriformes were classified as a large group called “Thecodontia” (Owen, 1859). It was thought that “Thecodontia” represented a “basal stock” in which Aetosauria, Crocodylia, Sauropodomorpha, Theropoda, Aves, Ornithischia, Pterosauria, and Phytosauria emerged (e.g., Charig, 1976: fig. 2). Through a number of publications, the ankle of “thecodonts” became very important for classification of various groups (Cruickshank, 1979; Chatterjee, 1982; Cruickshank and Benton, 1985), but the relationships among and between “thecodont” groups was not given much thought. A detailed history of precladistic studies was comprehensively reviewed by Sereno (1991a), Juul (1994), and Gower and Wilkinson (1996), including the group Pseudosuchia, and I will not repeat that here.
In the 1980s, cladistic methods reshaped our understanding of basal archosauriform relationships. The works of Gauthier (1984), Benton (1985), Benton and Clark (1988), and Gauthier et al. (1988) showed the following: (1) crocodylians and avians are each others' closest extant relatives, and they shared a common ancestor at some point in the Triassic; (2) many of the “thecodontians” are just outside Archosauria or belong on either the branch that leads to crocodylians or to avians; (3) dinosaurs are monophyletic. However, these studies only provided lists of synapomorphies supporting different clades (Gower and Wilkinson, 1996). Further, the authors did not provide a character matrix in print and almost entirely used suprageneric taxa. The absence of a numerical analysis did not allow the authors to identify weak portions of the tree and to test the homology of the character states. Nonetheless, this great stride in basal archosaur systematics provided a set of identified synapomorphies and a framework for numerical studies in the near future (fig. 1).
Fig. 1
Phylogenetic relationships of basal Archosauriformes: A, Gauthier (1984); B, Benton and Clark (1988); C, Juul (1994); D, Bennett (1996); E, Sereno (1991a). Suprageneric taxa are in bold.
In the early 1990s, each study on basal archosaurs (e.g., Sereno, 1991a; Parrish, 1993; Juul, 1994) included both a character list with discrete character states and a character-taxon matrix. The numerical phylogenetic analysis allowed testing of primary homology statements, and this led to the identification of homoplastic character states. However, limits on computing power and the contemporary cladistic methods led to the reliance of suprageneric taxa. The studies by Sereno (1991a), Parrish (1993), and Juul (1994) provided the characters for the next 10 years. Gower and Wilkinson (1996) examined these three numerical studies as well as those from the 1980s and found that a consensus of the major clades of archosauriforms had been reached, but the position of some taxa (e.g., ornithosuchids) remained controversial. Nearly all modern numerical analyses obtained the same “phylogenetic backbone” presented by Gower and Wilkinson (1996) and discussed by Brochu (2001). As demonstrated by Gower and Wilkinson (1996) all phylogenetic hypotheses show the following: (1) proterosuchians, erythrosuchians, Euparkeria, and proterochampsians are closely related to but lie outside Archosauria; (2) Archosauria consists of a major split between the crocodylian and avian lineages; (3) phytosaurs, aetosaurs, ornithosuchids, various “rauisuchians,” and crocodylomorphs are part of the crocodylian lineage; and (4) pterosaurs, Marasuchus, and dinosaurs are part of the avian lineage.
The most recent phylogenetic studies (Bennett, 1996; Benton, 1999, 2004; Nesbitt and Norell, 2006; Irmis et al., 2007a; Nesbitt, 2007; Brusatte et al., 2008) reused the pool of characters provided by previous studies. Furthermore, the usage of suprageneric taxa as terminal taxa continued in most analyses (but see Irmis et al., 2007a). Unfortunately, recent authors did not provide detailed character descriptions or rationale for scoring strategies as did Sereno (1991a), Juul (1994), and Bennett (1996). This led to heavily recycled characters, sometimes compounding scoring errors from the original matrices. Few, if any, new characters have been added to these analyses. For example, Benton's (1999) character list consisted only of characters previously used in the literature. Benton (2004) and Nesbitt and Norell (2006) added taxa to Benton (1999), yet the relationships of pseudosuchians changed easily when new taxa and characters were added (see fig. 2).
Fig. 2
Phylogenetic relationships of basal Archosauriformes based on the matrix of Benton (1999): A, Benton (1999); B, Benton (2004); C, Nesbitt and Norell (2006); Nesbitt (2007). Suprageneric taxa are in bold.
The above briefly summarizes the major basal archosaur analyses and attempts an illustration of our current understanding of the major relationships of basal archosauriform clades. I have identified the following four critical portions of the basal archosaur tree that are controversial: (1) the relationships of non-archosaurian archosauriforms, (2) “rauisuchians,” (3) the sister taxon to Crocodylomorpha, and (4) basal avian-line archosaur relationships. Specifically, the controversial relationships in these sections are discussed below.
Non-archosaurian Archosauriforms
Non-archosaurian archosauriforms represent the successive outgroups to Archosauria. Therefore, an understanding of character transformations in non-archosaurian archosauriforms is critical to the optimization of the ancestral character states of Archosauria. Most analyses to date use a suprageneric Proterosuchidae and Erythrosuchidae without providing detailed information on the terminal taxon.
Proterosuchus, by definition (Gauthier et al., 1988), is the basalmost member of Archosauriformes. Prior to the use of numerical analyses, Proterosuchus and other potential proterosuchians (Proterosuchidae) were grouped with Erythrosuchus and other potential erythrosuchians (Erythrosuchidae) in the Proterosuchia (e.g., Charig and Reig, 1970; Charig and Sues, 1976). Moreover, the proterosuchians were thought to give rise to the sauropodomorphs (Thulborn, 1975), and erythrosuchians were thought to give rise to the rauisuchians (Sill, 1974; Bonaparte, 1982). In the cladistic paradigm, Proterosuchia has been found to be paraphyletic grouping (but see Gower and Sennikov, 1996) in which erythrosuchians (usually Erythrosuchus is the only member scored) are found closer to Archosauria than proterosuchians (usually Proterosuchus is the only member scored) are to Archosauria (Gauthier, 1986; Benton and Clark, 1988; Juul, 1994; Bennett, 1996; Benton, 2004). Gower and Sennikov (1996) found a monophyletic Proterosuchia in a study utilizing character data only from the braincase of various proterosuchians and erythrosuchians. However, a paraphyletic Proterosuchia was found in a later study using the braincase characters of Gower and Sennikov (1996) in combination with cranial and postcranial characters (Gower and Sennikov, 1997). The monophyly of Proterosuchidae and Erythrosuchidae needs further testing.
The resolution of the sister taxon of Archosauria remains controversial. Both proterochampsians (Sereno, 1991a; Parrish, 1993; Juul, 1994; Benton, 1999, 2004) and Euparkeria (Benton and Clark, 1988) were found as the sister taxon to Archosauria. However, Proterochampsia was always scored as a suprageneric taxon, and it is not clear which proterochampsian taxa were scored. Sereno (1991a) cited the following two characters that are present in proterochampsians + Archosauria but not in Euparkeria: postaxial intercentra absent and continuous crural facets on the astragalus. However, Sereno (personal commun. in Gower, 1996) stated that Euparkeria has continuous crural facets on the astragalus. Therefore, the immediate outgroup to Archosauria remains poorly understood.
New discoveries of Triassic archosauriforms fueled our understanding of the earliest archosaurs. The absolute number of new non-archosaurian archosauriforms has increased slowly with only a handful of new taxa in the last 20 years (e.g., Tropidosuchus, Arcucci, 1990; Sarmatosuchus, Sennikov, 1994; Vancleavea, Long and Murry, 1995). Additionally, the discoveries of new material and more detailed descriptions of previously known non-archosaurian archosauriforms (e.g., Erythrosuchus, Gower, 2003) have proven most useful in studies of basal archosauriforms (Nesbitt et al., 2009a).
Rauisuchians
Rauisuchians are pseudosuchian archosaurs from all continents, save Antarctica, during the Triassic (Bonaparte, 1982; Gower, 2000). Rauisuchians represent an important faunal component in the evolution of the Triassic fauna. For example, forms such as Postosuchus and Shuvosaurus have many similarities to theropod dinosaurs (Nesbitt and Norell, 2006; Brusatte et al., 2008). Generalities regarding the potentially monophyletic clade remain difficult to state because it is unclear whether rauisuchians represent a mono-, para-, or even polyphyletic group (Gower, 2000). It was common for large (2–6 m) carnivorous archosaurs from the Triassic with large skulls, recurved teeth, and elongated limbs to be referred to various subgroups of rauisuchians (e.g., Prestosuchidae, Rauisuchidae, Poposauridae) by Romer (1971b), Sill (1974), and Chatterjee (1985). Rauisuchia and various subgroups have been grouped together based on only a few potential synapomorphies (e.g., additional sacral vertebrae, rugose ridge on ilium, perforate acetabulum) and the fact they did not easily fit into Dinosauria, Aetosauria, Phytosauria, or Crocodylomorpha. An understanding of the interrelationships of rauisuchians is essential to an understanding of the early evolution of Archosauria, the stability of relationships of taxa within Pseudosuchia, and the identification of the sister taxon of Crocodylomorpha.
The precladistic classification of rauisuchians varied considerably. Huene (1942) coined the term Rauisuchidae for the fragmentary specimens Rauisuchus and Prestosuchus from the Triassic sequence of Brazil. The more complete remains of Ticinosuchus (Krebs, 1965) and Saurosuchus (Reig, 1961) solidified the presence of a widespread group of Triassic archosaurs. Krebs (1963, 1965) argued that Ticinosuchus and Rauisuchus were more closely related to crocodylians than to any other group—a view that was opposed by various workers (e.g., Hughes, 1963; Romer, 1966, 1972b; Bonaparte, 1982) who thought that rauisuchids were proterosuchians. Romer (1966) coined Prestosuchidae for a grouping centered on Prestosuchus, but placed Rauisuchus and Saurosuchus into the Erythrosuchidae. Others presented a different composition of both Prestosuchidae and Rauisuchidae (e.g., Charig, 1967) without justification or a discussion of diagnostic characters. Other than Ticinosuchus, most specimens were known from less than 25% of the skeleton.
Chatterjee (1985) described Postosuchus kirkpatricki from two relatively complete partial skeletons from the Late Triassic of Texas. Even though Chatterjee (1985) hypothesized that Postosuchus was a close relative of carnosaurian theropods, he established a framework for rauisuchian relationships. Chatterjee (1985) allied Postosuchus with Poposaurus gracilis, Arizonasaurus, Teratosaurus, and Bromesgroveia and placed them into the Poposauridae, whereas, following Bonaparte (1981, 1984), he placed Rauisuchus, Fasolasuchus, Prestosuchus, Saurosuchus, Ticinosuchus and various other fragmentary forms into Rauisuchidae. Furthermore, Chatterjee (1985) coined Rauisuchia to incorporate Rauisuchidae and Poposauridae. Galton (1985) independently arrived at a similar division in his study of Bromesgroveia. Other studies such as Long and Murry (1995) revised the alpha taxonomy of Postosuchus kirkpatricki, but did not include a cladistic analysis.
By the mid-1980s, nearly all authors considered rauisuchian taxa part of Pseudosuchia. This classification was followed in early cladistic studies of archosaurs. The first major cladistic studies of Archosauria (Gauthier, 1984, 1986) treated Rauisuchia as a monophyletic clade similar to Aetosauria and Phytosauria. Benton and Clark (1988) used Prestosuchus and Ticinosuchus to represent Rauisuchidae and Postosuchus to represent Poposauridae. Gauthier (1986) found Rauisuchia as the sister taxon of Crocodylomorpha, whereas Benton and Clark (1988) found Poposauridae as the sister taxon to Crocodylomorpha. Benton and Clark (1988) found Aetosauria and Rauisuchidae in a monophyletic group termed Rauisuchia, and Rauisuchia was found as the sister taxon of Poposauridae + Crocodylomorpha.
The shift to numerical analyses tested the monophyly of Rauisuchia, Rauisuchidae, and Prestosuchidae as originally conceived. Parrish (1993) and Juul (1994) included a mixture of species-level and suprageneric taxa of pseudosuchians (fig. 2) and they both found a polyphyletic Rauisuchia. Parrish (1993) and Juul (1994) found that prestosuchids (Ticinosuchus, Saurosuchus, and Prestosuchus in Parrish, 1993) fell outside other traditional rauisuchians and Postosuchus kirkpatricki was the approximate sister taxon to Crocodylomorpha. The phylogenetic relationships of Benton and Walker (2002), Benton (2004), and Weinbaum and Hungerbühler (2007) found a paraphyletic grouping of rauisuchians (fig. 3). That hypothesis was supported by three studies of the braincase of pseudosuchians (Gower and Walker, 2002; Gower, 2002; Gower and Nesbitt, 2006).
Fig. 3
Phylogenetic relationships of Pseudosuchia with the incorporation of a diversity of “rauisuchians”: A, Parrish (1993); B, Weinbaum and Hungerbühler (2007); C, Benton and Walker (2002); D, Gower (2002). Suprageneric taxa are in bold.
Most recently, Nesbitt (2003), Nesbitt and Norell (2006), Nesbitt (2007), and Weinbaum and Hungerbühler (2007) focused on taxa variously considered poposaurs, poposaurids, and shuvosaurids ( = chatterjeeids). These four studies found a well-supported monophyletic clade of poposaurids with shuvosaurids as the most derived members within the clade.
Even though many recent basal archosaur phylogenies included various rauisuchian taxa, the uncertainty in the relationships led to much confusion in descriptions of new taxa (e.g., Sen, 2005) or redescriptions of existing specimens (e.g., Gebauer, 2004). Furthermore, no explicit phylogenetic definitions or diagnoses of Poposauria, Poposauridae, Rauisuchia, Rauisuchidae, or Prestosuchidae have been presented to date. Parrish (1993) named a number of poorly supported nodes with a unique taxon composition (e.g., Rauisuchiformes) and (possibly unintentionally) redefined Rauisuchia to include Crocodylomorpha. Confusion of the taxonomic history of “rauisuchians” has prevented some authors from publishing new forms. For example, “Mandasuchus” (Charig, 1956) was never fully described and was always considered closely related to Ticinosuchus (Parrish, 1993; Sen, 2005), but most of the similarities listed (“ilium is slightly horizontally inclined” attributed to Charig as a personal commun in Juul,1994) cannot be accurately evaluated in the crushed holotype of Ticinosuchus ferox.
Fortunately, there is a renewed interest in rauisuchian anatomy and relationships. The number of new “rauisuchian” taxa dramatically increased in the last 10 years and includes the following taxa: Batrachotomus (Gower, 1999, 2002; Gower and Schoch, 2009); Effigia (Nesbitt and Norell, 2006; Nesbitt, 2007); Polonosuchus silesiacus (Sulej, 2005, sensu Brusatte et al., 2009); Qianosuchus (Li et al., 2006); Postosuchus alisonae (Peyer et al., 2008); Arganasuchus (Jalil and Peyer, 2007); and Yarasuchus (Sen, 2005). Furthermore, new, more complete specimens of the following important specimens have been found: Poposaurus (Weinbaum and Hungerbühler, 2007); Saurosuchus (Alcober, 2000); and Arizonasaurus (Nesbitt, 2003, 2005a). These studies provided the groundwork for new phylogenetic studies.
Crocodylomorpha
The oldest members of the Crocodylomorpha appear in the fossil record at either the end of the Carnian or the beginning of the Norian (e.g., Hesperosuchus agilis and Trialestes romeri) as fleet-footed, quadrupedal predators that looked more like an odd theropod dinosaur than members of Crocodylia. The first representatives were small (1–2.5 m in body length). Crocodylomorpha is the only clade of pseudosuchians to survive the Triassic-Jurassic boundary.
The early members of Crocodylomorpha from the Triassic and the Early Jurassic were lumped into the Sphenosuchia prior to explicit phylogenetic analyses. Walker (1970, 1972, 1990) convincingly showed that sphenosuchians were very closely related to Crocodyliforms, and this hypothesis has been validated by cladistic studies. Since the advent of cladistic methodologies, basal crocodylomorph workers have argued whether the Triassic and Jurassic sphenosuchians represent a monophyletic clade or a paraphyletic group that comprises a series of successive sister taxa of Crocodyliformes. A monophyletic Sphenosuchia was found by Sereno and Wild (1992), Wu and Chatterjee (1993), Clark et al. (2000), and Sues et al. (2003), whereas Clark (in Benton and Clark, 1988), Parrish (1991), and Clark et al. (2004) found Sphenosuchia as a paraphyletic assemblage (fig. 4). The dataset of Clark et al. (2000), which originally found a monophyletic Sphenosuchia, was transformed to produce a paraphyletic Sphenosuchia with the addition of new taxa and new characters (Clark et al., 2004). Poor resolution in basal crocodylomorphs is a direct result of often fragmentary specimens and conflicting signals in the postcrania and cranium (as demonstrated by Clark et al., 2004).
Fig. 4
Phylogenetic relationships of basal Crocodylomorpha: A, Clark in Benton and Clark (1988); B, Wu and Chatterjee (1993); C, Clark et al. (2000); D, Sereno and Wild (1992); E, Parrish (1991); F, Clark et al. (2004). Suprageneric taxa are in bold.
Different in-group relationships in studies that find a monophyletic Sphenosuchia demonstrate that outgroup choice is critical to resolution of the debate. Clark (in Benton and Clark, 1988), Parrish (1991), Wu and Chatterjee (1993), and the dataset of Clark et al. (2000) all use real outgroups including Gracilisuchus, Postosuchus, and an aetosaur. Clark (in Benton and Clark, 1988), Parrish (1991), Wu and Chatterjee (1993), and the dataset of Clark et al. (2000) all found Postosuchus (variously labeled as Poposauria/Poposauridae) as the proximal outgroup taxon. Unfortunately, Postosuchus kirkpatricki (sensu Chatterjee, 1985) is a chimera of different Triassic archosaurs (Long and Murry, 1995; Weinbaum 2002). On the other hand, Sereno and Wild (1992) used an all (0) outgroup, a strategy deplored by most phylogenetic workers. What then is an appropriate outgroup for Crocodylomorpha?
As pointed out by Clark et al. (2000), rauisuchians are an appropriate proximal outgroup because of the results of Benton and Clark (1988), Parrish (1993), and Juul (1994). Gower and Walker (2002) and Gower (2002) proposed the unique hypothesis that aetosaurs represent the sister taxon of Crocodylomorpha based on synapomorphies in the braincase shared by the aetosaurs (mainly Stagonolepis) and crocodylomorphs (mainly Sphenosuchus and Crocodylus). Gower and Walker (2002) also provided two potential cranial characters uniting the two taxa. Their data were limited to cranial characters; however, the hypothesis represented a clear alternative to a rauisuchian sister taxon. Therefore, the question about the monophyly versus paraphyly of Sphenosuchia may rely on the choice of the proximal outgroup.
Basal crocodylomorph specimens remain rare in fossil collections, and most specimens consist only of vertebrae and partial limb bones (e.g., Parrish, 1991; Long and Murry, 1995). Fortunately, several new taxa are largely complete, and they have doubled the known diversity of basal crocodylomorphs from both the Triassic and the Jurassic. This continuously growing list includes Dromicosuchus (Sues et al., 2003), Litargosuchus (Clark and Sues, 2002), Hesperosuchus “agilis” (Clark et al., 2000), Kayentasuchus (Clark and Sues, 2002), Protosuchus haughtoni (Gow, 2000), and Junggarsuchus (Clark et al., 2004).
Avian-line Archosaurs
Avian-line archosaurs ( = Avemetatarsalia of Benton, 1999) consist of pterosaurs, dinosaurs, and a range of intermediate forms. The only surviving members of the clade are modern birds, one of the most speciose clades of all extant vertebrates (Padian and Chiappe, 1998). Avemetatarsalians first appeared in the Middle Triassic, but they remained a rare component until the origin of the dinosaurs in the Late Triassic. Even in the Late Triassic, avian-line archosaurs were dominated in number of taxa, body types, and overall abundance by the crocodylian-line archosaurs (Irmis et al., 2007a; Brusatte et al., 2008). Many cladistic studies have focused on the origin of Dinosauria and its closest relatives, and these are nearly in complete agreement with each other (Gauthier, 1986; Sereno, 1991a; Juul, 1994; Sereno, 1999; Benton, 1999, 2004; Langer and Benton, 2006; Ezcurra, 2006; Irmis et al., 2007a; Brusatte et al., 2008) (fig. 5).
Fig. 5
Recent hypotheses of the phylogenetic relationships of basal Dinosauria: A, Novas (1996); B, Ezcurra (2006); C, Langer and Benton (2006); D, Irmis et al. (2007a). Suprageneric taxa are in bold.
Beginning with early cladistic studies, pterosaurs were found as the basalmost clade among avian-line archosaurs; this argument was well documented by Padian (1984), Sereno (1991a), Juul (1994), Bennett (1996 in part), and many other studies. Only one study since the cladistic revolution found pterosaurs outside Archosauria (Peters, 2000; but see Hone and Benton, 2007). Pterosaurs share a number of ankle characters and hind limb characters with dinosaurs and their close relatives, but their divergent morphology in the earliest members of the clade has proven difficult when reconstructing character optimizations at Ornithodira and Archosauria (Bennett, 1996; Padian, 2009). The first pterosaurs appear in the fossil record in the Norian of central Europe (Wild, 1978; Dalla Vecchia, 2003) and Greenland (Jenkins et al., 1994), but the ghost lineage of Pterosauria suggests the clade diverged by the Ladinian (Sereno, 1991a). Further, the oldest pterosaurs (e.g., Eudimorphodon and Austriadactylus) fall well within the pterosaur clade (Unwin, 2003). This indicates that much of the early history of Pterosauria is missing.
With the exceptions of Irmis et al. (2007a) and Brusatte et al. (2008), authors have treated Pterosauria as a suprageneric taxon in comprehensive phylogenetic analyses (Sereno, 1991a; Bennett, 1996; Benton, 1999). Authors either scored from basal taxa, derived taxa known from complete material, or a combination of both. Unfortunately, the scorings of Sereno (1991a) and Benton (1999) cannot be found in any one taxon of pterosaur. Irmis et al. (2007a) and Brusatte et al. (2008) scored species-level pterosaur taxa and found pterosaurs as the sister taxon to all other avian-line archosaurs.
The controversial taxon Scleromochlus taylori, a small-bodied form from the Late Triassic of Scotland, was considered the most primitive pterosaur (Huene, 1914; Padian, 1984; Sereno, 1991a) or the sister taxon to Pterosauria + Aves (Benton, 1999). Although the taxon is represented by several nearly complete skeletons, all specimens are preserved as external molds and some of the tarsal elements are slightly larger than the grain size of the coarse sandstone in which they are preserved.
Basal dinosauromorphs and dinosauriforms lie closer to Dinosauria than to any other archosaur clade. Once thought of as “advanced thecodontians,” Lagerpeton and Marasuchus from the Middle Triassic of Argentina are known to represent the closest relatives of Dinosauria (Romer, 1971a, 1972a; Bonaparte, 1975; Arcucci, 1986; Sereno and Arcucci, 1994a, 1994b; Novas, 1996). These small-bodied taxa, although not completely known, bear ankle and hind limb synapomorphies found only in Dinosauria (Novas, 1996). Arcucci (1987) described Pseudolagosuchus, a larger form from the same fossils beds that produced Lagerpeton and Marasuchus, and Novas (1996) identified this important taxon as the sister taxon to Dinosauria.
Until recently, it was thought that dinosaurs quickly replaced the “dinosaur precursors” in the Triassic. However, new finds of primitive dinosauromorphs in the southwestern United States (Irmis et al., 2007a; Nesbitt et al., 2009b), reevaluations of purported Triassic dinosaurs (Nesbitt et al., 2007) and new finds of dinosauriforms (Dzik, 2003; Ferigolo and Langer, 2007) have shown that the closest relatives of dinosaurs evolved along with the dinosaurs for much of the Triassic. Furthermore, the bizarre, possibly quadrupedal dinosauriforms Silesaurus and Sacisaurus bear a suite of classical dinosaurian features, ornithischian dinosaurlike cranial features, and characters not found in any dinosaur. Most authors hypothesized that Silesaurus is a non-dinosaurian dinosauriform (Langer and Benton, 2006; Ezcurra, 2006; Nesbitt, 2007; Irmis et al., 2007a).
Owen (1842) conceived Dinosauria as consisting of the theropod Megalosaurus and the ornithischians Hylaeosaurus and Iguanodon. Many early workers were convinced that the different lineages of dinosaurs (e.g., sauropods) arose independently from a “basal stock” of Triassic “thecodontians” (e.g., Thulborn, 1971; Charig, 1976). In a seminal study, Bakker and Galton (1974) cemented Owen's (1842) original concept of Dinosauria and argued for the monophyly of the clade. Since 1974, most workers have agreed that a monophyletic Dinosauria comprises three major lineages, Ornithischia, Sauropodomorpha, and Theropoda and that Sauropodomorpha and Theropoda are sister taxa (Gauthier, 1984, 1986; Gauthier and Padian, 1985; Benton and Clark, 1988; Juul, 1994; Sereno, 1999).
Renewed interest in the origin of Dinosauria has led to the discovery of a greater diversity of basal members of each major dinosaur lineage. This includes the basal saurischians or theropods Herrerasaurus (Sereno and Novas, 1992, 1994b; Sereno, 1994; Novas, 1994) and Eoraptor (Sereno et al., 1993), the primitive sauropodomorphs Saturnalia (Langer et al., 1999) and Pantydraco (Yates, 2003; Galton et al., 2007), and the primitive ornithischian Eocursor (Butler et al., 2007). Combined with new specimens of basal dinosauromorphs and dinosauriforms, these new finds brought a wealth of anatomical data to the evolution of character states immediately outside of and within Dinosauria. Specifically, these finds helped optimize synapomorphies that can clarify a response to “What makes a dinosaur a dinosaur?” All numerical analyses provided a core set of dinosaurian synapomorphies, many of which overlapped. However, the absence of skulls and hands in the proximal outgroups of Dinosauria has prevented the optimization of many characters at Dinosauria. The new material of basal dinosauriforms, represented by both crania and postcrania, allows further testing of dinosaurian synapomorphies.
Objectives
This study investigates the evolutionary relationships of basal archosauriforms and places disparate clades of Triassic archosaurs into a comprehensive analysis. Since the advent of basal archosaur phylogenetic studies in the 1980s, the taxonomic sampling has more than doubled. Increasingly, the relationships of several basal archosaur clades (e.g., Dinosauria, Crocodylomorpha, Aetosauria) have been devoid of a larger phylogenetic context, and, thus, the chosen outgroups have affected in-group relationships. Here, all previous basal archosaur studies are combined, from studies examining the evolution of the ankle to studies looking at the relationships of basal crocodylomorphs. I employ rigorous character formulation; previously used characters are critically evaluated and in most cases modified or terminated, and new characters are also added. Nearly every character is fully described and put into a comparative context in an attempt to increase anatomical knowledge of basal archosaurs. Furthermore, each terminal taxon is carefully described. The resultant character list and taxon-character matrix represents a tripling of both character and taxa sampled. This explicit phylogenetic analysis contains scoring strategies and decisions regarding which specimens represent which species-level taxon, along with full synapomorphy lists. This approach records each step in the formulation of my phylogenetic hypothesis from bone features to analyzing phylogenetic trends. My record-keeping provides a framework for reproducing my results in future studies.
I address the following four major questions: (1) What is the sister taxon to Archosauria and what synapomorphies are found in the common ancestor of crocodylians and avians? (2) Are rauisuchians a mono-, para-, or polyphyletic group and what are their closest relatives? (3) What is the sister taxon to Crocodylomorpha? and (4) What characters support a monophyletic Dinosauria?
The answers to these questions provide a testable framework for asking further questions about the early diversification of Archosauria. Questions regarding the split between the crocodylian and avian lineages have important implications for the calibrations of molecular studies of extant archosaurs. Also, understanding the rate at which archosaur lineages evolved in their initial diversification gives insights about the tempo and mode of early archosaur diversification.
In addition, this study attempts to build a framework so that the relationships of incomplete specimens and even isolated specimens can be confidently added to studies of biogeography, abundance, paleoecology, extinction, and morphological rate change. The identification of a specimen is the first step in any evolutionary study, whereas the second and equally important step is putting that specimen into a comparative context. A comprehensive phylogeny is required for both. For example, Nesbitt et al. (2007) tested the identifications of early dinosaurs from the Upper Triassic of the western United States using the latest, most comprehensive diagnosis of Dinosauria, whereas Nesbitt and Stocker (2008) incorporated fragmentary fossils from a single quarry into a phylogenetic context to examine the validity of assemblage comparisons in the Chinle Formation of northern New Mexico. Furthermore, the explicit phylogeny allows the identification of homoplastic characters that may have been previously used to identify fragmentary fossils incorrectly.
Terminology
The phylogenetic definition of Archosauriformes is based on ancestry following Gauthier and Padian (1985), Gauthier (1986), Sereno (1991a), and other recent revisions (e.g., Senter, 2005). I present a summary of important taxa tied to phylogenetic definitions in figure 6. The phylogenetic taxonomy accepted here conforms to the most widely used clade names and remains the most logical to facilitate comparisons to previous phylogenetic hypotheses (see tree description for full definitions).
Fig. 6
Archosauriform stem and node clade names used in this study. Circles = nodes; chevrons = stem groups.
Here, I use Archosauria as first phylogenetically defined by Gauthier and Padian (1985) and Gauthier (1986) and not in the traditional sense of Romer (1966), Benton, and Gower (various works) and Juul (1994). Archosauria (sensu Gauthier and Padian, 1985) is equivalent to crown-group archosaurs and Avesuchia (Benton, 1999). Pseudosuchia (Zittel, 1887–1890, sensu Gauthier and Padian, 1985) is used interchangeably with crocodylian-line archosaurs, and “avian-line archosaurs” is used interchangeably with Avemetarsalia. The term “basal” generally describes the first few branches of a lineage with respect to later, more-derived members. For example, basal archosauriforms refers to non-archosaurian archosauriforms and members of Archosauria extending up into Crocodyliformes and within Dinosauria, whereas basal archosaurs excludes non-archosaurian archosauriforms.
Terms such as “rauisuchian” and “poposaurid” are usually avoided throughout this text because the monophyly of these clades was questioned recently (Gower, 2000). Instead, species-level taxa and specimen numbers are employed for explicitness and to test the monophyly of these groups. When used, the term “rauisuchian” is used in its traditional sense and includes taxa variously considered as members of Rauisuchidae, Prestosuchidae, Poposauridae, and Chatterjeeidae (Gower, 2000).
TERMINAL TAXA
Mesosuchus browni Watson, 1912
Referred material
SAM 6536, complete well-preserved skull and anterior half of the skeleton; SAM 7416, partial postcranial skeleton.
Key references
Watson, 1912; Broom, 1925; Dilkes, 1998.
Prolacerta broomi Parrington, 1935 (fig. 7E)
Fig. 7
Skull reconstructions of Triassic archosauriform terminal taxa: A, Proterosuchus fergusi in lateral and B, dorsal views; redrawn from Cruickshank (1972); C, Chanaresuchus bonapartei in lateral and D, dorsal views; modified from Romer 1972b; E, Prolacerta broomi in lateral view; based on BP/1/471; F, Erythrosuchus africanus in lateral and G, dorsal views; redrawn from Gower (2003); H, Euparkeria capensis in lateral and I, dorsal views; redrawn from Ewer (1965); J, Smilosuchus gregorii in lateral view; based on AMNH FR 3060. See appendix for anatomical abbreviations. Scale bars = 5 cm in A–B, F–G, J, and 1 cm in E, H–I.
Referred material
BP/1/471, complete skull; BP/1/2675, nearly complete skull with postcrania; BP/1/2676, nearly complete skeleton; UCMP 37151, skull; AMNH 9502, postcranial skeleton.
Key references
Parrington, 1935; Camp, 1945; Gow, 1975; Evans, 1988; Colbert, 1987; Gower and Sennikov, 1996; Dilkes, 1998; Modesto and Sues, 2004.
Proterosuchus fergusi Broom, 1903 (fig. 7A–B)
Referred material
TM 201, incomplete skull; RC 96, complete skull; BSP 514, nearly complete skull and anterior cervical vertebrae; NM QR 1484 (also listed as NMC 3016), complete skull and nearly complete articulated skeleton; NM QR 880, complete braincase and partial skull, partial postcrania; AMNH FR 2237, fragmentary postcranial skeleton with nearly complete articulated leg; BP/1/3993, nearly complete skull with braincase.
Key references
Broom, 1903; Cruickshank, 1972, 1979; Gow, 1975; Welman and Flemming, 1993; Welman, 1998.
Erythrosuchus africanus Broom, 1905 (fig. 7F–G)
Referred material
BP/1/ 5207, complete skull; SAM-K1098, maxilla; BMNH R3592, partial skull and skeleton; BMNH R3267a, incomplete postcranium.
Key references
Broom, 1905; Huene, 1911; Charig and Reig, 1970; Cruickshank, 1972; Gower, 1996, 1997, 2003.
Vancleavea campi Long and Murry, 1995
Occurrence
Mesa Redondo Member, Chinle Formation, Arizona; Monitor Butte Member, Chinle Formation, Utah; Blue Mesa Member, Chinle Formation, Arizona; Sonsela Member, Chinle Formation, Arizona; Petrified Forest Member, Chinle Formation, Arizona; Owl Rock Member, Chinle Formation, Arizona; “Siltstone Member,” Chinle Formation, New Mexico; Bull Canyon Formation, New Mexico; Redonda Formation, New Mexico; Tecovas Formation, Dockum Group, Texas.
Remarks
Vancleavea stands as one of the most bizarre archosauriforms recorded to date (Nesbitt et al., 2009a). The morphology of Vancleavea is unparalleled within Reptilia; it has four unique types of imbricated osteoderms covering the entire body, a short, highly ossified skull, relatively small limbs, and morphological features consistent with a semiaquatic lifestyle. Until recently, the taxon was only represented by a handful of incomplete specimens (Hunt et al., 2002, 2005). However, nearly complete specimens indicate that Vancleavea represents one of only few non-archosaurian archosauriforms from Laurasia (Parker and Barton, 2008; Nesbitt et al., 2009a). The long stratigraphic range of Vancleavea in the Chinle Formation suggests that it or similar taxa were present for much of the Late Triassic in western North America.
Key references
Long and Murry, 1995; Small and Downs, 2002; Hunt et al., 2002, 2005; Parker and Barton, 2008; Nesbitt et al., 2009a.
Chanaresuchus bonapartei Romer, 1971b (fig. 7C–D)
Referred material
PVL 4586, skull; PVL 4575, complete skull and nearly complete postcranial skeleton; PVL 4647, braincase and partial skull; MCZ 4035, complete skull and postcrania; MCZ 4036, skull and most of the postcranium.
Key references
Romer, 1971b, 1972b; Sues et al., 1976; Arcucci, 1990.
Tropidosuchus romeri Arcucci, 1990
Holotype
PVL 4601, nearly complete articulated skeleton without the distal portions of the forelimbs.
Referred material
PVL 4602, vertebral column, hind limbs, and partial skull; PVL 4603, complete vertebral column, posterior portion of the skull, osteoderms; PVL 4604, pectoral and forelimb elements; PVL 4605, much of an articulated skeleton including skull; PVL 4606, complete skull, presacral vertebrae, pelvic girdle, and hind limb elements; PVL 4624, hind limb elements.
Referred material
SAM 6050, partial skull; SAM 6047B, vertebrae, femur, pelvis, pectoral girdle; SAM 6049, dorsal, sacral, and caudal vertebrae, right hind limb, and partial pelvic and pectoral girdles; SAM 6047A, skull, vertebrae, and limb fragments; UMCZ T692, articulated foot with astragalus and calcaneum removed for study.
Key references
Broom, 1913; Ewer, 1965; Gower and Weber, 1998; Senter, 2003.
Parasuchus hislopi (Lydekker, 1885), sensu Chatterjee, 1978
Neotype
ISI R 42, nearly complete skull (Chatterjee, 1978: pl. 8) and articulated skeleton (see Chatterjee, 2001).
Referred material
ISI R 43, most of complete articulated skeleton lacking the forelimbs and the anterior portion of the skull.
Remarks
Parasuchus hislopi had confusing taxonomic history (Chatterjee, 1978) that is continued today (Lucas et al., 2007b). The nondiagnostic holotype was replaced by a neotype (ISI R 42) with approval from the ICZN (Opinion 2045) following the application of Chatterjee (2001). As a result, I score characters only from the two nearly complete skeletons described and illustrated by Chatterjee (1978). The two articulated skeletons represent the most complete phytosaurs known to date. Additionally, Parasuchus is important because it has been found as one of the most primitive phytosaurs in phylogenetic analyses of Phytosauria (see Lucas et al., 2007b, for references).
Key reference
Smilosuchus gregorii (Camp, 1930), sensu Long and Murry, 1995 (fig. 7J)
( = Machaeroprosopus gregorii Camp, 1930)
Referred material
USNM 18313, partial skull, complete mandible, and nearly complete postcranial skeleton; AMNH FR 3060, skull, mandible, pelvis, osteoderms, partial hind limb.
Remarks
Smilosuchus represents one of the largest phytosaurs (skull length >1.5 m) recovered from the Chinle Formation. As with most phytosaur specimens, the holotype of Smilosuchus consists of a skull and only fragments of the postcrania. Therefore, I rely on the nearly complete, disarticulated postcranial skeleton with associated skull of USNM 18313 for scoring postcrania. The ankle of USNM 18313 has figured prominently in studies of phytosaur locomotion (Parrish, 1986) and the origin of the “crocodile-normal” ankle type (Sereno, 1991a).
Key references
Camp, 1930; Gregory, 1962; Colbert, 1947; Ballew, 1989; Long and Murry, 1995; Stocker, 2008.
Pseudopalatus pristinus Mehl, 1928
Age
Mid-late Norian, Late Triassic; most specimens from Petrified Forest National Park, Arizona fall within 30 m of the Black Forest Bed which has been radiometrically dated at 213 ± 1.7 Ma (Riggs et al., 2003).
Occurrence
Petrified Forest Member of the Chinle Formation, Arizona; Bull Canyon Formation of the Dockum Group, New Mexico and Texas.
Referred material
UCMP 27235, partial skull and partially articulated postcranium, including much of the pes and manus; UCMP 34249, complete skull; various other isolated Pseudopalatus pristinus elements from the Canjilon Quarry (UCMP V2816); UCMP 34253, complete presacral column, sacrals, and anterior caudal vertebrae.
Remarks
Pseudopalatus pristinus occurs throughout the upper half of the Chinle Formation and Dockum Group and stands alone as one of the most completely known derived phytosaurs. As a result, it is constantly cited as biostratigraphically useful (see Lucas, 1998a). Additionally, Zeigler et al. (2002, 2003) proposed that Pseudopalatus pristinus represents a sexual morph opposite Pseudopalatus buceros. Here, I score material referable to Pseudopalatus from the Canjilon Quarry (UCMP V2816), particularly the articulated specimens UCMP 27235 and UCMP 34253 and the complete well preserved skull UCMP 34249.
Key references
Mehl, 1928; Ballew, 1989; Long and Murry, 1995; Hungerbühler, 2002.
Gracilisuchus stipanicicorum Romer, 1972c (fig. 8C–D)
Fig. 8
Skull reconstructions of pseudosuchian archosaur terminal taxa: A, Riojasuchus tenuisceps in lateral and B, dorsal views; redrawn from Sereno (1991a); C, Gracilisuchus stipanicicorum in lateral and D, dorsal views; redrawn from Romer (1971b); E, Arizonasaurus babbitti in lateral view; redrawn from Nesbitt (2005); F, Stagonolepis robertsoni in lateral and G, dorsal views; redrawn from Walker (1961); H, Revueltosaurus callenderi in lateral view; based on PEFO 34561. I, Effigia okeeffeae in lateral view; redrawn from Nesbitt and Norell (2006); J, Xilousuchus sapingensis in lateral view; based on IVPP V 6026. Shaded areas indicate incomplete preservation. See appendix for anatomical abbreviations. Scale bars = 1 cm.
Referred material
MCZ 4116, partially disarticulated skull; MCZ 4117, complete skull; MCZ 4118, partial skull, cervical vertebrae, and osteoderms; PVL 4597, nearly complete skull, presacral vertebrae, osteoderms, sacrum, pelvic girdle, nearly complete hind limb; PVL 4612, nearly complete skull, articulated presacral vertebrae.
Remarks
Gracilisuchus is known from at least five articulated skulls and much of the postcranial skeleton except for the forelimbs. The forelimb assigned to Gracilisuchus by Romer (1972c) is too small for the size of the holotype, and Sereno and Arcucci (1994b) considered it part of the undiagnostic holotype of “Lagosuchus.” Many of the specimens are either dorsoventrally or mediolaterally crushed. As a result, the articulations and orientations of the posterior skull bones have been hotly debated in the literature (Romer, 1972c; Brinkman, 1981; Parrish, 1993).
Gracilisuchus was first described as an ornithosuchid by Romer (1972c), whereas Brinkman (1981) recognized the completely “crocodile normal” ankle and suggested it was closer to crocodylians. Ever since, Gracilisuchus was found in a variety of positions among crocodylian-line archosaurs. Parrish (1993) found Gracilisuchus as a close relative of Postosuchus and crocodylomorphs, whereas Benton and Clark (1998) found it as a basal suchian. More recently, Benton (2004) found Gracilisuchus as the sister taxon to Phytosauridae. Gracilisuchus has also been used as an outgroup in phylogenetic analyses of basal crocodylomorph relationships (the dataset of Clark et al., 2000). Until recently, Gracilisuchus stood alone as potentially one of the oldest suchians.
Occurrence
Vertebrate Fossil Bed IV (Kannemeyeriid Zone), lower Kelamayi Formation, Taoshuyuanzi, about 30 km northwest of Turfan Basin, Xinjiang.
Remarks
Turfanosuchus is one of the oldest archosauriforms with a nearly complete skull and a partial skeleton. The partial skeleton was reassembled, the missing portions were sculpted, and the specimen was encased in plaster and then painted. The processing of the specimen for display purposes concealed details of the skeleton and obscured recognition of which bones were fossils and which were sculpted. For example, Young illustrated a nearly complete right manus and pes (Young, 1973: fig. 2). A recent inspection by me suggests that only the proximal portions of the metatarsals are preserved, whereas the manus and most of the pes are sculpted. Recently, Wu and Russell (2001) described reprepared material including the skull, femur, ilium, pubis, humerus, a newly discovered osteoderm, calcaneum, and astragalus. The specimens were fixed back to the mount after the completion of their study. The morphology of the astragalus could not be confirmed in this study and is not scored here.
The systematic position of Turfanosuchus has been debated recently and was included in only in only a few phylogenetic analysis thus far (Parrish, 1993; Dilkes and Sues, 2009). Parrish (1993) found Turfanosuchus well nested among crocodylian-line archosaurs. In a point-by-point response to the character scoring of Parrish (1993), Wu and Russell (2001) concluded that Turfanosuchus is neither a suchian nor a crurotarsan ( = crocodylian-line archosaur in their meaning). Even though the analysis of Parrish (1993) was fraught with problems, Wu and Russell's (2001) detailed discussion of why Turfanosuchus is not an archosaur warrants further comment (character number from Wu and Russell, 2001, in parentheses):
(4) Presence of palatal teeth: Even though palatal teeth are present in Turfanosuchus and many non-archosaurian diapsids, palatal teeth are present in the archosaur Eoraptor. Furthermore, the pterygoid teeth in Turfanosuchus are exceedingly small and well spaced; therefore, pterygoid teeth may not be recognized in poorly preserved taxa, disarticulated taxa, or taxa without palates preserved.
(5) Foramina for internal carotid arteries enter the body of the basisphenoid ventral to the basipterygoid processes: The foramina for the entrance of the internal carotid arteries enter ventrally in Turfanosuchus, Euparkeria (SAM 5867), proterochampsians (e.g., Chanaresuchus, PVL 4647), and other non-archosaurian archosauriforms. Previously, it was thought that the internal carotid arteries entered laterally in archosaurs (see Gower and Sennikov, 1996; Gower and Walker, 2002). However, new discoveries, including Arizonasaurus (Gower and Nesbitt, 2006), Qianosuchus (Li et al., 2006) and Silesaurus (Dzik, 2003), showed that a ventral entrance for the internal carotid arteries occurs within Archosauria.
(7) Calcaneal tuber shaft broader than tall: Wu and Russell (2001) confused the measurements of the tuber shaft with the dimensions of the distal end of the tuber. In fact, the shaft of the tuber is wider than tall (contra Wu and Russell, 2001).
(8) Calcaneal tuber not flared distally: The distal end of the calcaneum tuber of Turfanosuchus is flared (contra Wu and Russell, 2001).
Most recently, Dilkes and Sues (2009) found Turfanosuchus outside Archosauria giving support to the hypothesis of Wu and Russell (2001).
Key references
Young, 1973; Parrish, 1993; Wu and Russell, 2001.
Ornithosuchus longidens (Huxley, 1877), sensu Walker, 1964
Occurrence
Lossiemouth Sandstone Formation, various sandstone quarries in the Elgin area, Scotland (see Walker, 1964, for details).
Remarks
Ornithosuchus is one of the few archosaurs from the Late Triassic of Scotland known from both natural molds and preserved remains. The genus-level taxon has a complicated taxonomic history given the poor preservation of the specimens. Walker (1964) reviewed all species of Ornithosuchus and concluded that all the material from the Elgin area represents one species-level taxon, Ornithosuchus longidens. Walker's concept of Ornithosuchus was followed by all subsequent workers. Sereno (1991a) listed five autapomorphies of Ornithosuchus that are accepted here.
The relationships of Ornithosuchus are as complicated as its taxonomy history. Since the initial description, Ornithosuchus was considered an archosaur (in the contemporary usage) with possible affinities with dinosaurs, phytosaurs, and aetosaurs (Newton, 1894; Boulenger, 1903; Huene, 1914; Walker, 1964). In a modern cladistic framework, Ornithosuchus ( = Ornithosuchidae) was first found as one of the most basal avian-line archosaur clades (Gauthier, 1986; Benton and Clark, 1988), which was subsequently used as a basis to name the avian-line archosaur stem as Ornithosuchia (Gauthier and Padian, 1985). More recent analyses placed Ornithosuchus closer to crocodylians than to phytosaurs (Parrish, 1993; Benton, 1999), as the sister taxon of the Suchia (Sereno, 1991a), or within Suchia (Juul, 1994; Irmis et al., 2007a).
Key references
Huxley, 1877; Walker, 1964; Sereno, 1991a.
Riojasuchus tenuisceps Bonaparte, 1967 (fig. 8A–B)
Holotype
PVL 3827, complete skull, cervical, dorsal, sacral and caudal vertebrae, scapula, coracoid, humerus, distal portion of the radius and ulna, partial manus, ilium, pubis, femur, tibia, fibula, nearly complete pes.
Referred material
PVL 3828, nearly complete skull, cervical, dorsal, sacral and caudal vertebrae, scapula, coracoid, humerus, ulna, radius, pubis, ischium, ilium, femur, tibia, fibula, calcaneum; PVL 2826 cervical, dorsal, sacral and caudal vertebrae, coracoids, scapula fragments, humerus, ulna, radius, ilium, femur, and tibia; PVL 3814 vertebrae, humerus, and tibia.
Remarks
Riojasuchus is represented by nearly all portions of the skeleton. The well-preserved complete skull was well described by Bonaparte (1971), followed by Sereno (1991a). It is clearly the youngest member of the Ornithosuchidae and apparently represents a late surviving member of the clade.
Sereno (1991a) provided a list of autapomorphies that all differentiate Riojasuchus from Ornithosuchus. However, autapomorphies I, K, and L have a wider distribution. For example, aetosaurs (e.g., Aetosaurus, SMNS 5770) possess a sloping occiput much like that of Riojasuchus.
Key references
Bonaparte, 1967; 1971; Sereno, 1991a.
Stagonolepis robertsoni Agassiz, 1844 (fig. 8F–G)
Remarks
First regarded as a ganoid fish, Stagonolepis is one of the better known aetosaurs largely as the result of the work of Huxley (1877) and especially Walker (1961). Although casts produced from sandstone molds represent nearly all the specimens, Walker (1961) laboriously worked to produce a rather complete anatomy of Stagonolepis. Details of the pes and other bones are missing because of the preservation of the material. I urge future workers to score characters from the actual casts and molds of the material and not reconstructions of the material, even though it is tempting given Walker's fine work on the Lossiemouth Sandstone archosaurs. Here, I specifically use Stagonolepis so that the observations of the braincase by Gower and Walker (2002) could be included in a broader context. Furthermore, I assume all the aetosaur material from the Lossiemouth Sandstone Formation belongs to Stagonolepis robertsoni.
Lectotype
Specimen 16 (XVI), a nearly complete skull and postcranium that is part of the SMNS 5770 cluster, an assemblage of at least 25 specimens.
Referred material
SMNS 5771 (type locality and horizon), SMNS 18554 (articulated skeleton lacking skull and pectoral girdle; Blankenhorn Castle near Eibensbach); Middle Stubensandstein from Pfaffenhofen: SMNS 11837 (type of A. crassicauda), SMNS 12670 (collection of isolated dorsal plates and a fragment of the ventral osteoderms); SMNS 14882 (articulated tail portion with osteoderms and 14 caudal vertebrae).
Remarks
Aetosaurus was named by Fraas (1877) from an accumulation of at least 24 individuals that lie in almost complete articulation. Although Aetosaurus is known from well-preserved articulated material, the extensive osteoderm carapace or other skeletal elements conceal details of the vertebrae, braincase, palate, and pectoral and pelvic girdles. A detailed and useful review of the taxon by Schoch (2007) provided additional information on the skull, variation, and osteoderms. Even though specimens from outside the Lower Stubensandstein have been referred to Aetosaurus (Jenkins et al., 1994; Heckert and Lucas, 1998; Small, 1998) only the individuals numbered SMNS 5770 are scored here.
Key references
Fraas, 1877; Huene, 1920; Walker, 1961; Wild, 1989; Schoch, 2007.
Longosuchus meadei (Sawin, 1947), sensu Hunt and Lucas, 1990
Occurrence
Otis Chalk area, TMM 31025 (Quarry 1), TMM 31099 (Quarry 2), TMM 31100 (Quarry 3), TMM 31185 (Quarry 3A), TMM 31098 (site 3), TMM 31220 (sites 3, 4) “Pre-Tecovas horizon” (Long and Murry, 1995).
Lectotype
TMM 31185-97 postcrania (formerly 31185-84b). The well-preserved skull TMM 31185-98 belongs to the postcranial skeleton numbered TMM 31185-97 (Sawin, 1947), but because of ICZN rules, it is not part of the lectotype (Parker and Martz, 2010).
Referred material
TMM 31185-98, skull (formerly part of 31185-84b [Sawin, 1947; Hunt and Lucas, 1990]); TMM 31185-97 (formerly 31185-84a), axial, pelvic and pectoral elements, limb fragments; TMM 31185-84a, appendicular elements axial skeleton, many osteoderms, manus, pes; TMM 31100-435, two-thirds of an articulated tail.
Remarks
Originally named as a species of Typothorax (Sawin, 1947), Hunt and Lucas (1990) renamed the taxon as Longosuchus meadei based on the divergent morphology of the TMM specimens with those of Typothorax coccinarum. Most elements of the skeleton of Longosuchus are known from both articulated and disarticulated specimens and were collected from a limited geographic area near Otis Chalk (see Sawin, 1947). Two exquisitely preserved skulls, one partial and one nearly complete, preserve details of the palate, braincase, and details on the medial side of each preserved element (Sawin, 1947; Parrish, 1994). Furthermore, the specimens preserve manus and pedes in the collected material making Longosuchus the most complete, large bodied aetosaur. Unfortunately, many of the bones (e.g., pedes) described by Sawin (1947) are mounted in a reconstruction on display at the Texas Memorial Museum, at the University of Texas at Austin. Based on osteoderms, Lucas (1998b, 1998c) reported Longosuchus from the Timesgadiouine Formation of Morocco and Pekin Formation of the Newark Supergroup; none of these specimens is used to score the taxon here.
Key references
Parrish, 1994; Small, 2002; Parker, 2003.
Revueltosaurus callenderi Hunt, 1989 (fig. 8H)
Age
Middle-late Norian (Parker et al., 2005; most specimens from PEFO fall within 30 m of the Black Forest Bed which was radiometrically dated at 213 ± 1.7 Ma (Riggs et al., 2003).
Referred material
PEFO 34561, essentially complete skeleton; PEFO 34269, nearly complete skeleton; see Parker et al. (2005) for other specimens.
Remarks
The original description of Revueltosaurus was based on isolated teeth from the Upper Triassic deposits of the American Southwest (Hunt, 1989; Padian, 1990). As described by Hunt (1989), followed by Heckert (2002), Revueltosaurus shares an uncanny resemblance to the teeth of early ornithischians. The similarity of teeth of Revueltosaurus to ornithischians led to the proliferation of the naming of isolated diagnosable teeth similar to those of ornithischians from other Triassic deposits (Hunt and Lucas, 1994; Heckert, 2002). These isolated teeth formed the basis of our understanding of the early ornithischian record in North America and Europe. Parker et al. (2005) reported a partial skeleton referred to Revueltosaurus from the Petrified Forest Member of the Chinle Formation. They demonstrated that Revueltosaurus is a pseudosuchian, not a dinosaur. Consequently, Revueltosaurus illustrates the difficulty of assigning isolated teeth to a taxon; none of the “ornithischian-like” teeth from the Triassic of southwestern America can be confidently assigned to Ornithischia (Irmis et al., 2007b).
Much of the Revueltosaurus cranial and postcranial material originates from a monotypic bonebed. The specimens occur as isolated bones, complete associated specimens, or articulated skeletons. Thus, nearly the entire skeleton of Revueltosaurus is known. The phylogenetic position of the new, nearly complete specimens of Revueltosaurus have yet to be tested in a broad phylogenetic analysis of basal archosaurs. A full description of the skeleton is underway (Parker et al., in prep.).
Key references
Hunt, 1989; Heckert, 2002; Parker et al., 2005.
Ticinosuchus ferox Krebs, 1965 (figs. 9–10)
Fig. 9
A, The skull of Ticinosuchus ferox (PIZ T2817) with highlighted cranial elements (modified from Krebs 1965). The gray “piece” actually belongs underneath the piece with the left maxilla. B, Corrected skull of Ticinosuchus with the incorrectly placed piece removed. See appendix for anatomical abbreviations. Scale bar = 10 cm.
Fig. 10
Gut contents of Ticinosuchus ferox (PIZ T2817): A, drawing of the skeleton of Ticinosuchus (from Krebs, 1965); B, close up of the area posterior of the pelvis showing a small mass of scales (arrow); C, close up of the accumulation of bone fragments and scales; D–E, detailed photograph of fish scales. Scale bars = 10 cm in A–B, 1 cm in C, and 1 mm in D–E.
Referred material
PIZ T2471, six articulated caudal vertebrae with osteoderms; BES 189 at Museo Civico di Storia Naturale, Milano.
Remarks
Ticinosuchus was named by Krebs (1965) based on an essentially complete skeleton found near the Anisian-Ladinian boundary in marine sediments. The skeleton preserves most elements in either articulated or disarticulated state. However, as noted by Krebs (1965), many of the bones are heavily crushed, hidden by other elements, or have poorly preserved surfaces.
This has prevented comparison of individual elements to other pseudosuchian taxa, and many features cannot be scored into phylogenetic analyses. As a result, the phylogenetic position is poorly supported in the few analyses in which it has been included (e.g., Parrish, 1993; Benton, 2004).
I examined the skull region very carefully and have a few comments. Unfortunately, the blocks with skull elements appear to have been reassembled incorrectly. The skull must have been split longitudinally when collected, and a portion of the right side was placed incorrectly posterior to the left side of the skull (see fig. 9). This is apparent because the specimens from the “Grenzbitumen” horizon were collected in pieces, then reassembled later (Furrer, personal commun.). The medial surface of the right maxilla is exposed laterally, and there is a large gap between the anterior and posterior portions of the skull. Once this incorrectly placed piece is removed and the anterior and posterior portions of the skull are brought back together, the skull becomes much shorter (see fig. 9). Parts of the skull that were not originally identified include the left frontal and postfrontal in ventral view, the left parietal in lateral view, an upside-down left prearticular in medial view, the impression of the left angular, the left nasal in ventral view, and ?left lacrimal in ?medial view (see fig. 9).
The holotype of Ticinosuchus ferox preserves the remains of its last meal (fig. 10), and this represents one of the few examples of prey choice in any Triassic archosaur (see Nesbitt et al., 2006). A small collection of fish scales is present at the base of the tail posterior to the ilium and the proximal portion of the ischium. The three-dimensional structure consists of a tan matrix with randomly oriented fish scales (fig. 10D–E). Even though there are a few fish scales located throughout the matrix in which Ticinosuchus is entombed, the abundance of fish scales at the base of the tail far exceeds any other concentration on the slab. It is clear that Ticinosuchus included fishes in its diet, and it is unclear to which taxon or taxa the scales belong. Unfortunately, the identity of the fish as either freshwater or marine is not known at this time. The prey choice of Ticinosuchus may explain why the seemingly terrestrial carnivore would be found in marine sediments. It is clear that Ticinosuchus must have been living close to the shoreline. A similar taxon, Qianosuchus, was also found in marine sediments. Therefore, it is possible that Qianosuchus may have also lived near the shoreline.
Krebs (1965) described much of the skeleton of Ticinosuchus in detail; however, Parrish (1993) discussed a few features of the osteoderms. Parrish (1993) stated that Ticinosuchus has only one paramedian pair of osteoderms per vertebral segment. Nevertheless, it is clear from partially articulated segments of osteoderms, the small size of individual osteoderms, and the number of osteoderms preserved that there must be more than one paramedian pair of osteoderms per vertebral segment.
Pinna and Arduini (1978) referred a specimen (BES 189) from the Middle Triassic strata of Besano to Ticinosuchus. The specimen consists of partial forelimbs and pectoral girdle, an osteoderm, a tooth, and part of the mandible. The morphology of the osteoderm is consistent with that of Ticinosuchus, Prestosuchus, and Saurosuchus. The other bones do not bear any unique apomorphies for Ticinosuchus. Therefore, this taxonomic assignment is not followed. Only PIZ T2817 is scored for Ticinosuchus.
Holotype
IVPP V13899, a skeleton with distal part of forelimbs and posterior end of the tail missing.
Referred material
IVPP V14300, an incomplete skeleton with nearly complete skull; NMNS 000408/F003877, an incomplete skull.
Remarks
Li et al. (2006) described Qianosuchus from marine sediments from the Anisian of southern China. The taxon is the most completely known early archosaur and one of the most completely documented basal archosaurs to date given that it is represented by two nearly complete skeletons and a crushed skull in ventral view. Even though the specimens are essentially “slab-specimens,” the bones are nearly three-dimensionally preserved. Qianosuchus awaits a full anatomical description.
Li et al. (2006) hypothesized that Qianosuchus was semiaquatic based on tall neural spines of the caudal series, a thinned platelike scapula and coracoid, an elongate neck (the nine cervical vertebrae reaching 75% of the trunk length and, together with the skull, over 120% of the latter) with long and slender cervical ribs, and small-sized dorsal osteoderms in the neck and trunk regions, but absent in the tail region. The authors rightly pointed out that characters one and four are common in marine tetrapods. However, an elongated neck is also present in fully terrestrial archosaurs (e.g., Arizonasaurus, MSM 4590; Effigia, AMNH FR 30587; Hesperosuchus agilis, AMNH FR 6758), and a thinned plate-like scapula and coracoid seem to be an autapomorphy of the taxon with no clear ecological significance. As the authors noted, other features of the skeleton are typical of terrestrial archosaurs. The ecology of this important taxon is unclear because of the ambiguous mix of potentially semiaquatic and terrestrial features and that there are multiple skeletons of Qianosuchus from marine deposits.
Qianosuchus possesses an intriguing mix of character states commonly listed as “rauisuchian” and poposauroid apomorphies. Qianosuchus clearly bears a crocodylian-normal ankle similar to that of aetosaurs, “rauisuchians,” poposauroids, and crocodylomorphs. The taxon has a short pubis and ischium relative to the femur, at least four leaf-shaped osteoderms per vertebra in the presacral series, and typically carnivorous teeth, features found in Prestosuchus (UFRGS 0156-T; UFRGS 0152-T; BSP XXV 1-3/5-11/ 28-41/49), Ticinosuchus (PIZ T2817), and Saurosuchus (PVSJ 32). The elongated cervicals with elongated cervical ribs, the enlarged narial opening, a slot on the anterolateral surface of the maxilla for the posterior process of the maxilla, and the presence of three sacral vertebrae support a close relationship to poposauroids (302Nesbitt, 2005). The age, mix of “rauisuchian” and poposauroid character states, and mixed ecological signal makes Qianosuchus important to the early radiation of the crocodylian-line archosaurs.
Li et al. (2006) provided the following diagnosis: A medium-sized archosaurian, over 3 m in length, differing from all other archosaurians in having the following combination of derived features: low premaxilla bearing nine daggerlike teeth; posteriorly positioned external naris longer than any other skull opening and mainly enclosed by nasal dorsally and maxilla ventrally; external mandibular fenestra half oval in outline; neural spines in cervical vertebrae 2 to 9 longitudinally very broad, each with five pairs of small osteoderms on its top; neural spines of caudal vertebrae very tall, at least four times the height of the centra and longer than chevrons in midtail region; cervical ribs elongate, most of them over four times length of corresponding centra; scapula plate-like, hatchet shaped in outline.
Occurrence
Heshanggou Formation, Hazhen commune, Fugu County, northeastern Shensi Province, China (Wu, 1981).
Holotype
IVPP V 6026, maxillae, premaxilla, lacrimal, nasal, dentary, articular, surangular fragment, splenial, braincase, axis, presacral vertebrae 3–10, primordial sacral two, two distal caudal vertebrae, cervical rib, dorsal rib, clavicle, ungual.
Remarks
Xilousuchus sapingensis Wu, 1981, is one of the most completely known archosauriforms from the Early to Middle Triassic of China. Regardless of the exact age of the Heshanggou Formation, Xilousuchus lived along with early archosauriforms such as a Proterosuchus-like taxon and Fugusuchus, a taxon considered to be more closely related to Erythrosuchus than to other archosauriforms (Gower and Sennikov, 1996).
Xilousuchus was named from a single well-preserved partial skull and the anterior portion of the presacral vertebral series. As first described, Xilousuchus was referred to the Proterosuchia by Wu (1981), whereas Gower and Sennikov (1996) found it as an erythrosuchian based strictly on the braincase.
Xilousuchus sapingensis differs from all other archosauriforms except Lotosaurus, Ctenosauriscus, and Arizonasaurus in having posterior cervical vertebrae with neural spines that arc anteriorly at the distal end. It differs from Ctenosauriscus in having anteroposteriorly expanded neural spines on the midcervical vertebrae. It differs from Lotosaurus, but not Arizonasaurus, in having a deep pit at the anteroventral margin of the antorbital fossa in the maxilla. Xilousuchus differs from Arizonasaurus in having a deep pit ventral to the descending process of the opisthotic in the parabasisphenoid, the absence of a divided parapophysis of the posterior cervical vertebrae, and poor development of the posterior centrodiapophyseal lamina in the anterior cervical vertebrae.
Occurrence
Holbrook Member of the Moenkopi Formation, Arizona; Anton Chico Member of the Moenkopi Formation, New Mexico (Schoch et al., 2010).
Referred material
MSM 4590, skull and partial skeleton; see Nesbitt (2003, 2005a) and Schoch et al. (2010) for other specimens.
Remarks
Arizonasaurus represent one of the most completely documented sail-backed archosaurs from the Anisian. Additionally, Arizonasaurus is the most common reptile found in the Holbrook and Anton Chico Members of the Moenkopi Formation (Nesbitt, 2005b; Schoch et al., 2010). The holotype and the referred specimen (MSM 4590) share two characters: a uniquely shaped ascending process of the maxilla that is triangular in cross section and a deep pit at the posterior side of the base of the ascending process of the maxilla. Both these characters are present in Xilousuchus suggesting that the two taxa are closely related (see below). The partial “lacrimal” described by Nesbitt (2005) is actually the prefrontal.
Nesbitt (2003) found Arizonasaurus as a close relative of Poposaurus and Shuvosaurus ( = Chatterjeea) within Suchia. Other analyses found a similar position (Nesbitt and Norell, 2006; Irmis et al., 2007a; Weinbaum and Hungerbühler, 2007; Brusatte et al., 2008). Nesbitt (2003, 2005a) hypothesized that Arizonasaurus formed a clade with other sail-backed suchians including Ctenosauriscus, Lotosaurus, Bromsgroveia, and Hypselorhachis.
Arizonasaurus differs from all other archosaurs except Xilousuchus, Lotosaurus, Hypselorhachis, and Ctenosauriscus by the presence of a sail created by the elongation of the neural spines of the presacral vertebrae. It differs from Xilousuchus by the absence of a deep pit in the parabasisphenoid ventral to the descending process of the opisthotic. Arizonasaurus differs from Ctenosauriscus in anteroposteriorly wide neural spines of the midposterior cervical vertebrae. It differs from Lotosaurus by the presence of teeth.
Key references
Welles, 1947; Nesbitt, 2003, 2005a; Gower and Nesbitt, 2006; Schoch et al., 2010.
Poposaurus gracilis Mehl, 1915
Occurrence
Popo Agie Formation, Wyoming; Blue Mesa Member of the Chinle Formation, Arizona; Mesa Redondo Member of the Chinle Formation, Arizona; Tecovas Formation of the Dockum Group, Texas; Monitor Butte Member of the Chinle Formation, southern Utah.
Holotype
FMNH 357, two dorsal vertebrae, one caudal vertebra, a left ilium, the proximal portion of a left femur, a right femur, distal portion of the ischia.
Referred material
TTU-P 10419, vertebrae, pelvic elements; TMM 43683-1, vertebrae and nearly complete pelvis; various UCMP elements from A269 (see Long and Murry, 1995); YPM 57100, nearly complete skeleton lacking the skull.
Remarks
Poposaurus gracilis was named from a fragmentary specimen consisting of pelvic elements, the femora, and a few vertebrae (Mehl, 1915). The differences in morphology from other Triassic archosaurs led various authors to identify P. gracilis as an ornithischian (Nopsca, 1921), a stegosaur (Huene, 1950), a theropod (Colbert, 1961), and a pseudosuchian (Walker, 1969). New specimens of P. gracilis and other closely related taxa confirmed the pseudosuchian affinity of the taxon (Galton, 1977; Long and Murry, 1995; Nesbitt and Norell, 2006; Weinbaum and Hungerbühler, 2007). With the exception of a nearly complete skeleton lacking the skull (Joyce and Gauthier, 2006), nearly all specimens of P. gracilis consist of pelvic material, a few vertebrae, and partial limbs (Weinbaum and Hungerbühler, 2007).
The element that was identified as the pubes in the holotype (FMNH 357) is actually the ischium; therefore, the pubis is not represented in the holotype material. The element that was once identified at the pubis bears a large distal expansion ( = pubic boot), and this expansion has greatly influenced the interpretation of its relationships in older (Colbert, 1961) and more recent (Weinbaum and Hungerbühler, 2007) studies. Indeed, the ischium bears a greatly enlarged distal expansion. Ironically, new specimens confirm that a large distal expansion ( = pubic boot) is present in P. gracilis (TMM 43683-1; YPM 57100).
Dawley et al. (1979) described Heptasuchus, another “rauisuchian” from the same formation (Popo Agie Formation) as the holotype of P. gracilis. Later, Zawiskie and Dawley (2003) hypothesized that the skull of Heptasuchus belongs to the body of P. gracilis. Although only a few elements (e.g., pubis, ulna) are directly comparable between the unique specimen of Heptasuchus and P. gracilis, there are important differences between the pubes. Both taxa have a distal expansion of the pubis; however, the distal expansion in Heptasuchus is robust and rounded like that of Batrachotomus rather than the mediolaterally compressed distal expansion of P. gracilis (TMM 43683-1). Furthermore, the preserved portions of the skull of Heptasuchus (maxilla, premaxilla, braincase) are much like that of Batrachotomus and not much like those of the putative close relatives of Poposaurus such as Arizonasaurus and Effigia (Nesbitt, 2007). Furthermore, it is reasonable to assume that more than two paracrocodylomorph taxa exist in a single assemblage as demonstrated by the cooccurrence of Postosuchus and Poposaurus in the Placerias Quarry (Long and Murry, 1995) and Postosuchus and Shuvosaurus in the Post ( = Miller) Quarry (Long and Murry, 1995). Therefore, the hypothesis that Heptasuchus represents the skull of P. gracilis is rejected here.
Poposaurus gracilis possesses two autapomorphies: a thick lateral ridge posterior to the acetabulum and a pit on the proximal part of the ischium for reception of the convex ischial peduncle of the ilium (Weinbaum and Hungerbühler, 2007).
Key references
Mehl, 1915; Colbert, 1961; Galton, 1977; Long and Murry, 1995; Weinbaum and Hungerbühler, 2007.
Lotosaurus adentus Zhang, 1975
Referred material
IVPP V 48013, skull, articulated and disarticulated remains of at least ?10 individuals from a monotypic bonebed (unnumbered).
Remarks
Lotosaurus is a highly specialized archosaur from the Middle Triassic of China with elongated neural spines forming a sail, robust fore- and hind limbs, and a peculiar skull with an edentulous beak. In a preliminary description, Zhang (1975) noted that Lotosaurus may be related to other archosaur taxa with elongated neural spines (e.g., Ctenosauriscus) and others have followed this line of thought (e.g., Carroll, 1988). Nesbitt (2007) went further and described some of the features Lotosaurus shared with taxa such as Arizonasaurus and Effigia and found Lotosaurus to be closely related to these taxa in a position just outside “clade Y” ( = Shuvosauridae). As explained by Nesbitt (2007), if Lotosaurus is more closely related to Shuvosaurus and Effigia than to Arizonasaurus, the ctenosauriscids (as proposed by Nesbitt, 2005a) would be paraphyletic. A full description of Lotosaurus is currently underway.
Lotosaurus differs from all other archosaurs by the combination of a sail formed from the elongation of the neural spines of the presacral vertebrae and the presence of edentulous premaxillae, maxillae, and dentaries.
Age
Late Carnian, Late Triassic (Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).
Holotype
PVSJ 85, postcranium consisting of parts of five cervical vertebrae, the last four dorsal vertebrae, five sacrals, and the first eight caudal vertebrae (the dorsal, sacrals, and caudal vertebrae preserved in articulation), partial right ilium, both pubes (nearly complete), both ischia preserved in articulation, both femora (complete), various pieces of ribs, and indeterminate fragments. Additionally, a partial left coracoid and scapula, the proximal portion of the left humerus, and the proximal portions of both tibiae were also collected with the holotype but not described in the original description.
Remarks
Sillosuchus longicervix was described from an incomplete postcranial skeleton that is poorly preserved and crushed (Alcober and Parrish, 1997). Although rare in the Ischigualasto Formation, other specimens have been found, but await description. The deep pockets on the lateral side of the cervical and the anterior dorsal vertebrae are unparalleled among pseudosuchians. The anteroposteriorly elongate and oval pockets stretch for much of the length of the centrum and only a thin lamina of bone at the midline separates the lateral pockets. Much of the morphology of the pelvis of Sillosuchus is very similar to Shuvosaurus and Effigia. All three taxa share coossified ischia, a dorsally expanded ilium, a thin, anteriorly arching crest dorsal to the supraacetabular crest, anteroposteriorly elongated cervical centra, and four or more sacral vertebrae (Nesbitt, 2007).
The coracoid, part of the scapula, and the proximal portion of the humerus were collected with the holotype, but were not described in the original description. The coracoid bears an elongated postglenoid process like that of Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9001), but does not bear a deep fossa on the dorsal surface of the process as do Effigia and Shuvosaurus. The scapula is anteriorly expanded into a thin sheet of bone just like that of Effigia (AMNH FR 30587). Furthermore, the proximal portion of the head is poorly expanded, and even though the bone is not complete, the proximal portion of the humerus was probably not expanded more than twice the midshaft, another synapomorphy with Effigia and Shuvosaurus (Nesbitt, 2007). In summary, the undescribed forelimb material of the holotype of Sillosuchus is remarkably similar to that of Effigia and Shuvosaurus. Furthermore, the gracile humerus of Sillosuchus suggests that the forelimb of Sillosuchus was possibly similar to the short forelimbs of Effigia, and it may have had a similar forelimb to hind limb length. Therefore, Sillosuchus possibly was another bipedal taxon.
As remarked by Alcober and Parrish (1997), the presence of Saurosuchus and Sillosuchus in the Ischigualasto Formation, two relatively closely related taxa, adds ambiguity to the identification of isolated specimens of both taxa. For example, Sill (1974) tentatively assigned PVL 2267, an isolated cervical vertebra, to Saurosuchus. The presence of deep lateral pockets and anteroposterior elongation of the centrum indicate assignment to Sillosuchus rather than Saurosuchus. Furthermore, the anteroposteriorly short and dorsally tall cervical vertebrae found with the nearly complete skull of Saurosuchus (PVSJ 23) preclude assignment of PVL 2267 to Saurosuchus.
Sillosuchus is one of the larger pseudosuchians from the Triassic as indicated by the holotype (femur length = 47 cm) and the larger isolated cervical vertebra (PVL 2267) referable to Sillosuchus (see preceding paragraph). Each of the cervicals in the holotype measures about 8 cm long, whereas PVL 2267 measures 20 cm long. The length of PVL 2267 suggests that Sillosuchus could have reached an estimated length (from extrapolation of data from the holotype of Sillosuchus and Effigia) of 9–10 meters. This is further supported by other large isolated elements (PVL 2267; partial left ilium).
Nesbitt (2007) confirmed the hypothesis of Alcober and Parrish (1997) that Sillosuchus and Shuvosaurus ( = Chatterjeea) are closely related, and both are closely related to Poposaurus.
Sillosuchus longicervix possesses deep pockets ( = pneumatic recesses) on the lateral side of both the cervical and dorsal centra and coossified ischia with a highly dorsoventrally compressed distal end that differentiates it from all other archosaurs. Sillosuchus also possesses a unique suite of characters including: five coossified sacral vertebrae; small pubic boot; dorsally expanded ilium with a thin, anteriorly arching crest dorsal to the supraacetabular crest ( = rim). Alcober and Parrish (1997) listed two characters in the diagnosis: elongated cervical vertebrae and relatively short ischia. The cervical vertebrae of Sillosuchus are indeed elongated but proportionally are not more elongated than those of Shuvosaurus ( = Chatterjeea) and Effigia (AMNH FR 30587). Furthermore, the short ischia are not complete, and therefore the length cannot be assessed with certainty. That said, the preserved length of the ischia are probably relatively short relative to the pubis.
Key references
Alcober and Parrish, 1997; Nesbitt, 2007.
Effigia okeeffeae Nesbitt and Norell, 2006 (fig. 8I)
Occurrence
Coelophysis Quarry, “siltstone member” of the Chinle Formation, Ghost Ranch, northern New Mexico.
Holotype
AMNH 30587, nearly complete skull, much of the cervical dorsal, and sacral vertebrae and the first two caudal vertebrae, right pes, left and right femur, left and right tibia, left and right fibula, right and fragments of the left scapula, left and right coracoids, right humerus, right ulna, right radius, right manus, left and right ilium, left and right ischia, right pubis, gastralia, and dorsal ribs.
Referred material
AMNH FR 30588, femur, ilium, ischium, pubis, sacrum, nearly complete caudal series; AMNH FR 30589, partial skull and cervicals; AMNH FR 30590, proximal part of the femur.
Remarks
Nesbitt and Norell (2006) named Effigia from an articulated skeleton from the Coelophysis Quarry in northern New Mexico. The combination of a postcranial skeleton like that of “Chatterjeea” and an edentulous, highly apomorphic skull similar to Shuvosaurus showed that the skull of Shuvosaurus belongs to the body of “Chatterjeea.” Furthermore, the skeleton of Effigia bears an uncanny resemblance to that of theropods and more specifically, ornithomimids, even though it is more closely related to Crocodylia than Aves (Nesbitt and Norell, 2006). The realization of this convergence led Nesbitt et al. (2007) to critically examine the fossil record of early dinosaurs in North America and to conclude that many of the specimens once thought to be theropods actually belong to close relatives of Effigia.
In a superficial review of the taxonomy of Shuvosaurus, Lucas et al. (2007c) challenged the difference cited by Nesbitt and Norell (2006) and Nesbitt (2007) separating Effigia from Shuvosaurus. The two taxa are obviously closely related given their divergent morphology and numerous apomorphies between the two taxa (Nesbitt, 2007). However, the comments of 271272Lucas et al. (2007) must be addressed.
Of the six characters explicitly used to differentiate Effigia from Shuvosaurus by Nesbitt (2007), 271272Lucas et al. (2007) accepted differences between the maxilla, lacrimal, and squamosal, but stated “the biological significances” of the differences are unknown. It is not clear why 271272Lucas et al. (2007) required an understanding of “biological significance” for a difference to be valid. The absence of a posterior process of the maxilla in Effigia represents a genuine difference between the two taxa and nearly all other archosaurs. Furthermore, the squamosal of Lotosaurus (IVPP V 48013) also lacks a posterior squamosal process. The premaxillae of Shuvosaurus (e.g., TTU-P 9280) apparently lack any posterior process whereas that of Effigia has a small tonguelike process. The posterior process of the maxilla of Effigia is rather robust, and this suggests that the premaxillae of Shuvosaurus genuinely lack this process even though the preservation and preparation of the material of Shuvosaurus is poor. The relative sizes of the dentaries cannot be compared at present after the repreparation of the specimen. Prior to the disarticulation of the type skull of Shuvosaurus in 2005, the body of the dentary of Shuvosaurus extended well past the premaxilla-maxilla articulation, whereas in Effigia, the body of the dentary is anterior to and at the premaxilla-maxilla articulation (Chatterjee, 1993; Rauhut, 1997). However, after repreparation, some of the original bone on the posterior portion of the dentary was lost (S.J.N., personal obs.). Therefore, all six characters discussed by 271272Lucas et al. (2007) represent differences between Effigia and Shuvosaurus.
Furthermore, 271272Lucas et al. (2007) dismissed the differences in the postcrania of the two taxa listed by Nesbitt (2007). These include two characters: the anterior cervical centra have distinct keels (Long and Murry, 1995: fig. 163 A–D), whereas those of Effigia lack keels (Nesbitt, 2007: fig. 28D), and difference in the size of the coracoid foramen. Additionally, the only ulna of Shuvosaurus (TTU-P unnumbered) is proportionally much more stout than that of Effigia. Unfortunately, limited comparisons can be made at this time because much of the Shuvosaurus postcrania remains unprepared. As a result of the discussion presented above, Effigia and Shuvosaurus are separate terminal taxa here.
Effigia is distinguished from all other suchians except Shuvosaurus by the presence of an edentulous premaxilla, maxilla, and dentary, a posteriorly long anterodorsal process of the premaxilla, a long preacetabular process of the ilium that connects to the posterior process by a large thin flange, and a pubic boot that is 33% the length of the pubic shaft. It is distinguished from Shuvosaurus by the presence of both a dorsal and posterior process of the maxilla, relatively shorter dentary, the absence of posterior process of the squamosal, a small fossa on the posterolateral side of the squamosal, and the presence of a large pit on the posterior side of the lacrimal (from Nesbitt, 2007).
Key references
Nesbitt and Norell, 2006; Nesbitt, 2007.
Shuvosaurus inexpectatus (Chatterjee, 1993), sensu Nesbitt and Norell, 2006
= Chatterjeea elegans Long and Murry, 1995
Paratype
TTU-P 9281, anterior portion of dentries; TTU-P 9282, braincase and other cranial fragments.
Referred material
TTU-P 9001, much of a postcranial skeleton, hundreds of disarticulated and associated bones from the Post ( = Miller) Quarry (material referred to Chatterjee elegans).
Remarks
Chatterjee (1993) named Shuvosaurus inexpectatus based on associated bizarre cranial elements from the Post ( = Miller) Quarry. Chatterjee (1993) concluded that the large orbits, seemingly pneumatic braincase, and edentulous maxillae, premaxillae, and dentaries of the taxon allied it to ornithomimid dinosaurs. The resultant phylogenetic position indicated that much of the theropod diversity in the Cretaceous was the product of diversification in the Triassic (Rauhut, 1997). However, the absence of coelurosaurian or tetanuran synapomorphies made others (e.g., Rauhut, 2003) question Chatterjee's (1993) original assignment. Long and Murry (1995) named Chatterjeea elegans based on distinctive postcranial remains from the same quarry and suggested that Shuvosaurus may be the skull of Chatterjeea. Nesbitt and Norell (2006) used the articulated skeleton of Effigia to demonstrate that the skull of Shuvosaurus indeed belongs to the body of Chatterjeea. Here, the scorings of Shuvosaurus and Chatterjeea are combined, and only unambiguous material from the Post ( = Miller) Quarry is scored.
Long and Murry (1995) referred material to “Chatterjeea” throughout the Chinle Formation and the Dockum Group. Much of the material consists of isolated finds. Given that Effigia and Shuvosaurus are very similar but distinct taxa, most of these can be assigned only to the clade containing the two taxa.
Shuvosaurus inexpectatus is distinguished from all other suchians except Effigia by the presence of an edentulous premaxilla, maxilla, and dentary, a posteriorly long anterodorsal process of the premaxilla, a long preacetabular process of the ilium that connects to the posterior process by a large, thin flange, and a pubic boot that is 33% the length of the pubic shaft. It is distinguished from Effigia by the absence of both a dorsal and posterior process of the maxilla, relatively longer dentary, the presence of a posterior process of the squamosal, the absence of a small fossa on the posterolateral side of the squamosal, and the absence of a large pit on the posterior side of the lacrimal.
Key references
Chatterjee, 1993; Long and Murry, 1995; Rauhut, 1997; Nesbitt and Norell, 2006; Nesbitt et al., 2007; Nesbitt, 2007.
Prestosuchus chiniquensis Huene, 1938
Holotype
BSP XXV 1-3/5-11/ 28-41/49, splenial, anterior portion of the surangular, anterior portion of the angular, prearticular, right partial maxilla, fragmentary dentary, three incomplete cervical vertebrae, fragmentary ribs, one sacral vertebra, two sacral ribs, five anterior caudal vertebrae with chevron bones, 14 middle and posterior caudal vertebrae, right and left scapulocoracoid, interclavicle and clavicle, distal left humerus, right proximal and distal humerus, distal radius, fragmentary ulna, one manual phalanx, incomplete ilium, fragmentary ischia, pubes, and complete left hind limb (including femur, tibia, fibula, ankle, and pes).
Remarks
Huene (1938) named Prestosuchus chiniquensis for a mandible and cranial fragments and much of a postcranial skeleton. For the most part, P. chiniquensis was assigned to “Rauisuchia”; furthermore, Parrish (1993) found a clade containing P. chiniquensis, Ticinosuchus, and Saurosuchus. Desojo and Rauhut (2008) presented the following two autapomorphies of P. chiniquensis: anterior notch between the scapula and coracoid and longitudinal ridge on the dorsal surface of the ischium. Only the holotype is scored here for this terminal taxon.
Remarks
UFRGS 0156-T is a very large skull (88 cm) that was assigned to Prestosuchus chiniquensis by Barberena (1978) and more recently by Azevendo (1991). Parrish (1993) separated UFRGS 0156-T from P. chiniquensis in his phylogenetic analysis and found that no character scores separated the two. Thus, he combined the two in his final hypothesis. I separate P. chiniquensis and UFRGS 0156-T as terminal taxa. P. chiniquensis and UFRGS 0156-T are both from a similar stratigraphic position near the bottom of the Santa Maria sequence. Parrish (1993) scored characters of the calcaneum and pes of UFRGS 0156-T, but these elements are absent in UFRGS 0156-T.
Referred material
Maxillae, nasals, quadrate, partial quadratojugal, complete braincase, parietal, ectopterygoid, partial pterygoid, jugal, squamosal, anterior portion of the dentary, prearticular, articular, cervical, dorsal, sacral, and caudal vertebrae, osteoderms, scapula, coracoid, humerus, proximal portion of the ulna, complete pelvic girdle, femora, tibia, fibula, calcaneum, pes, chevrons.
Remarks
UFRGS 0152-T consists of an undescribed archosaur that possesses overlapping elements with both UFRGS 0156-T and Prestosuchus chiniquensis. Furthermore, UFRGS 0152-T is indistinguishable from UFRGS 0156-T and Prestosuchus chiniquensis. Even though the exact locality is not known, it was collected from the Santa Maria sequence.
Saurosuchus galilei Reig, 1959 (fig. 11D–E)
Fig. 11
Skull reconstructions of “rauisuchian” archosaur terminal taxa: A, Rauisuchus triradentes in lateral view; based on (BSP AS XXV-60-121); B, Polonosuchus silesiacus in lateral view; redrawn from Sulej (2005); C, Postosuchus kirkpatricki in lateral view; modified from Chatterjee (1985); D, Saurosuchus galilei in lateral and E, dorsal views; redrawn from Alcober (2000); F, Batrachotomus kuperferzellensis in lateral and G, dorsal views; redrawn from Gower (1999). Shaded areas indicate incomplete preservation. See appendix for anatomical abbreviations. Scale bars = 5 cm in C–G and 1 cm in A–B.
Age
Late Carnian, Late Triassic (Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).
Referred material
PVL 2198, partial maxilla, left ilium, both ischia, nine articulated dorsal vertebrae and fragments, part of the dermal armor, associated ribs and teeth; PVL 2557, two dorsal vertebrae, both sacrals, nine caudal vertebrae, right ilium and ischium, partial pubis, parts of right femur, tibia, fibula, complete right tarsus and foot, associated ribs and chevrons; PVSJ 32, skull and partial skeleton.
Remarks
Saurosuchus galilei was named from a nearly complete skull (Reig, 1959) from the Ischigualasto Formation, and isolated material from this unit has been referred to the taxon since (Sill, 1974). The holotype represented the first relatively complete skull material of any rauisuchian to date and stands as one of the most complete skulls of a “rauisuchian.” Of all the specimens referred to the taxon, only PVSJ 32, a complete skull and presacral vertebral column, can be confidently assigned to Saurosuchus galilei because all the autapomorphies of the taxon lie in the skull (see Alcober, 2000). Most of the isolated postcranial material from the Ischigualasto Formation was assigned to Saurosuchus without much explanation. Moreover, at least one other large paracrocodylomorph, Sillosuchus, is known from the Ischigualasto Formation. The absence of a coherent, supported assignment of the isolated material to Saurosuchus has led to confusion. For example, the cervical vertebra (PVL 2472) assigned to Saurosuchus belongs to a gigantic specimen of Sillosuchus (see above). Furthermore, there are proportional differences between the metatarsals of two pedes (PVL 2557 and PVL 2267) assigned to Saurosuchus. Moreover, metatarsal V of PVL 2557 is short and possesses a clear facet for articulation with a phalanx, whereas PVL 2267 possesses a long, tapered metatarsal V without a clear facet for articulation with a phalanx. The ilium found with PVL 2267 shares synapomorphies (e.g., elongated preacetabular process, concave ischial peduncle) with Sillosuchus and other poposauroids. Interestingly, the specimens that are possibly referable to Sillosuchus, along with the holotype of the taxon, are found in the lowest one-third of the Ischigualasto Formation (Sill, 1974; Alcober and Parrish, 1997), whereas Saurosuchus is from the upper two-thirds of the formation. The following examples demonstrate that all the material assigned to Saurosuchus may not belong to the taxon. Therefore, I score the cranial material, osteoderm, and axial column from the holotype (PVL 2062) and PVSJ 32 and score a few additional characters from PVL 2198 and the hind limb of PVL 2557.
Saurosuchus was only recently utilized in explicit phylogenetic analyses. It was found closely related to Prestosuchus and Ticinosuchus by Parrish (1993) and Benton (2004) and to lie outside a clade containing Postosuchus kirkpatricki, Tikisuchus, Batrachotomus, aetosaurs, and crocodylomorphs by Gower and Walker (2002) based on braincase characters. In all analyses, Saurosuchus was found as a crocodylian-line archosaur.
The following autapomorphies listed by Alcober (2000) are accepted here: sculptured skull roof and maxilla; ventral process of the lacrimal forms a slender pillar that abuts the jugal laterally; development of a crista on the dorsal supraoccipital; development of a robust, laterally expanded, capitate process of the laterosphenoid.
Alcober (2000) also listed three autapomorphies focused on the frontal and surrounding bones (reduced postfrontal hidden in dorsal view, thickening of the border of the frontal at the level of the orbital fenestra, and presence of a lateral process of the posterolateral frontal). All three of these characters are not unique among crocodylian-line archosaurs once the large bone dorsal to the orbit is reidentified as a palpebral fused to the frontal. A similar frontal-palpebral relationship is found in Postosuchus kirkpatricki (TTU-P 9000) and Polonosuchus silesiacus (ZPAL Ab III/563).
Key references
Reig, 1959; Sill, 1974; Alcober, 2000.
Batrachotomus kupferzellensis Gower, 1999 (fig. 11F–G)
Holotype
SMNS 52970, premaxillae, maxillae, nasals, frontal, postfrontals, parietals, squamosals, postorbitals, jugals, quadrates, dentaries, surangulars, articulars, right lacrimal, right prefrontal, left quadratojugal, left ectopterygoid, left prearticular, isolated teeth, three dorsal, a single sacral, three caudal vertebrae, single dorsal osteoderm, right ilium, femora, left tibia, left fibula.
Referred material
SMNS 80260–80339. See Gower (1999) for crania and Gower and Schoch (2009) for postcrania.
Remarks
Well-preserved material from different ontogenetic stages and a fully detailed description of the skull (Gower, 1999), the braincase (Gower and Walker, 2002) and postcrania (Gower and Schoch, 2009) make Batrachotomus the best represented suchian from the Ladinian and one of the most complete paracrocodylomorphs known from the Triassic. The taxon has served as a basis for comparison with all other paracrocodylomorphs. Unfortunately, Batrachotomus lacks good manus material, and much of the pes remains unknown.
Batrachotomus is different from all other suchians (Gower, 1999) and bears one clear autapomorphy: presence of a small depression on the lateral surface on the ventral portion of the postorbital.
Key references
Gower, 1999; Gower and Walker 2002; Gower and Schoch, 2009.
Fasolasuchus tenax Bonaparte, 1981
Holotype
PVL 3850, premaxillae, nasals, fragmentary maxillae and one fragmentary maxilla that includes 10 incomplete teeth, fragmentary pterygoid, unrecognized cranial element, a posterior dentary including the articular, six cervical vertebrae, six dorsal vertebrae, eight caudal vertebrae, incomplete ischium, proximal part of the pubis, complete radius and ulna, right femur, fibula, astragalus and calcaneum, several fragmentary vertebrae, ribs, and osteoderms.
Referred material
PVL 3851, left maxilla with a few teeth, left dentary with five teeth, articular region, axis, incomplete cervical centra, sacral centra, two sacral vertebrae.
Remarks
Bonaparte (1981) described Fasolasuchus from two associated skeletons from near the top of the Los Colorados Formation in Argentina. The limb bones and the maxilla indicate that Fasolasuchus was one of the largest suchians from the Triassic and may have reached 8–10 m in length (extrapolated from comparisons with Postosuchus and Saurosuchus). Only the articular is present in the two known specimens. Although the articulars do not share any unique morphology, the shape and size are very similar and both bear a medially directed process of the articular with a foramen that pierces it, two character states present in Arizonasaurus, Postosuchus kirkpatricki, Polonosuchus, Batrachotomus, Prestosuchus (UFRGS 0152-T), Stagonosuchus, and Rauisuchus. Some of the material described by Bonaparte (1981) such as the nasal could not be located at the time of this study.
Age
Late ?Carnian–early Norian, Late Triassic (Langer, 2005a), Alemoa local fauna (sensu Barberena et al., 1985, and Azevedo et al., 1990).
Holotype
BSP AS XXV-60-121, right premaxilla, right nasal, left jugal, right prefrontal/lacrimal, left squamosal, left surangular, right and left splenial, right ectopterygoid, left prearticular, left articular, right pterygoid, isolated teeth, atlas, axis; cervical, dorsal, and caudal vertebrae; ribs, chevrons, right scapula, right coracoid, left pubis, left ilium, right tibia, right fibula, right astragalus, and osteoderms.
Remarks
Rauisuchus was named for a partial, disarticulated skeleton consisting of skull elements and postcranial remains from the Santa Maria Formation, Brazil. According to von Huene (1942), the specimen was found in “Sanga 6” in the Alemoa area. The exact stratigraphic position of the specimen may never be known; however, it was found with other taxa in the “Alemoa local fauna” of Barberena et al. (1985) and Langer (2005a). Langer (2005a, 2005b) considered the “Alemoa local fauna” to correlate with the lower portion of the Ischigualasto Formation. If this correlation holds, Rauisuchus would be considered late Carnian because the base of the Ischigualasto Formation is dated at 228 Ma (Rogers et al., 1993). The type of Rauisuchus tiradentes is hypothesized to be one individual because all the bones preserving matrix have the same fine red mudstone adhering to them, there are no duplicated elements, and the size of the elements are congruent with those of other rauisuchians.
Other than the original description (Huene, 1942) and Parrish's (1993) scoring of the taxon into his cladistic analysis of pseudosuchians, Rauisuchus was largely ignored. Rauisuchus differs from all other suchians except Postosuchus kirkpatricki and Polonosuchus silesiacus in that it has a lateral, rugose ridge on the nasal, a rugose ridge on the dorsal portion of the squamosal, and it has an anteroventral process that splits the lower temporal fenestra into two portions. It differs from Postosuchus kirkpatricki and Polonosuchus silesiacus in that the axis is parallelogram shaped. Rauisuchus has two autapomorphies: ventral margin of the jugal bowed ventrally and ventrally pointed rugose ridge on the posterior portion of the squamosal.
Key references
Huene, 1942; Krebs, 1973; Parrish, 1993.
Polonosuchus silesiacus (Sulej, 2005), sensu Brusatte et al., 2009 (fig. 11B)
Holotype
ZPAL Ab III/563, right and left maxillae, premaxillae, nasals, prefrontals, palatines, quadrates, and fragments of dentary, left jugal, right lacrimal, quadratojugal, squamosal, pterygoid, surangular, articular, fragment of atlas articulated with axis and third cervical vertebra, 12 articulated caudal vertebrae, five caudal osteoderms, and pieces of cervical ribs.
Remarks
Sulej (2005) first described ZPAL Ab III/563 and assigned it to a new species-level taxon in the genus Teratosaurus because of similarities with Teratosaurus suevicus (BMNH 38646). In a superficial discussion, Lucas et al. (2007a) rejected all differences between the two taxa of Teratosaurus and stated that ZPAL Ab III/563 was referable to Teratosaurus suevicus. However, Lucas et al. (2007a) did little to discuss the anatomy of either taxon. Brusatte et al. (2009) showed that there are no clear apomorphies to unite ZPAL Ab III/563 and Teratosaurus suevicus exclusive of closely related taxa (e.g., Postosuchus kirkpatricki), found that ZPAL Ab III/563 and BMNH 38646 differed extensively, and, therefore, assigned ZPAL Ab III/563 to the new genus Polonosuchus. Polonosuchus was hypothesized to be closely related to Postosuchus kirkpatricki in phylogenetic analyses of basal archosaurs (Weinbaum and Hungerbühler, 2007; Brusatte et al., 2009).
Polonosuchus silesiacus differs from Postosuchus kirkpatricki by: ventral margin of the maxilla sinuous and highly convex in outline; first maxillary alveolus approximately equal in size to subsequent alveoli; nasal with bifurcated anterior end, including tapering premaxillary process that contacts the premaxilla; maxillary process of premaxilla terminating anterior to the caudal end of the external naris; absence of fossa on the dorsal surface of the nasal; absence of dorsoventral expansion of the anterior end of the dentary (from Brusatte et al., 2009).
Key references
Sulej, 2005; Brusatte et al., 2009.
Postosuchus kirkpatricki Chatterjee, 1985 (fig. 11C)
Key references
Chatterjee, 1985; Long and Murry, 1995; Weinbaum, 2002; Peyer et al., 2008.
Postosuchus alisonae Peyer et al., 2008
Occurrence
Mudstone of Lithofacies Association II sensu Hoffman and Gallagher (1989), south-central region of Durham subbasin of Deep River Basin, Newark Supergroup, West Genlee, Durham County, North Carolina, U.S.A. Equivalent to the lower Sanford Formation (Huber et al., 1993).
Holotype
UNC 15575, partial skeleton consisting of a few fragmentary cranial bones (nasal, frontal, squamosal, prootic, supraoccipital, left and right opisthotic, articular, angular, prearticular) and isolated teeth. The postcranial skeleton includes seven cervical, one dorsal, and four caudal vertebrae, with associated ribs and chevrons, partial sacral rib, cervical, dorsal, and caudal osteoderms, gastralia, right and partial left coracoid, partial left and right scapulae, interclavicle, clavicle, left and right humeri, radii, and ulnae, nearly complete right and partial left manus, distal ends of left and right pubes, left and right tibiae, fibulae, tarsi, and pedes (Peyer et al., 2008).
Remarks
Peyer et al. (2008) described a well-preserved partial skeleton of a suchian from the Late Triassic of the Newark Supergroup. The preserved portions of the skeleton are nearly identical to those of Postosuchus kirkpatricki except for the one clear autapomorphy stated above. Unfortunately, few comparisons can be made to Polonosuchus (ZPAL Ab III/543) because there are few elements that are shared by the known specimens of the two taxa, and those parts that do overlap either support a close relationship between Postosuchus kirkpatricki, Postosuchus alisonae, and Polonosuchus or represent plesiomorphies within Archosauria or more inclusive clades. Postosuchus alisonae remains one of only a few Triassic crocodylian-line archosaurs with articulated manus and pedes in the same individual.
Postosuchus alisonae is almost identical to Postosuchus kirkpatricki (see Peyer et al., 2008) in the overlapping elements. Postosuchus alisonae differs from all known suchians in the presence of a groove in the proximal portion of metacarpal I for contact with metacarpal II (Peyer et al., 2008).
Key references
CM 73372
= Postosuchus kirkpatricki Long and Murry, 1995; Weinbaum, 2002; Novak, 2004; Peyer et al., 2008.
Occurrence
Coelophysis Quarry, “siltstone member” of the Chinle Formation, Ghost Ranch, northern New Mexico.
Specimen
CM 73372, articulated postcranial skeleton including hind limbs, pelvis, dorsal, sacral, and caudal vertebrae, portions of the humerus, scapula, ulna, radius, partial manus, osteoderms, ribs, and gastralia.
Remarks
In a review of Postosuchus kirkpatricki, Long and Murry (1995) referred an articulated skeleton from the Coelophysis Quarry of New Mexico to P. kirkpatricki without specific justification. Weinbaum (2002), Novak (2004), and Peyer et al. (2008) accepted the identification of CM 73372 as P. kirkpatricki. Nevertheless, these authors failed to note any synapomorphies unique to P. kirkpatricki and CM 73372. All authors noted that the specimen represents a skeletally immature individual because none of the neural sutures are closed (see Brochu, 1996; Irmis, 2007). Weinbaum (2002) and Novak (2004) did note that the preacetabular process of the ilium was much longer than that of P. kirkpatricki.
Because there are no clear characters linking P. kirkpatricki to CM 73372 to the exclusion of other taxa, it is treated as a separate terminal taxon. CM 73372 differs from Postosuchus kirkpatricki and Rauisuchus in possessing a concave ventral margin of the ilium. Also, CM 73372 differs from P. alisonae in processing an asymmetrical distal end (in distal view) of metatarsal IV. CM 73372 and Polonosuchus overlap only in the caudal vertebrae, but do not differ.
Key references
Long and Murry, 1995; Weinbaum, 2002; Novak, 2004; Peyer et al., 2008.
Hesperosuchus agilis Colbert, 1952
Holotype
AMNH FR 6758, portions of the skull including the quadrate, maxillae, dentaries, portion of the premaxilla, part of the nasal, part of the jugal, part of the squamosal, partial braincase (opisthotic, basioccipital), cervical, dorsal, and caudal vertebrae, osteoderms, humerus, ulna, radius, partial radiale, parts of the manus, femora, tibiae, fibulae, partially articulated pes.
Remarks
Colbert (1952) named Hesperosuchus for a partially eroded, articulated specimen from the base of the Chinle Formation that was collected by Barnum Brown. The well-preserved specimen is three-dimensionally preserved, but many of the delicate elements are missing or unidentifiable. Colbert (1952) made a few errors in the identification of elements in his description, but Walker (1970) corrected these mistakes. For example, the “pterygoid” (Colbert, 1952: fig. 9) is actually a sacral rib from the first primordial sacral. Bonaparte (1971) suggested that there are two individuals in the holotype. However, there are no apparent duplications of any of the elements.
Most crocodylomorph-like bones and associated skeletons from the Chinle Formation and Dockum Group have been assigned to Hesperosuchus without specific justification (Parrish, 1991; Long and Murry, 1995; Clark et al., 2000). The better preserved specimens from the Coelophysis Quarry from the top of Chinle Formation have been separated out as a separate terminal taxon (see below). Here, I score only the holotype for this terminal taxon.
Much of the skeleton of Hesperosuchus was eroded before Barnum Brown recovered it in the 1930s. Brown and the AMNH preparators screen-washed thousands of pounds of matrix and recovered bone fragments from the resultant concentrate. Hundreds of bone fragments, teeth, and pieces of the holotype of Hesperosuchus were separated. Colbert's (1952) description focused on the material recovered in situ and the obvious bones collected on the surface. However, some of the bones described by Colbert belong to a dinosauromorph (e.g., the elongated metatarsals, one of the humeri), the sacral vertebra belongs to Vancleavea (Nesbitt et al., 2009a), and other material collected at the locality represents the remains of fishes, phytosaurs, amphibians, or other archosaurs. Fortunately, the preservation of the holotype of Hesperosuchus is unique among the other bones; the weakly weathered or in situ bones are a dark chocolate brown and the weathered bones are orange to yellow, whereas the other fragments are blue, black, tan, or dark grey. Furthermore, the outer surfaces of the bones of the holotype of Hesperosuchus are exquisitely preserved. These two factors allow the fragments of Hesperosuchus to be separated. As a result, parts of the skull (squamosal, nasal), osteoderms, pelvis, and manus were recovered. Furthermore, the screen-washed material was distributed throughout the fossil vertebrate collections at AMNH. I found parts of the holotype of Hesperosuchus with the aetosaurs and parareptiles.
Key references
Colbert, 1952; Walker, 1970; Parrish, 1991; Clark et al., 2000.
Hesperosuchus “agilis” Clark et al., 2000
Occurrence
Coelophysis Quarry, “siltstone member” of the Chinle Formation, Ghost Ranch, northern New Mexico.
Referred material
CM 29894, skull and anterior portion of the skeleton; YPM 41198, partially disarticulated skull, pubis, hind limb.
Remarks
Clark et al. (2000) described a well-prepared skull and partial skeleton from the Coelophysis Quarry at the top of the Chinle Formation and referred the specimen to Hesperosuchus agilis. However, the holotype of Hesperosuchus, from the Blue Mesa Member, near the base of the Chinle Formation and the specimen from the Coelophysis Quarry, from the top of the Chinle sequence, may be separated by as much as 20 million years. Clark et al. (2000) used the following two characters to refer CM 29894 to Hesperosuchus agilis: (1) deep anterior end of the dentary; and (2) the configuration of the maxillary tooth row with a rapid increase in size of the anterior teeth from the small, slender first to the very large fourth tooth. The first character is also in Postosuchus kirkpatricki (TTU-P 9000) and Polonosuchus (ZPAL Ab III/543), and I see little difference between the anterior portions of the dentaries of CM 29894, Dromicosuchus (UNC 15574), and Sphenosuchus (SAM 3014). The second character does not seem to be unique among suchians. Therefore, CM 29894 cannot be unambiguously assigned to Hesperosuchus agilis. CM 29894 and another identical crocodylomorph skull and partial skeleton, YPM 41198, are treated as a separate terminal taxon. Although there are no apparent differences in the holotype of Hesperosuchus and CM 29894, no unique characters link the two taxa exclusive of other crocodylomorphs. Therefore, they are treated as separate taxa.
Key references
Dromicosuchus grallator Sues et al., 2003 (fig. 12E)Fig. 13
Fig. 12
Skull reconstructions of crocodylomorph archosaur terminal taxa: A, Sphenosuchus acutus in lateral and B, dorsal views; redrawn from Walker (1990); C, Protosuchus richardsoni in lateral and D, dorsal views; modified from Crompton and Smith (1980); E, Dromicosuchus grallator in lateral view; based on UNC 15574. See appendix for anatomical abbreviations. Scale bars = 1 cm.
Fig. 13
Skull reconstructions of basal avian-line archosaur terminal taxa: A, Eudimorphodon ranzii in lateral view; redrawn from Wild (1978); B, Lesothosaurus dianosticus in lateral and C, dorsal views; redrawn from Sereno (1991a); D, Eoraptor lunensis in lateral view; redrawn from Sereno et al. (1993); E, Tawa hallae in lateral view; based on the holotype and referred specimens; F, Silesaurus opolensis in lateral view; redrawn from Dzik (2003); G, Herrerasaurus ischigualastensis in lateral and H, dorsal views; redrawn from Sereno and Novas (1994); I, Plateosaurus engelhardti in lateral view; redrawn from Yates (2003); J, Coelophysis bauri in lateral view; redrawn from Rauhut (2003). Shaded areas indicate incomplete preservation. See appendix for anatomical abbreviations. Scale bars = 1 cm.
Occurrence
Mudstone of Lithofacies Association II sensu Hoffman and Gallagher (1989), south-central region of Durham subbasin of Deep River Basin, Newark Supergroup, West Genlee, Durham County, North Carolina, U.S.A. Equivalent to the lower Sanford Formation (Huber et al., 1993).
Holotype
SAM 3014, nearly complete skull, cervical vertebrae, pectoral girdle, humeri, tibia, metatarsals.
Remarks
Sphenosuchus, originally described by Haughton (1915), was studied by Walker for over 30 years. From the beginning, Sphenosuchus was considered a close relative of crocodylians. In an unprecedented and unparalleled study of a basal archosaur, Walker (1990) disassembled, and in astonishing detail, prepared the entire skull. He revealed particulars of the braincase that united Sphenosuchus with crocodyliforms that were later used by Gower and Walker (2002) and Gower (2002) in a braincase study of basal archosaurs. The divergent postcranium of Sphenosuchus formed the basis of an argument for a long-limbed clade, Sphenosuchia, at the base of Crocodylomorpha (Sereno and Wild, 1992; Wu and Chatterjee, 1993).
Key references
Haughton 1915; Walker, 1970, 1990; Clark et al., 2000.
Dibothrosuchus elaphros Simmons, 1965
Referred material
CUP 2489, partial postcranial skeleton; IVPP V 7907, complete skull and partial postcranium including the cervical vertebrae and osteoderms, humerus, ulna, radius, scapula, coracoid, manus, ilium.
Remarks
Dibothrosuchus was originally described from incomplete skull fragments and partial limb bones by Simmons (1965). Wu and Chatterjee (1993) referred a complete, well-prepared skull and the anterior portion of a skeleton to the taxon, and their referral is accepted here. Like Sphenosuchus, Dibothrosuchus is known from an articulated skull with a well-preserved braincase, a combination that is rare among basal archosaurs. Although many fragmentary specimens were referred to Dibothrosuchus, I score only IVPP V 7907 for this analysis.
Key references
Simmons, 1965; Wu, 1986; Wu and Chatterjee, 1993; Clark et al., 2000.
Terrestrisuchus gracilis Crush, 1984
Occurrence
Fissure fills in the Carboniferous limestone of the Pant-y-ffynon Quarry, Cowbridge, Glamorgan, Wales.
Remarks
Terrestrisuchus was named for and based on material collected from fissure fills in a Carboniferous limestone in Wales. The abundant taxon is known from dozens of specimens, from articulated and disarticulated crania, and postcrania. A few of the three-dimensionally preserved bones were prepared out of the matrix and formed the basis of the description by Crush (1984). Although much of the skull was described by Crush (1984), portions of the skull, including much of the braincase, nasals, and premaxilla, remain unknown. Originally, the holotype and referred material were housed at University College, London, but they were transferred to and reside at the Natural History Museum (BMNH).
Soon after Terrestrisuchus was named, Benton and Clark (1988) proposed that the taxon may be synonymous with Saltoposuchus from the Stubensandstein (Norian) of Baden-Wurttemberg, Germany. Benton and Clark (1988) rightfully criticized the single maxillary character cited by Crush (1984) to separate the two taxa. Sereno and Wild (1992) defended the position that the two taxa should be separated, but as demonstrated by Clark et al. (2000), many of the differences named are noncomparable between the two taxa. Clark et al. (2000) cited a few differences but were unsure in the end whether the taxa were different. Most recently, Allen (2003) suggested that Terrestrisuchus was a juvenile of Saltoposuchus. Given the uncertainties of the taxonomy of the two taxa, I score only material of Terrestrisuchus described by Crush (1984).
Key references
Crush, 1984; Sereno and Wild, 1992; Clark et al., 2000; Allen, 2003.
Litargosuchus leptorhynchus Clark and Sues, 2002
Holotype
BP/1/5237, complete skull and much of an articulated postcranium missing the manus and pedes.
Remarks
The well-preserved, though crushed, skeleton of Litargosuchus represents one of the most complete non-crocodyliform crocodylomorphs from Gondwanaland. The skull bears similarities to crocodyliforms, but as in Kayentasuchus, it has a mix of “sphenosuchian” and crocodyliform character states. The limb proportions are long relative to the axial column like that of Terrestrisuchus. Although incomplete, the ulnare and radiale appear to be the longest of any crocodylomorph. The postcranium has yet to be described formally, but is included in my scoring of the taxon.
Occurrence
Willow Springs, middle of the silty facies of the Kayenta Formation, northern Arizona (Clark and Sues, 2002).
Holotype
UCMP 131830, partial skull roof, left facial portion, partial mandible, parial ilium, complete femur, and other postcranial elements.
Remarks
Clark and Sues (2002) named Kayentasuchus for an associated skeleton from the Kayenta Formation. The taxon bears a mix of synapomorphies of the non-crocodyliform crocodylomorphs and crocodyliforms. As a result, the incorporation of Kayentasuchus into the phylogenetic analysis of Clark et al. (2000) led to a large polytomy at the base of Crocodylomorpha (Clark and Sues, 2002; Clark et al., 2004). Kayentasuchus joined an ever-growing list of crocodylomorphs from the Kayenta Formation, including an Edentosuchus-like taxon (Clark, 1994), Eopneumatosuchus colberti (Crompton and Smith 1980), Calsoyasuchus valliceps (Tykoski et al., 2002), and an undescribed protosuchid (TMM 43648-1; 474Tykoski, 2005).
Occurrence
Upper Elliot Formation, Orange River Valley, Qacha's Nek Providence, Lesotho (Nash, 1975).
Remarks
Orthosuchus was named for a nearly complete, three-dimensionally prepared skeleton from near the top of the upper Elliot Formation of Lesotho (Nash, 1968, 1975). As mentioned by Clark (in Benton and Clark, 1988), the holotype is dorsoventrally crushed and, as a result, some characters discussed by Nash (1975) are the result of crushing. Between the holotype and the two referred specimens, most of the anatomy of Orthosuchus can be scored.
Orthosuchus was found as a crocodyliform more closely related to Protosuchus than to Crocodylus in all explicit phylogenies of basal crocodyliform relationships (Benton and Clark, 1988; dataset of Pol et al., 2004, 2009). Clark (in Benton and Clark, 1988) listed the following character states that are shared with Protosuchus: ventrolateral contact of otoccipital with quadrate relatively broad (Busbey and Gow, 1984); squamosal relatively thick; vomer transversely broad, not rodlike.
Key references
Nash, 1968, 1975; Benton and Clark, 1988.
Protosuchus haughtoni (Busbey and Gow, 1984), sensu Gow, 2000
= Baroqueosuchus haughtoni Busbey and Gow, 1984
Referred material
BP/1/4770, complete skull and partial postcranium; SAM-K-8026, complete skull, articulated presacral column and osteoderms, partial forelimb, articulated tail.
Remarks
Protosuchus haughtoni is known from an exceptionally well-preserved, three-dimensional skull and partial postcranium. Protosuchus haughtoni from South Africa is remarkably similar to Protosuchus richardsoni from the Early Jurassic of Arizona (Clark, 1986; Gow, 2000). I score only BP/1/4770 and SAM-K-8026 for this taxon.
Protosuchus haughtoni differs from P. richardsoni in the following: (1) a midline ridge and paired ridges lateral to it present on the basisphenoid (these are absent in P. richardsoni); (2) junction of maxillae in palate ends well anterior to the maxillary tooth rows; and (3) the large foramen in the maxilla within the anterior notch is not recorded for P. richardsoni (Gow, 2000).
Referred material
MCZ 6727, three-dimensionally preserved skull and nearly complete skeleton; UCMP 131827, posterior portion of a skull and disarticulated skeleton; UCMP 130860, complete skull split longitudinally; UCMP 36717, postcranial skeleton.
Remarks
Remains of Protosuchus richardsoni are known from a limited number of closely spaced localities along Ward's Terrace in the sandstones of the Moenave Formation in Arizona. The taxon is represented by nearly every skeletal element in extraordinary detail including portions rarely preserved (braincase and palate). P. richardsoni forms part of the definition of Crocodyliformes and lies at a critical junction between basal crocodylian-line archosaurs and Crocodylia.
Key references
Brown, 1933; Colbert and Mook, 1951; Crompton and Smith, 1980; Clark, 1986.
Alligator mississippiensis Daudin, 1809
Specimens
AMNH (herpetology collection) 43316, skull and skeleton; AMNH 40583, articulated skull; AMNH 40584, disarticulated skull.
Remarks
The entire anatomy of Alligator has been described in full detail (e.g., Owen, 1850). Additionally, various authors (e.g., Witmer, 1997) used Alligator and avians as end members to phylogenetically bracket Archosauria and infer behavior, soft tissue anatomy, and function in basal archosaurs. I use Alligator to represent Mesoeucrocodylia.
Dimorphodon macronyx (Buckland, 1829), sensu Owen, 1870
= Pterodactylus macronyx Buckland, 1829
Referred material
BMNH R 1035, much of a skull and skeleton; BMNH 41212, postcrania; YPM 350, partial skeleton; YPM 9182, partial skeleton (see Padian, 1983).
Remarks
Dimorphodon is one of the oldest pterosaurs known from well-preserved material. In the most recent pterosaur phylogenies, Dimorphodon (or Dimorphodontidae) was found as one of the basalmost taxa either outside Anurognathidae + Pterodactyloidea (Unwin, 2003) or outside Campylognathoididae + Pterodactyloidea (Kellner, 2003). Some of the material assigned to Dimorphodon is three-dimensionally prepared, which is rare among basal pterosaurs. This permits unprecedented examination of the ankle, femur, metatarsals, proximal tarsals, humerus, and tibia and fibula (Padian, 1983).
Key references
Buckland, 1829; Owen, 1870; Padian, 1983; Unwin, 1988.
Eudimorphodon ranzii Zambelli, 1973 (fig. 12A)
Holotype
MCSNB 2888, complete skull, articulated postcranium missing the caudal region, pelvis, and most of the hind limbs.
Referred material
MCSNB 8950, articulated skeleton missing the skull and tail; MCSNB 3496, partial skeleton, foot, pelvis.
Remarks
Eudimorphodon was the first pterosaur to be described from the Triassic and was named from a largely articulated specimen including a nearly complete skull. Nearly all pterosaur workers found Eudimorphodon within the Campylognathoididae (Kellner, 2003; Unwin, 2003; Dalla Vecchia, 2009; but see Andres et al., 2010), outside Rhamphorhynchidae + Pterodactyloidea. The divergent morphology of the dentition and skull bones, Triassic age, and the relatively derived position within Jurassic pterosaur clades illustrates that much of the early evolution of Pterosauria remains hidden. Here, I score most characters from the holotype, and a few other characters (pelvis, foot, sternum) are scored from MCSNB 8950 and MCSNB 3496.
Referred material
PVL 4619, articulated sacrum, pelvis, and partial right and left hind limbs; PVL 4625, articulated vertebral column including dorsal, sacral, and anterior caudal vertebrae, left pelvis, and left femur; PVL 5000, proximal left femur; MCZ 4121, partial right and left femora.
Remarks
Lagerpeton was named for a hind limb (Romer, 1971a), and referred material consists of the pelvic girdle and posterior presacral, sacral, and proximal caudal vertebrae (Bonaparte, 1984; Arcucci, 1986; Sereno and Arcucci, 1994a). In the most recent review of the taxon, Sereno and Arcucci (1994a) provided a detailed description of the hind limb and highlighted synapomorphies shared with dinosauriforms. Therefore, Lagerpeton possesses an important mix of plesiomorphic archosaurian character states and derived dinosaurian characters.
Lagerpeton, Dromomeron romeri, and Dromomeron gregorii form the Lagerpetidae (Nesbitt et al., 2009b) at the base of Dinosauromorpha. Consequently, most of the unique features of the femur, tibia, and ankle of Lagerpeton cited by Sereno and Arcucci (1994a) are now synapomorphies of Lagerpetidae. The thin, aliform ridge for the attachment of the caudifemoralis musculature ( = fourth trochanter) differentiates Lagerpeton from both D. romeri and D. gregorii.
Key references
Romer, 1971a, 1972a; Bonaparte, 1984; Arcucci, 1986; Sereno and Arcucci, 1994a.
Dromomeron gregorii Nesbitt et al., 2009b
Paratypes
TMM 31100-464, right femur; TMM 31100-1308, right femur; TMM 31100-1234, right femur; TMM 31100-764, right femur; TMM 31100-278, right tibia; TMM 31100-1314, left tibia.
Remarks
Nesbitt et al. (2009b) described a second taxon of Dromomeron from the base of the Dockum Group; D. gregorii and D. romeri are separated stratigraphically. D. gregorii shows that non-dinosaurian dinosauromorphs were present throughout much of the Late Triassic sediments in the southwestern United States. Like D. romeri, D. gregorii is currently known only from hind limb material. D. gregorii and D. romeri are found as sister taxa in a clade with Lagerpeton to the exclusion of all other archosaurs in Nesbitt et al. (2009b).
Dromomeron gregorii differs from Dromomeron romeri in possessing a distinct ridge for the attachment of the M. caudifemoralis longus ( = 4th trochanter), the presence of an anterior trochanter and trochanteric shelf, robust proximal and distal ends of the femora, the intercondylar groove of the distal femur is reduced to a slit in larger specimens (possible autapomorphy), and the lack of an anteromedial concavity on the distal end of the tibia.
Age
Mid to late Norian (Litwin et al., 1991; Lucas, 1998a; Heckert et al., 2005; Parker, 2007; Irmis et al., 2007a)
Paratypes
A right femur, GR 219, and a left tibia, GR 220, may belong to the same individual as the holotype. Additional material includes GR 221, a partial left femur; GR 234, a complete right femur; GR 222, a complete left tibia; and GR 223, a complete astragalocalcaneum.
Referred material
GR 235, partial articulated skeleton; GR 236, isolated right tibia (cnemial crest crushed); NMMNH P-35379, complete astragalocalcaneum; AMNH FR 2721, distal portion of a femur; AMNH FR 30648, distal portion of a right tibia; AMNH FR 30649, distal portion of a right tibia.
Remarks
Irmis et al. (2007a) named and briefly described Dromomeron romeri, the first non-dinosaurian dinosauromorph discovered since Lagerpeton. The holotype femur bears characters that were thought to be autapomorphies of Lagerpeton. The discovery of Dromomeron in the Norian of North America, along with non-dinosaurian dinosauriforms and dinosaurs, shows that primitive dinosauromorphs coexisted with dinosaurs. Only hind limb elements are known from this taxon at present.
Dromomeron romeri differs from Dromomeron gregorii and all other basal dinosauromorphs in possessing the following autapomorphies: (1) absence of a fourth trochanter; (2) presence of a sharp ridge on the anteromedial edge of the distal end of the femur; (3) presence of a lateral tuberosity on the anterolateral edge of the distal end of the femur; and (4) a large crest on the anteromedial edge of the astragalus and associated anteromedial concavity on the distal tibia.
Key references
Irmis et al., 2007a; Nesbitt et al., 2009b.
Marasuchus lilloensis (Romer, 1971a), sensu Sereno and Arcucci, 1994b
= Lagosuchus lilloensis Romer, 1971a
Holotype
PVL 3871, partial articulated skeleton including the posterior portion of the vertebral column (from the last dorsal vertebra to the 25th caudal vertebra), left scapulocoracoid, humerus, radius, ulna, fragmentary right pelvis, left ilium, left pubis, partial right and left hind limbs.
Referred material
PVL 3870, partial skeleton including the maxilla and partial braincase, vertebral column from the atlas to the anterior caudal vertebrae, articulated pelvis and hind limbs lacking only the distal phalanges and unguals; PVL 3872, partial braincase and articulated vertebral column from the atlas to the ninth presacral vertebra; PVL 4670, articulated anterior caudal vertebrae with chevrons; PVL 4671, articulated anterior caudal vertebrae with chevrons; PVL 4672, articulated vertebral column from atlas to the 17th presacral vertebra.
Remarks
Romer (1971a, 1972a) described two incomplete long-limbed forms from the Middle Triassic of Argentina, “Lagosuchus talampayensis” (UNLR 09) as the genotype and “Lagosuchus” lilloensis (PVL 3871) as a second species. Sereno and Arcucci (1994b) demonstrated that the holotype of “Lagosuchus talampayensis” (UNLR 09) is not diagnostic, but referred specimens of “Lagosuchus talampayensis” as well as “Lagosuchus” lilloensis (PVL 3871) are diagnosable. Therefore, Sereno and Arcucci (1994b) coined a new genus-level taxon, Marasuchus, to replace the nondiagnostic Lagosuchus. Subsequent workers followed Sereno and Arcucci (1994b) in this taxonomic usage.
Marasuchus holds a critical phylogenetic position as a proximal outgroup to Dinosauria in a number of studies (Sereno and Arcucci, 1994b; Novas, 1996; Benton, 1999; Irmis et al., 2007a). The anatomy was well documented by Bonaparte (1975) and Sereno and Arcucci (1994b). Unfortunately, most of the skull and the manus are missing. Here, I rely almost exclusively on PVL 3870 and 3871 for scoring.
Rauhut (2003) proposed the following characters autapomorphies of Marasuchus: (1) posterior cervical neural spines project anterodorsally; and (2) neural spines of mid- to posterior dorsal vertebrae contact each other dorsally.
Key references
Romer, 1971a, 1972a; Bonaparte, 1975; Sereno and Arcucci, 1994b.
Lewisuchus admixtus Romer, 1972d
Holotype
UNLR 1, posterior portion of the skull, maxilla, dentary, articular (now apparently lost), cervical and dorsal vertebrae, scapulocoracoid, and humerus.
Remarks
Lewisuchus was named by Romer (1972d) based on a partial skull and articulated anterior half of a skeleton. The posterior half of the skull, maxilla, dentary, and postcranium were found in the same nodule, but not articulated (Romer, 1972d). Romer (1972d) argued that the cranial material and postcrania belong to the same individual based on agreement of size and the “thecodont nature” of the material. I agree with Romer's argument and argue that none of the elements is duplicated in the specimen. The maxilla and dentary agree in size and the maxilla differs from those of any other archosauriform from the Chañares assemblage. Furthermore, the maxilla bears a large antorbital fossa that is present on the dorsal process of the maxilla, which is a character present only in archosaurs. The femur described by Romer (1972d) is actually a tibia as observed by Arcucci (1998). The size of the hind limb agrees with the rest of the specimen. Romer (1972d) illustrated and described the posterior portion of a mandible. At the time of this study, these elements seem to be lost.
Romer (1972d) considered Lewisuchus a pseudosuchian (at that time, Pseudosuchia was a wastebasket taxon) and made comparisons to “coelurosaurs” (1972 usage), Hesperosuchus, and “Teleocrater” (a taxon never formally described). Little else was said about the taxon until Parrish (1993) included it in his phylogeny of pseudosuchians. Parrish found it more closely related to crocodylomorphs than to “prestosuchids.” This result was a direct result of Parrish (1993) having scored a “crocodile-normal” astragalus for Lewisuchus. Arcucci (1998) declared that this astragalus belongs to a much smaller proterochampsian, and her assessment is followed here. Moreover, Arcucci (1997, 1998) stated that Pseudolagosuchus and Lewisuchus are the same taxon (see below). Hutchinson (2001a) accepted that the two were synonymous.
Key references
Romer, 1972d; Parrish, 1993; Arcucci, 1997, 1998.
Pseudolagosuchus majori Arcucci, 1987
Occurrence
Chañares Formation, near the town of Rio Los Chañares, Departamento Lavalle, Provincia de La Rioja, Argentina.
Holotype
PVL 4629, complete articulated left femur, tibia, fibula, more poorly preserved astragalus and calcaneum, complete pubis, fragments of presacral vertebrae, and ribs.
Referred material
PVL 3454, fragment of the distal portion of the femur, distal two-thirds of tibia and fibula articulated with the astragalus, proximal portion of fibula, incomplete metatarsal, two poorly preserved sacral vertebrae connected to both ilia; MACN 18954, three disarticulated vertebrae (probably dorsals), five articulated caudal vertebrae, distal portion of femur, distal portions of articulated tibia and fibula, articulated astragalus and calcaneum; UNLR 53, distal fragments of tibia and fibula, proximal tarsals, and various articulated caudal vertebrae.
Remarks
Pseudolagosuchus was named for a partial articulated pelvic girdle and much of a hind limb by Arcucci (1987). Only the pelvic girdle, hind limb, sacrals, a few dorsal vertebrae, and proximal caudal vertebrae are known from this taxon. Arcucci (1987) recognized that the proximal tarsals were similar to that of Marasuchus and dinosaurs, and this was later supported by synapomorphies listed by Novas (1996). Nesbitt et al. (2007) suggested that Pseudolagosuchus shares femoral synapomorphies with Silesaurus to the exclusion of other avian-line archosaurs.
Key references
Arcucci, 1987; Novas, 1996; Nesbitt et al., 2007.
Asilisaurus kongwe Nesbitt et al., 2010
Paratypes
NMT RB21, anterior cervical vertebra; NMT RB10, left scapulocoracoid; NMT RB11, sacrum; NMT RB12, proximal portion of an ischium; NMT RB13, ilium; NMT RB14, proximal portion of the pubis; NMT RB15, anterior portion of a skull; NMT RB16, proximal portion of the left humerus; NMT RB17, left astragalus; NMT RB18, right calcaneum; NMT RB19, proximal portion of a left femur; NMT RB20, right tibia. Additional material from the type locality referable to Asilisaurus is under preparation.
Remarks
During a recent collection effort in the Manda Beds, the remains of a small dinosauriform were collected (Sidor et al., 2008). The remains were locally abundant, and articulated segments and isolated bones were collected from a handful of localities over a 3 km2 area. Fragments of the dentary and two complete astragali confirm a close relationship with Silesaurus, a non-dinosaurian dinosauriform. Asilisaurus kongwe represents the oldest avian-line archosaur yet discovered and shows that many of the basal avian-line archosaur clades were present by the end of the Anisian.
The taxon bears the following unique combination of characters: anterior portion of the dentary tapers to a sharp point, teeth absent from the anterior portion of the dentary, teeth ankylosed into the alveoli, distinctly convex dorsal margin of the dentary, Meckelian groove positioned at the dorsoventral midpoint of the medial surface of the dentary, peg-like teeth with extremely small, and poorly developed serrations.
Eucoelophysis baldwini Sullivan and Lucas, 1999
Age
Mid to late Norian (Litwin et al., 1991; Lucas, 1998a; Heckert et al., 2005; Irmis et al., 2007b).
Holotype
NMMNH P-22298, incomplete postcranial material consisting of two dorsal and four incomplete caudal vertebrae, nearly complete right pubis, partial right ischium, ilium fragment, fragmentary femora, proximal half of the left tibia, incomplete right metatarsals II and IV, complete metatarsal III, phalanges, unidentified bone fragments, and possibly an incomplete left scapulocoracoid.
Remarks
The incomplete specimen of Eucoelophysis was found in a multitaxic assemblage in the Petrified Forest Member, Chinle Formation, New Mexico. Originally described as a coelophysoid theropod dinosaur by Sullivan and Lucas (1999), Eucoelophysis is now considered to be a non-dinosaurian dinosauriform (Nesbitt et al., 2005; Ezcurra, 2006; Irmis et al., 2007a; Nesbitt et al., 2007; Brusatte et al., 2008). In explicit phylogenetic analyses, Ezcurra (2006) found Eucoelophysis as the sister taxon to Dinosauria, whereas Irmis et al. (2007a) found Eucoelophysis in a clade with Silesaurus as the sister taxon to Dinosauria.
The studies of Ezcurra (2006) and Nesbitt et al. (2007) agreed for the most part. However, because the hind limbs were the only elements of Eucoelophysis that were definitely associated (within a multitaxic quarry), Nesbitt et al. (2007) considered the hind limbs and metatarsals the only definite material pertaining to the holotype of Eucoelophysis. Nesbitt et al. (2007) hypothesized that the pubis does not go to the hind limbs, whereas Ezcurra (2006) scored the pubis as part of Eucoelophysis in his data matrix. If the pubis character scores of Eucoelophysis are removed, Eucoelophysis, Silesaurus, and Dinosauria form a polytomy in Ezcurra's (2006) matrix.
The following autapomorphies were listed by Ezcurra (2006): (1) noninvasive pleurocoels in the dorsal vertebrae; (2) strongly marked U-shaped ischio-acetabular groove in pubis (Sullivan and Lucas, 1999); (3) absence of femoral trochanteric shelf of femur; (4) cnemial crest distinctively offset from the tibial shaft, cranially straight, and without lateral notch; and (5) femoral fourth trochanter reduced. The first character does not occur in any of the vertebrae of Eucoelophysis. The second character is present in the pubis, but this element cannot be unambiguously shown to belong to Eucoelophysis. Staurikosaurus (MCZ 1669), some basal theropods (e.g., Dilophosaurus, UCMP 37302), Sacisaurus (MCN PV10019), Lagerpeton (PVL 4619), basal ornithischians (e.g., Scutellosaurus), and basal sauropodomorphs (save Saturnalia) all lack a trochanteric shelf. The proximal end of the tibia, including the cnemial crest, is eroded the absence of a posterior notch cannot be assessed. Furthermore, the shaft of the tibia is incomplete; therefore, it cannot be assumed that it is straight. The fourth trochanter of Eucoelophysis is reduced relative to other archosaurs. Nesbitt et al. (2007) cited an appressed surface of the tibia as an apomorphy of Eucoelophysis.
Key references
Sullivan and Lucas, 1999; Nesbitt et al., 2005; Ezcurra, 2006; Nesbitt et al., 2007.
Sacisaurus agudoensis Ferigolo and Langer, 2007
Occurrence
Santa Maria 2 sequence. Top of the Alemoa Member of the Santa Maria Formation or base of the Caturrita Formation.
Referred material
Dentaries (MCN PV10042, PV10043, PV10044, PV10061, PV10048); MCN PV10050, maxilla; MCN PV10051, postorbital; vertebrae (MCN PV10028, PV10029, PV10032, PV10090, PV10097); MCN PV10033, scapula; MCN PV10100, ilium; pubes (MCN PV10023, PV10024); MCN PV10025, ischium; femora (MCN PV10009, PV10010, PV10011, PV10013, PV10014, PV10015, PV10016, PV10018, PV10019, PV10063, PV10075); MCN PV10020, tibia.
Remarks
Sacisaurus was described by Ferigolo and Langer (2007) from a multitaxic bonebed from the Santa Maria sequence. The holotype was picked from a collection of tens of individuals (counted from femora) as a distinct dentary, and all crania and postcrania were subsequently referred to the taxon (Ferigolo and Langer, 2007). I agree with the authors for most of their assignments because of the similarity of most of the material to Silesaurus. However, remains of a basal saurischian dinosaur were found in the same bed among the disarticulated skeletons of Sacisaurus (S.J.N., personal obs). An ectopterygoid (MCN PV10049) assigned to Sacisaurus appears too large for that taxon and possibly belongs to a saurischian. As described by Ferigolo and Langer (2007), Sacisaurus is very similar to Silesaurus, a non-dinosaurian dinosauriform.
Ferigolo and Langer (2007) provided the following diagnosis: dinosauriform differing from other known basal members of the group, except Silesaurus opolensis and ornithischians, for the presence of an edentulous mandibular rostral portion. This jaw segment differs from that of S. opolensis because its front tip is not dorsally curved, and from that of ornithischians because it does not form a typically single (unpaired) predentary, but articulates to its counterpart in the midline.
Holotype
ZPAL Ab III/361, dentaries, braincase, pterygoid, frontals, quadrate, surangular, nearly complete presacral column, sacrum, caudal vertebrae, scapulocoracoid, radii, ulnae, complete pelvic girdle, and hind limbs.
Referred material
ZPAL AbIII/362, braincase, cervical, dorsal, sacral, and caudal vertebrae, partial pectoral girdle and forelimb, partial pelvic girdle and hind limbs; ZPAL AbIII/363, associated pelvic girdle; ZPAL AbIII/364, braincase, presacral vertebrae, ribs, partial forelimbs, complete articulated hind limbs.
Remarks
Since Dzik's (2003) initial description, Silesaurus has revolutionized the understanding of the systematics of basal avian-line archosaurs. Silesaurus is known from well-preserved material from nearly all parts of the skeleton (Dzik, 2003) except some of the more delicate bones of the skull (Dzik and Sulej, 2007). The material derives from a single horizon in a single locality (Krasiejów) and occurs as both isolated elements and nearly complete skeletons. The well-preserved three-dimensional specimens allow a nearly unparalleled examination of morphological features.
The divergent morphology of Silesaurus strongly contrasts with the typical basal dinosaurian and avian-line archosaur bauplan. The elongated forelimbs are proportionally longer than those of basal dinosaurs. The manus is largely missing, but fragments of metacarpals and phalanges suggest the manus was quite small. Furthermore, the dentition closely resembles that of ornithischians, and the anterior portion of the dentary tapers to a sharp point.
The odd mixture of features elsewhere present in herbivorous dinosaurs features has led to controversy concerning the systematic position of Silesaurus. Dzik (2003) did not place Silesaurus in a phylogenetic analysis, but suggested that it was closely related to, but did not represent, a true dinosaur. Subsequently, Dzik and Sulej (2007) suggested that Silesaurus represents a basal ornithischian based on new material. However, this was not based on a phylogenetic analysis either. In explicit phylogenetic analyses, Langer and Benton (2006), Ezcurra (2006), and Irmis et al. (2007a) found Silesaurus as the sister taxon to Dinosauria. Given this important systematic position, Silesaurus polarizes dinosaurian synapomorphies and is of extreme interest.
Silesaurus differs from all other archosaurs by the combination of the following characters: (1) edentulous anterior portion of the dentary that tapers to a point well above the dental margin; (2) maxillary and dentary tooth crowns expanded above root with small denticles; and (3) femur with notch on the proximal end.
Key references
Dzik, 2003; Ezcurra, 2006; Nesbitt et al., 2007; Irmis et al., 2007a.
Pisanosaurus mertii Casamiquela, 1967
Age
Late Carnian–early Norian, Late Triassic (Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).
Holotype
PVL 2577, tooth-bearing elements, vertebrae, incomplete hind limb, impression of the pelvis.
Remarks
Since its discovery, Pisanosaurus was considered the most primitive ornithischian (Casamiquela, 1967; Bonaparte, 1976; Weishampel and Witmer, 1990; Sereno, 1991b; Irmis et al., 2007a; Butler et al., 2007, 2008b). However, questions about the association of the material plagued certainty regarding the validity of the taxon. Our current understanding of the taxon can be better understood only with the discovery of a new specimen. However, Pisanosaurus is almost always found as the basalmost member of Ornithischia (Langer and Benton, 2006; Butler et al., 2007, 2008b; Irmis et al., 2007a) because of the combination of ornithischian synapomorphies and archosaur plesiomorphies, such as the anteroventrally directed pubis.
Key references
Casamiquela, 1967; Bonaparte, 1976; Weishampel and Witmer, 1990; Sereno, 1991b; Irmis et al., 2007a; Butler et al., 2008b.
Heterodontosaurus tucki Crompton and Charig, 1962
Occurrence
Clarens Formation ( = Cave Sandstone) and upper Elliot Formation, Herschel, Cape Province, South Africa.
Remarks
Although Heterodontosaurus is only represented by two unambiguous specimens, SAM-K-1332 remains one of the most well-preserved and most complete dinosaurs known to date. Santa Luca (1980) fully described the postcrania of SAM-K-1332, but a full description of the skull has yet to be published.
The phylogenetic position of Heterodontosaurus, though highly debated in the literature, is critical to the understanding of early dinosaur and ornithischian relationships. As summarized by Butler et al. (2008b), Heterodontosaurus was has been classified as a basal ornithopod, as the sister taxon to Margincephalia, as the sister taxon to Margincephalia + Ornithopoda, and as one of the basalmost ornithischians. Most recently, Heterodontosaurus was found as a basal ornithischian near Pisanosaurus (Butler et al., 2008b). This position better reflects the fossil record of Ornithischia and suggests that some of the “odd” features (e.g., the hand) of Heterodontosaurus present in non-ornithischian dinosaurs (e.g., Herrerasaurus) may represent plesiomorphies of Dinosauria rather than autapomorphies of Heterodontosaurus. A further discussion of these potentially plesiomorphic features were presented by Butler et al. (2008b).
Butler et al. (2008b) provided the following diagnosis of Heterodontosaurus: dorsal process of premaxilla does not form contact with nasals; anterior, accessory opening present within the antorbital fossa; squamosal-quadratojugal contact is anteroposteriorly broad; paroccipital processes are very deep dorsoventrally; paired, deep recesses on the ventral surface of the basisphenoid; basisphenoid processes are extremely elongated; cingulum is completely absent on cheek teeth; ischium with elongate flange on lateral margin.
Key references
Crompton and Charig, 1962, Santa Luca et al., 1976; Santa Luca, 1980; Butler et al., 2008b.
Lesothosaurus dianosticus Galton, 1978 (fig. 12B–C)
Syntypes
BMNH RUB17, mostly disarticulated remains of at least two individuals, one larger than the other, including most of one articulated skull; BMNH RUB 23, partial skull, nearly complete, disarticulated skull; BMNH R11004, partially articulated posterior skull and anterior neck, including the braincase, parietals, right squamosal, right quadrate, posterior portion of the right lower jaw, axis and third cervical, partial postcranium; SAM-PK-K401, partial postcranium, including proximal ischia, partial postcranium, including proximal ischia.
Remarks
Lesothosaurus was first described by Galton (1978) for well-preserved crania and postcrania from the upper Elliot Formation. Sereno (1991a) added further details to Galton's (1978) original description, and assigned other material to the taxon. All basal dinosaur and ornithischians analyses agree that Lesothosaurus is one of the basalmost ornithischians. Only Butler et al. (2008b) found heterodontosaurids more basal than Lesothosaurus and found Lesothosaurus as the sister taxon to all thyreophorans.
Butler (2005) provided the following diagnosis for Lesothosaurus: anterior premaxillary foramen present; slot in maxilla for lacrimal present; six premaxillary teeth present; absence of diastema between the premaxillary and maxillary teeth; maxillary teeth lack apicobasally extending ridges on their lingual and labial faces; manual phalanges lacking prominent intercondylar processes; ilium with well-developed supraacetabular flange and ventromedially angling brevis shelf visible in lateral view; dorsal groove on the ischial shaft present; shaft of ischium twists through 90° along its length, forms an elongate symphysis with the opposing ischial blade, and lacks a tab-shaped obturator process; prepubic process short and mediolaterally flattened rather than rodlike and does not extend beyond the end of the preacetabular process of the ilium; postcranial osteoderms absent.
Key references
Thulborn, 1970, 1972; Santa Luca, 1984; Norman et al., 2004; Butler, 2005.
Scutellosaurus lawleri Colbert, 1981
Occurrence
Silty facies of the Kayenta Formation, Rock Head and other nearby localities (e.g., Gold Spring), northern Arizona (Colbert, 1981).
Holotype
MNA 175, nearly complete, associated skeleton including the dentition-bearing parts of the skull, cervical, dorsal, sacral, and caudal vertebrae, hundreds of osteoderms, much of the pectoral and pelvic girdles, and portions of the fore- and hind limbs.
Referred material
MNA 1752, partial disarticulated skeleton; UCMP 130580; UCMP 170829; TMM 43687-16; MCZ 8592; MCZ 8799.
Remarks
Scutellosaurus is one of the better-known basal ornithischians, known from at least 10 partial skeletons. All specimens originate from a small set of localities in the silty facies of the Kayenta Formation. Scutellosaurus was consistently found as one of the earliest undoubted members of Thyreophora in phylogenetic analyses of ornithischians (Sereno, 1999; Norman et al., 2004; Butler et al., 2008b). As stated by Irmis et al. (2007b), Scutellosaurus is the oldest confirmed ornithischian in North America.
Autapomorphies listed by Butler et al. (2008b) include: dorsal and ventral margins of the preacetabular process of the ilium are drawn out medially into distinct flanges that converge upon one another anteriorly; elongate tail of comprising at least 58 caudal vertebrae.
Key references
Colbert, 1981; Rosenbaum and Padian, 2000; Butler et al., 2008b.
Eocursor parvus Butler et al., 2007
Occurrence
Damplaats Farm, Ladybrand District, Free State, Republic of South Africa, upper part of the lower Elliot Formation (Butler et al., 2007).
Holotype
SAM-PK-K8025, disarticulated partial skeleton including parietal, supraoccipital, basisphenoid, parasphenoid, right dentary, surangular and angular, isolated cheek tooth, fragmentary cervical, dorsal, sacral and caudal vertebrae, scapulae, humeri, radius, six manual phalanges, ilia, ischia, pubes, femora, tibiae, fibulae, right metatarsals II and III, and three pedal phalanges.
Remarks
Eocursor is known from crania and postcrania from the lower Elliot Formation. This stratigraphic position makes Eocursor the earliest most complete ornithischian currently known (Butler et al., 2007). Butler et al. (2007) found Eocursor near the base of Ornithischia.
Butler et al. (2007) differentiated Eocursor from other ornithischians by: an accessory fossa present on the lateral surface of the basisphenoid, posterior to the canal for the internal carotid artery; maximum transverse expansion of the distal end of the humerus is only 50% of maximum transverse expansion of proximal humerus; and pubic obturator foramen subcircular and enlarged (maximum dorsoventral diameter of foramen is twice the maximum diameter of proximal pubic shaft).
Age
Late Carnian, Late Triassic (Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).
Holotype
PVL 2566, dorsal, sacral, and caudal vertebrae, ilium, pubis, ischium, right femur, metatarsals, phalanges, left astragalus.
Referred material
PVSJ 373, well-preserved articulated skeleton, lacking skull and most cervical and caudal vertebrae; PVSJ 407, nearly complete articulated skeleton with skull and mandible.
Key references
Reig, 1963; Sereno and Novas, 1992; Novas, 1994; Sereno, 1994; Langer and Benton, 2006.
Staurikosaurus pricei Colbert, 1970
Holotype
MCZ 1669, incomplete skeleton including partial mandibular rami, almost complete vertebral column including six cervical vertebrae, most of the trunk and caudal series, the complete sacrum, two fragments of the scapulocoracoid, a bone fragment of uncertain affinities attributed to the humerus (Galton, 2000), almost complete ilia, pubes, ischia, femora, and the left tibia and fibula.
Remarks
Staurikosaurus was named by Colbert (1970) for a unique specimen from the Triassic Santa Maria sequence in southern Brazil. The age and provenance makes Staurikosaurus a very important specimen for answering questions about early dinosaur diversification, relationships, and early evolution. The partially articulated skeleton preserves much of the axial column and pectoral girdle but lacks forelimbs, most of the skull, and the ever-important pes. Unfortunately, the surfaces of the bones are poorly preserved, and the identification of some of the more incomplete elements found with the specimen continue to be debated (see Galton, 2000).
Researchers have generally agreed that Staurikosaurus is a dinosaur, but placement within Dinosauria remains controversial. Staurikosaurus was found as the sister taxon of Herrerasaurus in phylogenetic analyses (Novas, 1992; Sereno, 1999; Rauhut, 2003; Langer, 2004; Langer and Benton, 2006) or suggested as a more basal dinosaurian taxon (Galton, 1977; Brinkman and Sues, 1987). Out of the possible autapomorphies of Staurikosaurus listed by Bittencourt and Kellner (2005), none seems to be restricted to the taxon. I agree with Rauhut (2003) that a postacetabular process of the ilium abbreviated and straight posteriorly is an autapomorphy of the taxon.
Key references
Colbert, 1970; Galton, 1977, 2000; Bittencourt and Kellner, 2005.
Saturnalia tupiniquim Langer et al., 1999
Holotype
MCP 3844-PV, a well-preserved, semiarticulated skeleton including most of the presacral vertebral series, both sides of the pectoral girdle, right humerus, partial right ulna, right radius, both sides of the pelvic girdle with the sacral series, left femur, and most of the right hind limb.
Referred material
MCP 3845-PV, skeleton including the posterior part of the skull with braincase, the natural cast of a mandibular ramus–bearing teeth, presacral series including posterior cervical and anterior trunk vertebrae, both halves of the pectoral girdle, right humerus, right side of the pelvic girdle and most of the right hind limb; MCP 3846-PV, an incompletely prepared skeleton, from which a partial tibia and foot, as well as some trunk vertebrae, are known.
Remarks
Saturnalia is one of the oldest and most completely known sauropodomorphs. Although well described in a series of papers (Langer et al., 1999; Langer, 2003; Langer et al., 2007), much of the material, including the skull, has yet to be fully prepared. Saturnalia was found as the basalmost sauropodomorph in all phylogenetic analyses that included the taxon. Thus, it is very important to studies of basal dinosaurs.
Key references
Langer et al., 1999, 2003, 2007; Langer, 2005a; Langer and Benton, 2006.
Eoraptor lunensis Sereno et al., 1993 (fig. 12D)
Age
Late Carnian, Late Triassic (Rogers et al., 1993, adjusted for the new Triassic timescale of Muttoni et al., 2004).
Remarks
Eoraptor remains one of the most controversial basal dinosaurs discovered. It is known from an entire articulated skeleton. Nevertheless, the poor preservation of the surface of the bone, missing details of the skull, crushing of the some of the elements, and covered elements led to conflicting interpretations (compare Sereno et al., 1993, to Langer and Benton, 2006).
Sereno et al. (1993) found Eoraptor as the basalmost theropod sister taxon to Herrerasaurus + Neotheropoda. Other studies focused on the interrelationships of theropods (e.g., Rauhut, 2003) found Eoraptor as the sister taxon to Herrerasaurus + Neotheropoda. Most recently, Langer and Benton (2006) found Eoraptor as the sister taxon to Eusaurischia. A detailed description of the taxon is currently in progress.
Eoraptor possesses two potential autapomorphies: a leaf-shaped premaxillary and anterior maxillary crowns, and a ventral process of the postorbital flexed sharply anteriorly in the ventral portion (from Rauhut, 2003).
Key references
Sereno et al., 1993; Langer and Benton, 2006; Sereno 2007.
Efraasia minor (Galton, 1973), sensu Yates, 2003
Occurrence
Middle Löwenstein Formation, Weisser Steinbruch (Quarry), Pfaffenhofen, Germany, lower Löwenstein Formation, Goesel Quarry, Ochsenbach, Germany, (Yates, 2003).
Holotype
SMNS 11838, dorsal vertebrae, one sacral vertebra, right manus, partial left manus, pubes, right femur, tibia, and fibula, and partial right pes.
Remarks
In a revision of sauropodomorph taxa from the Triassic of Germany, Yates (2003) assigned the sauropodomorphs from Weisser Steinbrunchh, Pfaffenhofen, Germany, to the taxon Efraasia. Efraasia represents one of the more plesiomorphic sauropodomorphs (Yates, 2003). Recent phylogenetic analyses of basal sauropodomorph relationships (Yates, 2007; Upchurch et al., 2007) found Efraasia diverging before the split of prosauropods (Plateosaurus-like taxa) and the lineage leading to Sauropoda.
Efraasia minor possesses two autapomorphies, interbasipterygoid web with a central tubercle and a hypertrophied semilunate-shaped pubic tubercle projecting laterally from the proximal pubis (Yates, 2003).
Key references
Huene, 1908; Galton, 1973; Yates, 2003.
Plateosaurus engelhardti Meyer, 1837 (fig. 12I)
Reference material
SMNS 13200, a nearly complete skull and skeleton. (The original syntypes are not diagnostic [Yates, 2003]).
Referred material
AMNH FR 6810, disarticulated skull and complete skeleton; AMNH FR various specimens from the Plateosaurus Quarry. See Yates (2003). Numerous skeletons from SMNS and GPIT.
Remarks
Plateosaurus is one of the best-known Triassic dinosaurs, and it is represented by hundreds of specimens ranging from nearly complete skeletons to isolated elements. It is unclear which species name, Plateosaurus engelhardti or Plateosaurus longiceps, should be applied to the Plateosaurus Quarry specimens given the incomplete and nondiagnostic syntypes of Plateosaurus engelhardti (Meyer, 1837). Here, I follow Yates (2003) and consider all specimens from the Plateosaurus Quarry as Plateosaurus engelhardti. I score only specimens from the Plateosaurus Quarry and have referred to them as Plateosaurus engelhardti.
Plateosaurus engelhardti has the following character states: a dorsal end of the lacrimal with a broad, weakly rugose, lateral sheet covering the posterodorsal corner of the antorbital fenestra; short jugal with a dorsoventrally deep suborbital bar; palatine with a centrally located, ventral, peglike process; interbasipterygoid septum deep, filling the whole of the space between the basipterygoid processes, and with paired central processes' stout metacarpal V with a convex proximal articular surface; broad proximal caudal neural spines (proximodistal width greater than 40% of their height); and laterally compressed distal ischial expansions (from Yates, 2003).
Key references
Meyer, 1837; Huene, 1926; Galton, 2000; Yates, 2003.
Tawa hallae Nesbitt et al., 2009c (fig. 12E)
Age
Middle Norian (Litwin et al., 1991; 265266Lucas, 1998; Heckert et al., 2005; Parker, 2006; 235Irmis et al., 2007). The HQ has been dated to ∼215 to 213 million years ago (Mundil et al., 2008).
Remarks
A group of six to seven individuals of Tawa skeletons were found in a small area in an extensive multitaxic assemblage. The individuals differ in ontogenetic stage; the smallest fibula is 70% the length of the largest fibula. The well-preserved specimens vary in completeness, from isolated elements to nearly complete articulated skeletons. The skeletons have yet to be completely prepared, and a complete description of this important taxon is underway.
Coelophysis bauri (Cope, 1887), sensu Colbert, 1989 (fig. 12J)
Occurrence
Coelophysis Quarry, “siltstone member” of the Chinle Formation, Ghost Ranch, northern New Mexico.
Holotype
AMNH 7224, complete skeleton missing the tail (the tail is reconstructed from other individuals).
Referred material
AMNH 7223 (see Colbert, 1989) and any coelophysoid material from the Coelophysis Quarry, including CM 31374, a complete skull.
Remarks
Coelophysis bauri refers only to the small theropod collected from the Coelophysis Quarry at Ghost Ranch. Even though it was cited as represented by a thousand skeletons (Schwartz and Gillette, 1994), few of the original specimens are fully prepared, and all of the specimens were subjected to crushing and distortion. Despite the distortion, Coelophysis remains the most completely known basal theropod available for study.
Coelophysis differs from Eoraptor, Herrerasaurus, and Staurikosaurus in the more elongated dorsal vertebrae, five fused sacral vertebrae, dolichoiliacic ilium, presence of a small lateral projection on the distal end of the tibia, and the functionally tridactyl foot with a metatarsal I that is attached to metatarsal II and does not reach the ankle joint. It differs from Gojirasaurus in the relatively lower neural spines of the dorsal vertebrae and the significantly smaller size, from Liliensternus in the absence of a broad ridge that extends from the posterior end of the diapophyses to the posterior end of the vertebral centra in cervical vertebrae and the smaller size, from Procompsognathus in the larger overall size and the lower metatarsal III : tibia ratio, from Shuvosaurus in the lack of any of the derived cranial features of the latter taxon, and from the slightly younger, but very similar Syntarsus in the lack of a postnasal fenestra. (based on Padian 1986, Colbert, 1989, AMNH 7223 and 7224).
Key references
Colbert, 1989; Rauhut, 2003; Nesbitt et al., 2006.
Dilophosaurus wetherelli (Welles, 1954), sensu Welles, 1970
= Megalosaurus wetherelli Welles, 1954
Occurrence
Lower portion of the silty facies of the Kayenta Formation, Moenkopi Wash, northern Arizona (Welles, 1984).
Referred material
UCMP 37303, premaxilla, maxilla, mandibles, vertebrae, articulated manus; TMM material figured by Tykoski (2005a).
Remarks
Dilophosaurus is one of the best-known early theropods and is known from a variety of material housed at UCMP, TMM, and MNA. All material referable to the taxon originated from the silty facies of the Kayenta Formation on Ward's Terrace. Basal theropod phylogenies placed Dilophosaurus as either the basalmost coelophysoid (Gauthier, 1986; Rowe, 1989; Rowe and Gauthier, 1990; Tykoski and Rowe, 2004) or closer to the tetanurans than to coelophysoids (Rauhut, 2003; Smith et al., 2007; Yates, 2007) in a clade containing Dracovenator, Zupaysaurus, and “Dilophosaurus” sinensis.
Rauhut (2003) listed the following autapomorphies for Dilophosaurus: lacrimal with thickened dorsoposterior rim; cervical neural spines with distinct central “cap”; an anterior and posterior “shoulder”; scapular blade with squared distal margin.
Neotype
UUVP 6000, a complete skull and partial skeleton only lacking first caudal vertebra, chevrons, ribs, forearms, and some digits of the pes (Madsen, 1976).
Remarks
Allosaurus is one of the best Jurassic theropods known to date. The taxon is represented by many articulated and disarticulated elements found throughout the Morrison Formation in North America. Allosaurus has been used in many phylogenetic analyses examining the relationships of theropods (e.g., Turner et al., 2007; Rauhut, 2003).
Allosaurus fragilis possesses the following unique characters: distinct “step” in the ventral margin of the jugal, leading to a significant ventral displacement of the posterior part in relation to the anterior portion; neomorphic bone ( = antarticular of Madsen, 1976); well-developed notch in the anteroventral margin of the prearticular (Rauhut, 2003).
Key references
Marsh, 1877; Madsen, 1976; Brusatte and Sereno, 2008.
Velociraptor mongoliensis Osborn, 1924
Referred material
IGM 100/24, complete skull and a few postcranial elements; IGM 100/25, complete skeleton; IGM 100/976, partial skeleton with a fragmentary skull and partial postcranium; IGM 100/986, fragmentary skeleton consisting of cranial and postcranial fragments; IGM 100/982, nearly completely preserved skeleton.
Remarks
Velociraptor is one of the best-understood maniraptoran theropods from the Cretaceous. It is known from complete skulls and skeletons. Velociraptor has been used in many phylogenetic analyses examining the relationships of theropods (e.g., TWiG; Turner et al., 2007) and has been critical to understanding the theropod-bird link (Padian and Chiappe, 1998). Barsbold and Osmólska (1999) present a thorough diagnosis of Velociraptor based on the skull.
CHARACTER DESCRIPTIONS
Cranium
1. Premaxilla, anterodorsal process ( = nasal process), length: (0) less than the anteroposterior length of the premaxilla; (1) greater than the anteroposterior length of the premaxilla (figs. 14, 19) (Nesbitt and Norell, 2006).
Fig. 14
Premaxillae of archosauriforms: A, the skull of Effigia okeeffeae (AMNH FR 30587) in right lateral view; B, the skull of Dromicosuchus grallator (UNC 15574) in right lateral view; C, left premaxilla of Plateosaurus engelhardti (AMNH FR 6810) in lateral view; D, left premaxilla of Postosuchus kirkpatricki (TTU-P 9000) in lateral view. Numbers refer to character states. Scale bars = 1 cm.
Nearly all archosauriforms have a short anterodorsal process of the premaxilla; the process forms the anterior and sometimes the anterodorsal corner of the external naris. In contrast, Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280) each have an elongated anterodorsal process of the premaxilla that extends posteriorly, dorsal to the external nares (Nesbitt, 2007). The length of the anterodorsal process is measured from the ventral edge of the external naris.
2. Premaxilla, posterodorsal process ( = maxillary process, = subnarial process), length: (0) less than or about the same as the anteroposterior length of the premaxilla; (1) greater than the anteroposterior length of the premaxilla (figs. 14, 19) (new).
The posterodorsal process of the premaxilla in most archosauriforms is shorter than or about the same as the anteroposterior length of the premaxilla; however, the length of the posterodorsal process varies widely in archosauriforms. This character attempts to describe the long length of the posterodorsal process of the premaxilla in a subset of suchians. In Rauisuchus (BSP AS XXV-60-121), Saurosuchus (PVSJ 32), Postosuchus kirkpatricki (TTU-P 9000), and Polonosuchus silesiacus (ZPAL Ab III/563), the posterodorsal process of the premaxilla is longer than the anteroposterior length of the premaxilla. The length of the posterodorsal process is measured from the ventral edge of the external naris.
3. Premaxilla, posterodorsal process ( = maxillary process, = subnarial process): (0) wide, platelike; (1) thin (figs. 14–15, 17, 19–20) (modified from Gauthier, 1986; Rauhut, 2003; Langer and Benton, 2006; Smith et al., 2007).
Fig. 15
Maxillae of archosauriforms: A, left maxilla of Postosuchus kirkpatricki (TTU-P 9000) in lateral view; B, left maxilla of Xilousuchus sapingensis (IVPP V 6026) in lateral view; C, partial maxillae of Sphenosuchus acutus (SAM 3014) in ventral view highlighting the palatal processes of the maxillae; D, left maxilla of Euparkeria capensis (SAM K 6047) in lateral view. The rest of the skull has been removed in the figure for comparison purposes; E, left maxilla of Fasolasuchus tenax (PVL 3851) in medial view; F, close up of the posterior maxillary teeth of CM 29894 (referred to as Hesperosuchus “agilis”). Numbers refer to character states. Arrow indicates anterior direction. Scale bars = 1 cm in B–D, F and 5 cm in A, E.
The maxillary process of the premaxilla broadly contacts the nasal at the posterodorsal portion of the external naris in archosaurs ancestrally (Gauthier, 1986). The posterodorsal process is thin in basal theropods (e.g., Coelophysis bauri, CM 31374) and basal sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810). This morphology contrasts with that of non-eusaurischian archosauriforms, ornithischians (e.g., Heterodontosaurus, SAM-PK-1332), and Herrerasaurus (PVSJ 407). As noted by Smith et al. (2007), the exact configuration of the maxillary process of the premaxilla relative to the maxilla and the nasal is variable within Theropoda.
4. Premaxilla, posterodorsal process ( = maxillary process, = subnarial process): (0) fits between the nasal and the maxilla or lies on the anterodorsal surface of the maxilla; (1) overlaps anterodorsal surface of nasal; (2) vertical, strongly sutured to maxilla; (3) fits into slot of the nasal. (fig. 14) (modified from Parrish, 1993; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
This character was originally used to describe the unusual posterior process of the premaxilla in basal crocodylomorphs (Parrish, 1993). In non-archosaurian archosauriforms, non-crocodylomorph crocodylian-line archosaurs, and basal avian-line archosaurs (e.g., Lesothosaurus, Herrerasaurus), the posterior process of the premaxilla fits between the nasal and the maxilla. In taxa with a short posterior process of the premaxilla (e.g., Effigia, inferred for Arizonasaurus), the process lies on the anterodorsal edge of the maxilla as with other taxa scored as (0). In the non-crocodyliform crocodylomorphs Dromicosuchus (UNC 15574), Hesperosuchus “agilis” (CM 29894), Sphenosuchus (SAM 3014), and Dibothrosuchus (IVPP V7907), the posterior process lies on the lateral process of the nasal and not between the nasal and the maxilla (Clark et al., 2000). Crocodyliformes are scored as (2) following Clark et al. (2000). The posterodorsal process fits into a distinct slot within the nasal (state 3) in Turfanosuchus (IVPP V 3237) and Revueltosaurus (PEFO 33788).
5. Premaxilla, posterodorsal process ( = maxillary process, = subnarial process): (0) extends posteriorly to the external naris; (1) restricted to the ventral border of the external naris (figs. 16, 20) (Langer and Benton, 2006).
Fig. 16
Skulls of basal archosauriforms in lateral view: A, Prolacerta broomi in lateral view; B, Erythrosuchus africanus in lateral view; C, Smilosuchus gregorii in lateral view; D, Proterosuchus fergusi in lateral view; E, Chanaresuchus bonapartei in lateral view; F, Euparkeria capensis in dorsal view. Numbers refer to character states. Scale bars = 5 cm B–D and 1 cm in A, E, F.
In non-archosaurian archosauriforms and most crocodylian-line archosaurs, the posterodorsal process of the premaxilla extends posterior to the external naris. Within crocodylian-line archosaurs, Qianosuchus (IVPP 13899), Arizonasaurus (MSM P4590), Xilousuchus (IVPP V 6026), and Effigia (AMNH FR 30587), the posterodorsal process of the premaxilla is restricted to the ventral border of the external naris. As discussed by Langer and Benton (2006), the posterodorsal process is restricted to the ventral border of the external naris in basal sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810) and basal theropods (Coelophysis bauri, CM 31374), whereas the posterodorsal process of Herrerasaurus (PVSJ 407) and ornithischians (e.g., Heterodontosaurus, SAM-PK-1332) extends posterior to the external naris.
6. Premaxillary teeth, number: (0) 3; (1) 4; (2) 5; (3) 6+; (4) 0 (figs. 14, 17) (Nesbitt and Norell, 2006).
Fig. 17
Skulls of crocodylian-line archosaurs in lateral view: A, Revueltosaurus callenderi in lateral view; B, Stagonolepis robertsoni in lateral view; C, Riojasuchus tenuisceps in lateral view; D, Effigia okeeffeae in lateral view. Shaded area indicates incomplete preservation. Numbers refer to character states. Scale bars = 1 cm.
The number of premaxillary teeth is somewhat variable among basal archosauriforms, and this character attempts to support small clades. Premaxillary teeth are absent in Lotosaurus (IVPP 48013), Effigia (AMNH FR 30587), and Shuvosaurus (TTU-P 9280). Only a few basal archosaur taxa have three premaxillary teeth, and this includes Euparkeria (SAM 5867), Heterodontosaurus (SAM-PK-1332), Ornithosuchus (BMNH R3143), Riojasuchus (PVL 3827), and Gracilisuchus (MCZ 4117). “Rauisuchians” have four premaxillary teeth (e.g., Postosuchus kirkpatricki, TTU-P 9000; Batrachotomus, SMNS 80260), whereas Revueltosaurus (PEFO 34561), Stagonolepis (BMNH R4787), Poposaurus (YPM 57100), Xilousuchus (IVPP V 6026), Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Dibothrosuchus (IVPP V 7907), and Alligator have five. Taxa with elongated premaxillae, such as phytosaurs, Qianosuchus (IVPP V 13899), and Proterosuchus (NM QR 1484) have many (8–25) premaxillary teeth.
7. Premaxilla, teeth: (0) present along entire length of the premaxilla; (1) absent in the anterior portion of the premaxilla (modified from Heckert et al., 1999; Parker, 2007).
Premaxillary teeth in archosauriforms are usually distributed along the length of the premaxilla. The aetosaurs Aetosaurus (SMNS 5770 S-4) and Stagonolepis (BMNH R4787) do not have premaxillary teeth in the anterior portion of the premaxilla, whereas Desmatosuchus (Small, 2002) does not have premaxillary teeth.
8. Premaxilla: (0) nearly horizontal; (1) downturned (fig. 16) (Gower and Sennikov, 1997).
Gower and Sennikov (1997) cited a downturned premaxilla as a synapomorphy of Sarmatosuchus + Proterosuchus. They described the following criteria for a downturned premaxilla: (1) the anterodorsal process is directed toward the anterodorsal edge of the posterodorsal process, in lateral view; and (2) the long axis of the palatal process is at a more acute angle to the ventral margin of the premaxilla than is present in taxa without downturned premaxillae (Gower and Sennikov, 1997). I follow these criteria here. The only known Permian archosauriform, Archosaurus (PIN 1100/55), has a downturned premaxilla.
Sereno (1991a) used a similar character to describe the premaxillae of Riojasuchus and Ornithosuchus. Therefore, Riojasuchus and Ornithosuchus are also scored as (1).
9. Premaxilla, narial fossa: (0) absent or shallow; (1) expanded in the anteroventral corner of the naris (figs. 14, 20) (modified from Sereno, 1999; Langer and Benton, 2006; Irmis et al., 2007a).
Langer and Benton (2006) discussed this character in detail and found that an expanded narial fossa on the anteroventral corner of the naris is found almost exclusively in theropods, Herrerasaurus (PVSJ 407), Eoraptor (PVSJ 512), and sauropodomorphs. However, even though Langer and Benton (2006) scored the suprageneric ornithischian terminal taxon as (0), a deep narial fossa is present in Heterodontosaurus (SAM-PK-1332). The narial fossa described for Batrachotomus (Gower, 1999) seems to be an autapomorphy of the taxon.
10. Premaxilla, length: (0) shorter than the maxilla; (1) longer than the maxilla (modified from Sereno, 1991a).
The length of the premaxilla nearly is universally shorter than the maxilla in archosauriforms. However, in phytosaurs (e.g., Parasuchus hislopi, ISI R 42) the greatly elongated premaxilla is longer than the maxilla. In some forms such as Mystriosuchus planirostris, the premaxilla is at least twice as long as the maxilla (Hungerbühler, 2002).
11. Premaxilla-maxilla, subnarial gap between the elements in lateral view: (0) absent; (1) present (figs. 14, 19) (Gauthier, 1986; Langer and Benton, 2006).
Following Langer and Benton (2006: fig. 4), a subnarial gap between the premaxilla and the maxilla is present in the basal theropods Coelophysis bauri (CM 31374) and Dilophosaurus (UCMP 37302). The condition in Eoraptor (PVSJ 512) is more similar to Coelophysis bauri (CM 31374) than to Herrerasaurus (PVSJ 407), so I score it as (1). A clear subnarial gap is present in crocodylomorphs (e.g., Dibothrosuchus, IVPP V 7907; Protosuchus richardsoni, MCZ 6727), Vancleavea (GR 138), and Heterodontosaurus (SAM-PK-1332). In these taxa, the gap receives an enlarged dentary tooth.
12. Premaxilla-maxilla, subnarial foramen between the elements: (0) absent; (1) present and the border of the foramen is present on both the maxilla and the premaxilla; (2) present and the border of the foramen is present on the maxilla but not on the premaxilla; (3) present and the border of the foramen is present on the premaxilla but not on the maxilla (figs. 14, 17, 19) (modified from Benton and Clark, 1988; Parrish, 1993; Juul, 1994; Benton, 1999).
Benton and Clark (1988) were the first to use the presence of a slitlike fenestra/foramen between the maxilla and premaxilla to diagnose the clade Rauisuchidae in a phylogenetic context. Parrish (1993) and other basal archosaur workers also termed the opening the subnarial foramen in their character lists. The differences in the shape of the gap between the maxilla and the premaxilla, the scoring inconsistencies among the various authors, and the scant distribution of an opening between the maxilla and premaxilla in various archosaurs (see Juul, 1994; Gower, 2000) led Gower (2000) to question the validity of this character as a synapomorphy of a clade of “rauisuchians.”
The morphology of the subnarial foramen was reported in a variety of “rauisuchians.” Here, I evaluate the distribution of the feature in archosauriforms in order to clarify possible homologies. Some non-archosauriform archosauromorphs (e.g., Mesosuchus, Prolacerta), and non-archosaurian archosauriforms (e.g., Proterosuchus) possess a large anteriorly directed foramen on the anterior portion of the maxilla. Generally, a similar opening is not present in the same position in Erythrosuchus + Archosauria. It is not clear what vessels passed through this opening, but it is possible that they were the same as the elements that passed through the openings between the maxilla and premaxilla in other archosaurs. Juul (1994), followed by Gower (2000), reported a round opening between the maxilla, premaxilla, and nasal in the erythrosuchian Shansisuchus; the nasal does not contribute to the opening in any other taxon examined. Juul (1994) reported a similar opening in Erythrosuchus; however, in a thorough description of Erythrosuchus, Gower (2003) did not find a similar opening. Even though there is not a large opening, the premaxilla of Erythrosuchus (BPI 4526) bears a deep groove that originates on the posterior portion at the border of the articulation with the maxilla, dorsal to the premaxillary peg. A similar groove in the premaxilla is present in Revueltosaurus (PEFO 34561). As described by Gower (2003), the anterior portion of the maxilla does not have an indentation for an opening between the maxilla and the premaxilla. This also is the case in Revueltosaurus; there is no evidence of a foramen in the maxilla alone. However, a foramen between the maxilla and the premaxilla is formed only when the two elements are present. In both Erythrosuchus and Revueltosaurus, the foramen opens anteriorly.
Among crocodylian-line archosaurs, aetosaurs (e.g., Aetosaurus, SMNS 5770 S-7; Longosuchus, TMM 31185–98) lack any opening between the maxilla and premaxilla. Among other suchians (e.g., “rauisuchians”), the opening between the maxilla and premaxilla is variable. Effigia bears an opening between the maxilla and the premaxilla, but there is very little lateral exposure. The condition in Effigia is opposite that of Erythrosuchus and Revueltosaurus; there is no evidence of an opening on the premaxilla, but there is a posteriorly directed groove on the maxilla (Nesbitt, 2007). I concur, following Juul (1994) and Gower (2000), that there is no opening between the maxilla and premaxilla in Prestosuchus (UFRGS 0156-T). I argue that the two examples of a slitlike gap between the maxilla and premaxilla in both Luperosuchus (UNLR 4) and the holotype of Saurosuchus galilei (PVL 2062) are taphonomic features resulting from disarticulation and preservation rather than real morphology. The holotype of Luperosuchus fractus is poorly preserved, and the nasal is separated from the maxilla for the length of each element. The holotype of Saurosuchus galilei (PVL 2062) has a long slit between the maxilla and premaxilla, whereas a newly referred specimen (PVSJ 32; Alcober, 2000) has a long slit on the left side, but the right side has a tightly bound maxilla and premaxilla. The right maxilla of PVSJ 32 is clearly disarticulated because there are numerous cracks displacing the posterior process. The left side of PVSJ 32 illustrates the real morphology of the maxilla-premaxilla contact and indicates that there is no slitlike gap in Saurosuchus.
A small foramen shared between the premaxilla and maxilla is present in Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), and Batrachotomus (Gower, 1999). The small foramina in P. kirkpatricki, T. silesiacus, and Batrachotomus are nearly identical to each other; the maxilla and premaxilla both form part of the border of the subnarial foramen. Furthermore, a small foramen similar to that of P. kirkpatricki, T. silesiacus, and Batrachotomus is possibly also found in the basal crocodylomorphs Dromicosuchus (UNC 15574) and Hesperosuchus (CM 29894). In these crocodylomorphs, a dentary tooth fits in a small foramen in the maxilla and not in the gap between the premaxilla and maxilla. In contrast, a dentary tooth fits into a large gap between the premaxilla and maxilla of Dibothrosuchus (Wu and Chatterjee, 1993), and the condition is not clear in Sphenosuchus (Walker, 1990). Rauisuchus is scored the same as Postosuchus kirkpatricki even though the maxilla is not known; half a foramen is present on the well-preserved premaxilla and it is clear that the foramen would open laterally.
It is clear that the multitude of morphologies of the opening between the maxilla and the premaxilla are different among various taxa. The homologies of the different morphologies are not clear, and it is not obvious if the different openings transmit the same vessels and thus, has an underlining homology among archosauromorphs. Therefore, the character states are expanded from past iterations of this character to incorporate the details discussed above.
13. Premaxilla-maxilla, two-tooth diastema between the posterior premaxillary teeth and the anterior maxillary teeth: (0) absent; (1) present (fig. 17) (Sereno, 1991a).
A two-tooth diastema is present in both Ornithosuchus (BMNH R3143) and Riojasuchus (PVL 3827).
14. Maxilla, facial portion anterior to anterior edge of antorbital fenestra: (0) shorter than posterior portion; (1) equal in length or longer than portion posterior to anterior edge of fenestra (figs. 16, 19–20) (character states reversed from Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Clark et al., 2004; Sues et al., 2003; Clark et al., 2004).
The facial portion of the maxilla anterior to the anterior edge of the antorbital fossa is much shorter than the length of the posterior portion of the maxilla posterior to the anterior edge of the antorbital fenestra in non-archosaurian archosauriforms, basal avian-line archosaurs, and most non-crocodylomorph crocodylian-line archosaurs. An exception includes Qianosuchus (IVPP V 13899) and phytosaurs. Crocodylomorphs (e.g., Hesperosuchus “agilis,” CM 29894; Protosuchus richardsoni, MCZ 6727) have 50% or more of the length of the maxilla anterior to the anterior edge of the antorbital fenestra (Clark, 1986). A similar condition in theropods is also present (Rauhut, 2003).
15. Maxillary teeth, posterior edge of posterior maxillary teeth: (0) concave or straight; (1) convex (fig. 15) (modified from Sues et al., 2003; Clark et al., 2004).
The posterior edge of archosauriform taxa with carnivorous-like (mediolaterally compressed, recurved, serrated) teeth is either concave or straight posterior margins in the posterior portion of the maxilla. In Postosuchus kirkpatricki (TTU-P 9000), Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Litargosuchus (BP/1/5237), Kayentasuchus (UCMP 131830), Protosuchus richardsoni (AMNH FR 3024) and Alligator, the are either bulbous or have a posterior convex margin on posterior maxillary teeth. The posterior maxillary teeth of phytosaurs (e.g., Parasuchus hislopi, ISI R 42) also have convex posterior margins. Ornithischian-like teeth in Revueltosaurus (PEFO 34561), aetosaurs (e.g., Aetosaurus, SMNS 5770 S-4), Silesaurus (ZPAL Ab III/361/26), and sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810) are scored as (1).
16. Maxilla, posterior process: (0) articulates ventral to the jugal; (1) articulates into a slot on the lateral side of the jugal (fig. 17) (new).
In most archosauriforms, the maxilla simply articulates ventral to the jugal. Revueltosaurus (PEFO 34561) and aetosaurs (e.g., Desmatosuchus and Aetosaurus) share a complex jugal and maxilla articulation. In these taxa, a small, tapering posterior process of the maxilla fits into a groove in the lateral side of the jugal. Conversely, the jugal has two small, posteriorly tapering processes, one dorsal and one ventral, that project anteriorly into slots in the maxilla.
17. Maxilla, dentition in posterior portion: (0) present; (1) absent (new).
Dentition in the posterior portion of the maxilla is present in most archosaurs. Dentition in the posterior portion of the maxilla of Orthosuchus (SAM-PK-409) and Erpetosuchus (BMNH R3139) is absent. Edentulous taxa are scored as (?) here.
18. Maxilla, dentition: (0) present; (1) absent (fig. 17) (Nesbitt and Norell, 2006).
In nearly all archosauriforms studied here, the maxillae bear a dentition. Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9280), and Lotosaurus (IVPP V 48013) have an edentulous maxilla.
19. Maxilla, anterior extent: (0) posterior to the anterior extent of the nasals; (1) anterior to the nasals (fig. 16) (Sereno, 1991a).
In nearly all basal archosauriforms, the anterior end of the nasals lies anterior to the maxilla. However, in phytosaurs (e.g., Smilosuchus gregorii USNM 18313), the maxilla stretches well anterior to the nasals. In these forms the nasal and external nares lie completely dorsal to both the antorbital fenestra and the main body of the maxilla.
20. Maxilla, anterolateral surface: (0) smooth; (1) slot for the premaxillary process (fig. 15) (new).
In most archosauriforms, the anterolateral surface of the maxilla is smooth. In contrast, the anterolateral surface of the maxilla bears a distinct slot for the posterodorsal process of the premaxilla in Arizonasaurus (UCMP 36232; MSM P4590), Qianosuchus (IVPP V 14300), and Xilousuchus (IVPP V 6026).
21. Maxilla, ventral portion (ventral to the lacrimal): (0) dorsoventral height greater than mediolateral length; (1) mediolateral length greater than dorsoventral height (new).
In nearly all archosauriforms, the width of the ventral surface of the posterior portion of the maxilla is much less than the height of the posterior process. In contrast, the width of the ventral surface of the posterior process is greater than the height in Effigia (30587), Lotosaurus (IVPP V 48013), Erpetosuchus (BMNH R3139), and Parringtonia (BMNH R8646). A wide posterior portion of the maxilla is also present in Stagonolepis (Walker, 1961).
22. Maxilla, interdental plates: (0) separate; (1) fused (fig. 15) (new).
The interdental plates are separated in nearly every archosauriform with dentition in the maxilla examined here. In non-archosauriform archosauromorphs (e.g., Prolacerta, BP/1/2675; Mesosuchus, SAM 6536) and the basalmost archosauriform (Proterosuchus, BSP 514), interdental plates are absent. In these taxa, the teeth are ankylosed to the bone of attachment (see character 174). Therefore, the presence of teeth that are free from the bone of attachment (thecodont dentition) seems to coincide with the origin of interdental plates.
Among crocodylian-line archosaurs, only Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), Teratosaurus suevicus (BMNH 38646) and Fasolasuchus (PVL 3851) have fused interdental plates. All the interdental plates are fused in P. kirkpatricki, T. suevicus and Fasolasuchus whereas the interdentral plates of T. silesiacus are fused in the posterior half of the maxilla. The interdental plates fuse into a continuous sheet of bone that covers the medial side of each alveolus. Taxa without interdental plates are scored as inapplicable.
23. Maxilla, buccal emargination: (0) absent; (1) present (fig. 20) (Butler, 2005; Irmis et al., 2007a; Irmis et al., 2007a).
In most archosauriforms, a buccal margin on the lateral side of the maxilla is absent. Other than ornithischians, Revueltosaurus (PEFO 34561) is the only other taxon to have a buccal emargination. According to Butler et al. (2008b), a buccal emargination is present in nearly all basal ornithischians.
24. Maxilla, anterodorsal margin: (0) separated from the external naris by the premaxilla; (1) borders the external naris (figs. 15, 17, 19–20) (modified from Gauthier, 1986; Langer and Benton, 2006).
The anterodorsal margin of the maxilla of most archosauriforms is separated from the external naris by the posterodorsal premaxillary process. However, in the suchians Arizonasaurus (UCMP 36232; Nesbitt, 2005a) and Effigia (AMNH FR 30587; Nesbitt, 2007) and the aetosaurs Aetosaurus (Schoch, 2007), Desmatosuchus (TTU-P 9024; Small, 2002), Stagonolepis (Walker, 1961), and Neoaetosauroides (PVL 4363; Desojo and Baez, 2007) the maxilla creates part of the posterior border of the external naris.
25. Maxilla, anterodorsal margin at the base of the dorsal process: (0) convex or straight; (1) concave (figs. 15, 17, 19) (modified from Langer and Benton, 2006).
Non-archosaurian archosauriforms, Revueltosaurus (PEFO 34561), Gracilisuchus (MCZ 4117), Turfanosuchus (IVPP V 3237), Riojasuchus (PVL 3827), “rauisuchians” (Postosuchus kirkpatricki, TTU-P 9000; Saurosuchus, PVSJ 32), basal crocodylomorphs (Hesperosuchus “agilis,” CM 29894; Protosuchus richardsoni, AMNH FR 3024), basal ornithischians (Lesothosaurus, BMNH R8501), and basal theropods (Coelophysis bauri, CM 31374) have either straight or convex anterodorsal margins of the maxilla. As scored by Langer and Benton (2006) basal sauropodomorphs have a distinctly concave anterodorsal margin of the maxilla at the base of the dorsal process. In these taxa (e.g., Plateosaurus, AMNH FR 6810), the posterodorsal process of the premaxilla separates the maxilla from the external naris. In Arizonasaurus (MSM P4590), Qianosuchus (IVPP V 13899), Xilousuchus (IVPP V 6026), Lotosaurus (IVPP V 48013), Effigia (AMNH FR 30587), Batrachotomus (SMNS 52970), basal pterosaurs (Eudimorphodon, MCSNB 2888), and aetosaurs (e.g., Aetosaurus, SMNS 5770 S-7), the anterodorsal margin of the dorsal process is concave.
26. Maxilla, lateral surface: (0) smooth; (1) sharp longitudinal ridge present; (2) bulbous longitudinal ridge present (fig. 15, 19) (Gower, 1999; Weinbaum and Hungerbühler, 2007).
Character state (0) applies to a smooth lateral surface of the maxilla in taxa with or without an antorbital fossa. Some taxa with an antorbital fossa may have a slight embankment marking the transition from the antorbital fossa to the lateral surface of the maxilla (e.g., Saurosuchus, PVSJ 32). However, this embankment is never raised above the lateral side of the maxilla in taxa scored as (0). Character state (1) and (2) describe ridges that occur at the same location, the transition from the antorbital fossa to the lateral side of the maxilla, but are well raised above/lateral to the lateral surface of the maxilla.
The maxillae of Postosuchus kirkpatricki (TTU-P 9000) and Polonosuchus silesiacus (ZPAL Ab III/563) share a bulbous ridge (state 2) on the lateral side of the maxilla that extends onto the jugal. The ridge separates the antorbital fossa from the rest of the maxilla. The ridge in Postosuchus kirkpatricki (TTU-P 9000) is more bulbous and extends further anteriorly than that of Polonosuchus silesiacus (ZPAL Ab III/563).
As discussed by Smith et al., (2007), in Eoraptor (PVSJ 512), Coelophysis bauri (CM 31374), Coelophysis rhodesiensis, Syntarsus kayentakatae, Zupaysaurus, Liliensternus liliensterni, and a number of other theropods, a distinct raised and sharp ridge on the alveolar border of the maxilla paralleling the tooth row is present (state 1). This horizontal ridge marks the ventral extent of the maxillary antorbital fossa, but also is raised lateral to the alveolar margin.
27. Maxilla, posterior portion ventral to the antorbital fenestra: (0) tapers posteriorly; (1) has a similar dorsoventral depth as the anterior portion ventral to the antorbital fenestra; (2) expands dorsoventrally at the posterior margin of the maxilla (figs. 15, 17, 19) (new).
The posterior portion of the maxilla tapers posteriorly in most saurischians, ornithosuchids (e.g., Riojasuchus, PVL 3827), Xilousuchus (IVPP V 6026), Arizonasaurus (MSM P4590), Effigia (AMNH FR 30587), Prolacerta (UCMP 37151; Modesto and Sues, 2004), Gracilisuchus (MCZ 4117), and Chanaresuchus (PVL 4575). In basal ornithischians, Postosuchus kirkpatricki (TTU-P 9000), Batrachotomus (SMNS 52970; Gower, 1999), Saurosuchus (PVSJ 32; Alcober, 2000), Hesperosuchus (CM 29894), Dromicosuchus (UNC 15574), Protosuchus (MCZ 6727), and Sphenosuchus (Walker, 1990), the posterior process of the maxilla is rectangular and nearly the same dorsoventral height as the anterior portion of the posterior process. The posteriormost portion of the maxilla expands dorsoventrally relative to the anterior portion ventral to the antorbital fenestra in aetosaurs, Revueltosaurus (PEFO 34561), Proterosuchus (BPS 514), Erythrosuchus (BPI 5207), Euparkeria (SAM 5867), Turfanosuchus (IVPP V 3237; Wu and Russell, 2001), and phytosaurs (Ballew, 1989; Hungerbühler, 2002).
28. Maxilla, promaxillary foramen: (0) absent; (1) present (Carpenter, 1992; Rauhut, 2003; Tykoski, 2005b; Smith et al., 2007).
The promaxillary foramen is a small opening located at the base of the dorsal process of the maxilla within the antorbital fossa (Rauhut, 2003). Typically, the promaxillary foramen is hidden in lateral view (Tykoski, 2005b). Rauhut (2003) scored non-archosaurian archosauriforms, crocodylian-line archosaurs, basal avian-line archosaurs, Herrerasaurus, Eoraptor, Coelophysis bauri, C. rhodesiensis, and Torvosaurus as (0) and nearly all theropods (except the taxa listed above) as (1). Sereno (2007) stated that Herrerasaurus also possess a promaxillary foramen with little discussion or justification. Sereno (2007) stated that the large promaxillary foramen in Herrerasaurus is clearly visible in lateral view. The basal theropod Coelophysis bauri (CM 31374) and closely related taxa (Liliensternus lilisterni; Tykoski, 2005b) lack a promaxillary foramen, whereas the feature is present and visible in lateral view in Dilophosaurus (Welles, 1984), Zupaysaurus (Arcucci and Coria, 2003; Ezcurra, 2006), and “Syntarsus” kayentakatae (Tykoski, 1998).
29. Maxilla, dorsal ( = ascending) process: (0) tapers posterodorsally; (1) remains the same width for its length (fig. 15, 17, 19) (new).
The dorsal process of the maxilla gradually tapers posterodorsally in most archosauriforms. In Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), Teratosaurus suevicus (BMNH R 38646), Revueltosaurus (PEFO 34774), aetosaurs (Aetosaurus, SMNS 5770), Riojasuchus (PVL 3827), and Prestosuchus (UFRGS 0152-T; UFRGS 0156-T), the dorsal process of the maxilla maintains the same dorsoventral height posteriorly. In taxa scored as (1), the suture between the lacrimal and the dorsal process of the maxilla is nearly oriented dorsoventrally.
30. Antorbital fenestra, anterior margin: (0) gently rounded; (1) nearly pointed (figs. 15, 19) (modified from Benton and Clark, 1988; Alcober, 2000; Benton and Walker, 2002; Weinbaum and Hungerbühler, 2007).
Many basal archosaur workers previously used this character without discussion. The original wording of the character “antorbital fenestra shape: elliptical or circular (0), triangular, and with elongate narrow anterior point” is open to interpretation and difficult to score. This reformulation focuses on the anterior margin of the antorbital fenestra rather than the opening as a whole. Taxa scored as (0) (e.g., non-archosaurian archosauriforms, aetosaurs, Herrerasaurus, Gracilisuchus, and Effigia) have gently rounded anterior ends of the antorbital fenestra with a radius of curvature similar to the posterior end of the antorbital fenestra. In contrast, the anterior end of the antorbital fenestra is pointed and has a smaller radius relative to the posterior end of the antorbital fenestra in the taxa scored as (1) (e.g., Postosuchus kirkpatricki, TTU-P 9000; Saurosuchus, PVSJ 32; Riojasuchus, PVL 3827). In these taxa, the antorbital fenestra is triangular or wedge shaped. Furthermore, the dorsal process of the maxilla projects posterodorsally 45° or less. Although the antorbital fenestra of Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), and Protosuchus (MCZ 6727) are anteroposteriorly elongated and not triangular as those of Postosuchus and Saurosuchus, they are scored as (1) because the anterior end of the margin of the antorbital fenestra terminates in a small radius of curvature like those of the other taxa scored as (1).
31. Maxilla, anterolateral surface, large anteriorly opening foramen: (0) present; (1) absent (fig. 15) (Modesto and Sues, 2004; Nesbitt et al., 2009a).
A large anteriorly opening foramen is present on the anterolateral surface of the maxilla, just ventral to the base of the dorsal process, in Prolacerta (BP/1/471; Modesto and Sues, 2004), Proterosuchus (RC96; Welman, 1998), Lotosaurus (131827), and Euparkeria (SAM 6049), as well as in non-archosauriform archosauromorphs such as Protorosaurus (Modesto and Sues, 2004) and Mesosuchus (Dilkes, 1998). A similar opening is not present in the same position in Erythrosuchus, Vancleavea, proterochampsians, and nearly all Archosauria. A similar opening between the premaxilla and the maxilla (in suchian taxa such as Revueltosaurus, S.J.N., personal obs., and Batrachotomus, Gower, 1999, and saurischian dinosaurs, Sereno and Novas, 1994, termed the subnarial foramen) may transmit the same vessels as the feature described above but does not seem to be homologous (see character 12).
32. Maxilla, palatal processes: (0) do not meet at the midline; (1) meet at the midline; (2) meet at the midline and expand anteriorly and posteriorly. ORDERED (fig. 15) (modified from Parrish, 1993; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004; Nesbitt et al., 2009a).
The palatal process of the maxilla is present in all archosauriforms in this study except for Proterosuchus. The structure expands anteromedially from the anteromedial edge of the maxilla on the medial side. The palatal processes of the maxillae do not meet in Erythrosuchus (Gower, 2003), Vancleavea (Nesbitt et al., 2009a), proterochampsians, and phytosaurs. Gow (1970) reported that there is medial contact of the palatal processes in Euparkeria; however, the medial edge of the well-prepared palatal process of SAM 6050 does not have an articular surface, as do other taxa with medial contact of the palatal processes. Gow (1970) did not provide a specimen number, so it is not clear to which specimen he referred; it is likely that he obtained his observations from SAM 6050 because it is the only example of a prepared palatal process. The palatal processes of the maxillae of Revueltosaurus (PEFO 34561), Arizonasaurus (MSM P4590), Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), Fasolasuchus (PVL 3850), Batrachotomus (SMNS 52970), and the crocodylomorphs Hesperosuchus “agilis” (YPM 41198), Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V7907), Protosuchus richardsoni (AMNH FR 3024) and Alligator meet at the midline. This is also true of the avian-line archosaurs (e.g., Allosaurus, Madsen, 1976; Silesaurus, ZPAL Ab III/361/26).
The palatal processes of the maxillae of crocodylians meet at the midline and expand posteriorly to form an extensive secondary palate (Brochu, 2003). Parrish (1993) scored basal crocodylomorphs as having a “secondary palate.” Scoring a “secondary palate” in basal archosaurs is difficult given the range of morphologies between that of basal crocodylomorphs and those of crocodylians. Therefore, the ambiguous term “secondary palate” is abandoned for this character state and the actual morphology is described. The palatal processes of the maxillae of crocodylomorphs (Hesperosuchus “agilis,” YPM 41198; Sphenosuchus, SAM 3014; Dibothrosuchus, IVPP V 7907; Protosuchus richardsoni, AMNH FR 3024; Alligator) expand anteriorly and posteriorly.
33. Nasal-prefrontal, contact: (0) present; (1) absent (fig. 17) (modified from Sereno, 1991a).
Sereno (1991a) cited the absence of nasal and prefrontal contact as a synapomorphy of Riojasuchus (PVL 3827) and Ornithosuchus (BMNH R 2409, 3562, 3142; Walker, 1964). Here, Sereno's (1991a) observations are confirmed.
34. Nasals, posterior portion at the midline: (0) convex or flat; (1) concave (fig. 21) (new).
The posterodorsal surface of the nasal of most archosauriforms is either flat or slightly convex. Among crocodylian-line archosaurs, this describes Prestosuchus (UFRGS T-156), Saurosuchus (PVSJ 32), Aetosaurus (SMNS 5770), Revueltosaurus (PEFO 34561), Riojasuchus (PVL 3827), Effigia (AMNH FR 30587), Gracilisuchus (MCZ 4117), and Protosuchus richardsoni (AMNH FR 3024). In contrast, the anterodorsal surface of the nasals is depressed in Rauisuchus (BSP AS XXV-60-121), Postosuchus kirkpatricki (TTU-P 9000), Batrachotomus (SMNS 80260; Gower, 1999), Dromicosuchus (UNC 15574), Hesperosuchus “agilis” (CM 29894), Dibothrosuchus (IVPP V 7907), and Sphenosuchus (SAM 3014). In these taxa, an ellipsoid depression is formed at the midline at the posterior portion of the nasals. The depression may be a consequence of a lowered portion of the nasal relative to the raised lateral nasal ridge in Rauisuchus (BSP AS XXV-60-121), Postosuchus kirkpatricki (TTU-P 9000), and Batrachotomus (SMNS 80260; Gower, 1999). However, small lateral ridges present on the posterolateral portion of the nasals in Sphenosuchus (SAM 3014) suggest the features are homologous in a subset of “rauisuchians” and crocodylomorphs. The nasal of Fasolasuchus (PVL 3580) could not be located (as of 2005). Nevertheless, Bonaparte (1981: fig. 1) showed a depressed region on the posteromedial portion. Therefore, Fasolasuchus is scored as (1).
35. Nasal, dorsolateral margin of the anterior portion: (0) smoothly rounded; (1) distinct anteroposteriorly ridge on the lateral edge (figs. 19, 21) (new).
In nearly all archosauriforms (e.g., Euparkeria, Dromicosuchus), the dorsolateral edge of the nasals is smoothly convex. In contrast, a distinct lateral and rugose ridge is located on the dorsolateral edge of the nasals of Postosuchus kirkpatricki (TTU-P 9000), Batrachotomus (SMNS 52970), Polonosuchus silesiacus (ZPAL Ab III/563), and Rauisuchus (BSP AS XXV-60-121). In Postosuchus kirkpatricki, Batrachotomus, and Polonosuchus silesiacus, the rugose ridge continues posteriorly on the lateral margin of the lacrimal, palpebral (Postosuchus and Polonosuchus), postorbital, and the squamosal.
36. Nasal: (0) does not possess a posterolateral process that envelops part of the anterior ramus of the lacrimal; (1) possesses a posterolateral process that envelops part of the anterior ramus of the lacrimal (fig. 20) (Yates, 2003; Langer and Benton, 2006).
Eusaurischians, and Eoraptor (PVSJ 512) have a small posteriorly directed process that invades the lacrimal (Langer and Benton, 2006: fig. 5). This process is located at the dorsal margin of the antorbital fossa. Langer and Benton (2006) scored Herrerasaurus (based on PVSJ 407) as (0), and this is followed here.
37. Nasal: (0) does not form part of the dorsal border of the antorbital fossa; (1) forms part of the dorsal border of the antorbital fossa (modified from Sereno et al., 1994; Langer and Benton, 2006; Irmis et al., 2007a).
Langer and Benton (2006) showed that the nasal forms the dorsal border of the antorbital fossa in a variety of basal archosaurs instead of only allosaurids (Sereno et al., 1994). The portions of the maxilla and the lacrimal without an antorbital fossa separate the antorbital cavity from the nasal in taxa scored as (0). This condition is present in crocodylomorphs, Effigia (AMNH FR 30587), Revueltosaurus (PEFO 34272), phytosaurs (e.g., Smilosuchus, USMN 18313), Riojasuchus (PVL 3827), basal ornithischians (e.g., Heterodontosaurus, SAM-PK-1332), and non-archosaurian archosauriforms with antorbital fenestrae. Although Langer and Benton (2006) stated Euparkeria should be scored as (1), a thin portion of the lacrimal in SAM 5207 separates the nasal from the antorbital fossa. The antorbital fossa reaches the nasal in Aetosaurus (SMNS 5770 S-7), Gracilisuchus (MCZ 4117), Turfanosuchus (IVPP V 3237), Polonosuchus silesiacus (ZPAL Ab III/563), Postosuchus kirkpatricki (TTU-P 9000), Fasolasuchus (PVL 3851), Batrachotomus (SMNS 52970), and as listed by Langer and Benton (2006), Eoraptor, and saurischians.
38. Lacrimal: (0) does not fold over ( = overhang) the posterior/posterodorsal part of the antorbital fenestra; (1) folds over ( = overhangs) the posterior/posterodorsal part of the antorbital fenestra (fig. 20) (modified from Sereno, 1999; Langer and Benton, 2006).
A “folded over” lacrimal possesses a deep pocket or fossa unobservable in lateral view in the posterodorsal portion of the antorbital fenestra. Langer and Benton (2006) found that state (1), which was previously thought to be only in theropods (Sereno, 1999), is present in all basal saurischians including basal members of Theropoda and Sauropodomorpha. State (0) is present in all other archosauriforms in this study.
39. Lacrimal, height: (0) significantly less than the height of the orbit, and usually fails to reach the ventral margin of the orbit; (1) as high as the orbit, and contacts the jugal at the level of the ventral margin of the orbit (fig. 20) (Rauhut, 2003).
Rauhut (2003) thoroughly discussed this character. The plesiomorphic condition in archosauriforms is to have a lacrimal that is dorsoventrally elongated at the anterior portion and meets the jugal well above the ventral portion of the orbit. In contrast, the lacrimal of Eoraptor (PVSJ 512), sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810), and theropods (e.g., Coelophysis bauri, CM 31374) has an inverted L shape with a dorsoventrally shallow anterior process, and it meets the jugal at the ventral portion of the orbit (Rauhut, 2003). Among crocodylian-line archosaurs, crocodylomorphs also have lacrimals that meet the jugal at the base of the orbit.
The character is essentially equivalent to one used by Gower and Sennikov (1997) (jugal, anterior process: [0] slender and tapering [1] broad and dorsally expanded anteriorly) for basal archosauriform relationships.
40. Prefrontal, ventromedial process: (0) absent; (1) present (fig. 18) (Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Gower and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
Fig. 18
Cranial character states in archosaurs: A, right squamosal and the dorsal portion of the quadrate of Saurosuchus galilei (PVSJ 32) in lateral view; B, right squamosal of Postosuchus (UCMP 27441) in lateral view; C, skull of Litargosuchus leptorhynchus (BP/1/5237) in dorsal view; D, frontals of Hesperosuchs agilis (AMNH FR 5867) in dorsal view; E, partial right frontal and postfrontal of Postosuchus (UCMP 27479) in dorsal view. The suture between the frontal and prefrontal is highlighted; F, the prefrontal of Longosuchus meadei (TMM 31185-98) posterior view from within the orbit. Arrow indicates anterior direction. Shaded areas indicate incomplete preservation. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
Gower and Walker (2002) used the presence of a ventromedial process of the prefrontal to support a close relationship between aetosaurs and crocodylomorphs. A medial, expanded flange of the prefrontal is clearly present in the crocodylomorphs Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V 7907), Protosuchus richardsoni (UCMP 131827), and Alligator, and in the aetosaurs Longosuchus (TMM 31185-98) and Stagonolepis (Witmer, 1997). Clark et al. (2000) used a similar character in their analysis of crocodylomorphs (prefrontal not underlying anterolateral edge of frontal to a significant degree [0] or with distinct posterior process underlying frontal dorsal to orbit [1]). A ventromedial process is not present in any other archosauriform studied here.
41. Prefrontal: (0) does not contact the palate; (1) contacts the palate (Wu and Chatterjee, 1993).
In the basal archosauriforms studied here, the prefrontal touches the palate only within crocodylomorphs. Even though examination of this character requires exquisite preservation and meticulous preparation, the prefrontal does not touch the palate in Sphenosuchus (SAM 3014; Walker, 1990), whereas the prefrontal touches the palate in Dibothrosuchus (IVPP V 7907; Wu and Chatterjee, 1993), Protosuchus richardsoni (UCMP 131827), and Alligator.
42. Frontal, dorsal surface: (0) flat; (1) with longitudinal ridge along midline (fig. 17) (Wu and Chatterjee, 1993; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
In the crocodylomorphs Hesperosuchus (AMNH FR 6758, CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), and Dibothrosuchus (IVPP V 7907), as cited by Clark et al. (2000), a ridge is formed by both frontals. The frontals of Batrachotomus (SMNS 52970) and Postosuchus kirkpatricki (TTU-P 9000) also have similar ridges. In both Batrachotomus and Postosuchus there is a ridge on the medial edge of each frontal. When in articulation, the frontals have two parallel ridges separated by a small gap at the midline. The ridge in Batrachotomus (SMNS 52970) is much more robust and developed in comparison with Postosuchus (UCMP 27479).
43. Frontal, anterior portion: (0) about as wide as the orbital margin or has a transversely aligned suture with the nasal; (1) tapers anteriorly along the midline (fig. 18) (new).
In most archosauriforms, the frontal meets the nasal in a transverse suture and/or the frontal is about the same mediolateral width as that of the orbital margin. This is apparent in Euparkeria (SAM 5867), phytosaurs (e.g., Smilosuchus, UCMP 27200), ornithosuchids (e.g., Riojasuchus, PVSJ 3827), aetosaurs (e.g., Aetosaurus SMNS S-16), Gracilisuchus (MCZ 4117), Qianosuchus (IVPP V 13899), Effigia (AMNH FR 30587), and in basal avian-line archosaurs (e.g., Plateosaurus, AMNH FR 6810; Allosaurus, Madsen, 1976). The frontals taper anteriorly along the midline in Postosuchus kirkpatricki (TTU-P 9000), Revueltosaurus (PEFO 34561), Batrachotomus (SMNS 52970), Saurosuchus (PVSJ 32), Ticinosuchus (PIZ T2817), and in crocodylomorphs (e.g., Hesperosuchus “agilis,” CM 29894; Sphenosuchus, SAM 3014; Litargosuchus, BP/1/5237).
44. Postfrontal: (0) present; (1) absent (Gauthier, 1986; Benton and Clark, 1988; Juul, 1994; Bennett, 1996; Novas, 1996; Benton, 1999; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004; Langer and Benton, 2006; Nesbitt, 2007; Irmis et al., 2007a).
Within Archosauromorpha, the postfrontal is present plesiomorphically at the posterodorsal margin of the orbit. This is the case in all archosaur groups except for Crocodylomorpha (Benton and Clark, 1988), Dinosauria (Langer and Benton, 2006), Effigia (AMNH FR 30587), and Shuvosaurus (TTU-P 9280) and the non-archosaurian archosauriform Proterochampsia. The postfrontal has been scored as absent in Erpetosuchus by both Benton and Walker (2002) and Olsen et al. (2000). However, the specimen scored (BMNH R3139) by Benton and Walker (2002) is preserved as a mold; thus, sutures are extraordinarily difficult to discern. Furthermore, a suture between the postfrontal and surrounding bones may not be necessarily expressed as a distinct surface feature. Therefore, it is not clear whether a postfrontal was present or absent in Erpetosuchus.
45. Quadratojugal: (0) forms less than 80% of the posterior border of the lower temporal fenestra; (1) more than 80% of the posterior border of the lower temporal fenestra (figs. 17, 19) (modified from Benton and Clark, 1988; Parrish, 1993).
Fig. 19
Skulls of crocodylian-line archosaurs and crocodylomorphs in lateral view: A, Saurosuchus galilei in lateral in lateral views; B, Batrachotomus kuperferzellensis in lateral view; C, Postosuchus kirkpatricki in lateral view; D, Dromicosuchus grallator in lateral view; E, Sphenosuchus acutus in lateral view; F, Protosuchus richardsoni in lateral view. Shaded areas indicate incomplete preservation. Numbers refer to character states. Scale bars = 5 cm in A–C and 1 cm in D–F.
The quadratojugal forms 80% or less of the posterior border of the lower temporal fenestra in most archosaurs. Consequently, taxa scored as (0) have elongated ventral processes of the squamosal, thus making a character measuring the length of a ventral process of the squamosal redundant (Parrish, 1993: char. 39; Clark et al., 2000: 11; Olsen et al., 2000: 11; Benton and Walker, 2002: 11; Sues et al., 2003: 11; Clark et al., 2004: 11). In non-archosaurian archosauriforms, Saurosuchus (PVSJ 32), Prestosuchus (UFRGS 0156-T), Revueltosaurus (PEFO 34561), Turfanosuchus (IVPP V 3237), Gracilisuchus (MCZ 4117), Qianosuchus (IVPP V 13899), Lotosaurus (IVPP 131827), and avian-line archosaurs (e.g., Plateosaurus, AMNH FR 6810; Herrerasaurus, PVSJ 407), the quadratojugal forms less than 80% of the posterior border of the lower temporal fenestra. Taxa scored as (1) include the aetosaurs Aetosaurus (SMNS 5770 S-8) and Desmatosuchus (Small, 2002), Postosuchus kirkpatricki (TTU-P 9002), Polonosuchus silesiacus (ZPAL Ab III/563), and Rauisuchus (BSP AS XXV-60-121; inferred from the length of the squamosal), and crocodylomorphs (e.g., Hesperosuchus “agilis,” CM 29894; Protosuchus richardsoni, UCMP 131827). Even though the ventral process of the squamosal of Batrachotomus (SMNS 80260) is broken, the preserved length indicates that the quadratojugal would not make up 80% of the posterior border of the lower temporal fenestra. As described above, the taxa scored as (1) have either short processes or do not have ventral processes of the squamosal.
46. Quadratojugal, shape: (0) L-shaped; (1) subtriangular (fig. 16) (Sereno, 1991a).
The quadratojugal of basal archosaurs is L-shaped. Alternatively, the quadratojugal of phytosaurs forms a triangular plate among the squamosal, quadrate, and jugal (Case, 1929; Colbert, 1947; Gregory, 1962; Westphal, 1976; Ballew, 1989; Hungerbühler, 2002). A subtriangular quadratojugal is present in all phytosaur taxa included here.
47. Quadratojugal, lateral surface: (0) without a ridge marking the posteroventral corner of the lower temporal fossa; (1) with a ridge marking the posteroventral corner of the lower temporal fossa (fig. 16) (Nesbitt et al., 2009a).
The main body of the quadratojugals of Tropidosuchus (PLV 4604) and Chanaresuchus (PVL 4575) has a distinct lower temporal fossa marked by a sharp ridge. All other taxa in this analysis have a nearly smooth quadratojugal without a distinct ridge.
48. Squamosal, posterior end: (0) does not extend posterior to the head of the quadrate; (1) extends posterior to the head of the quadrate (figs. 16, 18) (Nesbitt et al., 2009a).
The squamosals of Mesosuchus (SAM 6536), Prolacerta (BP/1/471), Proterosuchus (BSP 514), and Erythrosuchus (BP/1/ 5207) terminate posteriorly just dorsal to the posterior edge of the head of the quadrate. In contrast, the squamosals of Euparkeria (SAM 5867), the proterochampsians Chanaresuchus (PVL 4586) and Tropidosuchus (PVL 4606), and members of crown-group Archosauria (e.g., Arizonasaurus, Herrerasaurus) have a posteriorly expanded squamosal process that terminates well posterior of the quadrate. This character may be a subdivision the character referred to as the development of the “archosaur otic notch” of Romer (1956) and Gauthier (1984).
49. Squamosal: (0) without distinct ridge on dorsal surface along edge of supratemporal fossa; (1) with distinct ridge on dorsal surface along edge of supratemporal fossa (figs. 18, 19);
= Lateral projecting flange on the squamosal absent (0) or present (1) (Bonaparte, 1982; Parrish, 1993);
= Squamosal not significantly overhanging lateral temporal region (0) or with broad lateral expansion overhanging lateral temporal region (1) (Clark et al., 2000: char. 10; Olsen et al., 2000: 10; Benton and Walker, 2002: 10; Sues et al., 2003: 10; Clark et al., 2004: 10);
= Squamosal overhanging quadrate and quadratojugal laterally: absent (0), present, and contacting the lower temporal fenestra dorsally (1), present, but excluded from the rim of the lower temporal fenestra by postorbital and quadratojugal (2) (Benton and Clark, 1988; Juul, 1994: 74; Benton, 1999: 8).
This character was used in numerous analyses (see above) and in several forms that seem to describe the same morphology. The squamosals of archosauriforms typically do not have a distinct ridge on the dorsolateral margin of the squamosal. This includes Euparkeria (SAM 5867), phytosaurs, Revueltosaurus (PEFO 34561), aetosaurs, Turfanosuchus (IVPP V 3237), Prestosuchus (UFRGS 0156-T), Effigia (AMNH FR 30587), Arizonasaurus (MSM P4590), Saurosuchus (PVSJ 32), and avian-line archosaurs. A distinct ridge on the dorsolateral margin of the squamosal is clearly present in Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), Rauisuchus (BSP AS XXV-60-121), and Batrachotomus (SMNS 80260). Here, I agree with Parrish (1993) and homologize the dorsally expanded ridge in the taxa listed above with that of the crocodylomorphs Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), Terrestrisuchus (BMNH R7591), Dibothrosuchus (IVPP V 7907), Litargosuchus (BP/1/5237), and crocodyliforms included here. In these taxa, the lateral ridge originates from the same portion of the squamosal, and the lateral edge of the extended portion is rugose. The dorsal margin of the squamosal of Gracilisuchus (MCZ 4117) does not have a laterally expanded ridge.
50. Squamosal, transverse length of dorsal exposure: (0) less than the mediolateral width of the upper temporal fenestra; (1) equal to or greater than the mediolateral width of the upper temporal fenestra (fig. 18) (new).
In most basal archosauriforms, the transverse length of the squamosal lateral to the upper temporal fenestra is far less than the maximum transverse width of the upper temporal fenestra. In contrast, the transverse length of the squamosal lateral to the upper temporal fenestra in Litagrosuchus (BP/1/5237), Kayentasuchus (UCMP 131830), Protosuchus richardsoni (UCMP 131827; BP/1/4746), and Orthosuchus (SAM-PK-409) is equal to or greater than the mediolateral width of the upper temporal fenestra.
51. Squamosal: (0) without ridge on lateral side of the ventral process; (1) with ridge on lateral side of the ventral process (fig. 18) (new).
The ventral process of squamosal (if present) is nearly flat and smooth in nearly all archosauriforms except Batrachotomus (SMNS 80260), Saurosuchus (PVSJ 32), and Prestosuchus (UFRGS 0156-T). In these taxa, a ridge on the ventral process originates ventral to the articulation of the parietal and arcs anteroventrally (Gower, 1999).
52. Squamosal, anteroventral process: (0) absent; (1) present and perforates the lower temporal fenestra; (2) present and contacts the postorbital bisecting the lower temporal fenestra. ORDERED (figs. 18, 19) (new).
The ventral process of the squamosal extends ventrally or anteroventrally in basal archosauriforms. In Prestosuchus (UFRGS 0152-T) and Saurosuchus (PVSJ 32), a small anterior process on the ventral process penetrates the lower temporal fenestra. In Postosuchus kirkpatricki (TTU-P 9000; TTU-P 9002), Polonosuchus silesiacus (ZPAL Ab III/563), and Rauisuchus (BSP AS XXV-60-121), an anterior process on the ventral process contacts the postorbital, and as a result, the process bisects the lower temporal fenestra. The resultant circular opening dorsal to the anteroventral process is formed completely by the squamosal and the postorbital. A sliver of the anteroventral process forms the dorsal border of the lower temporal fenestra in Postosuchus kirkpatricki, Polonosuchus, Silesiacus, and Rauisuchus.
53. Squamosal, dorsolateral edge: (0) without longitudinal groove; (1) with longitudinal groove (fig. 19) (Clark et al., 2000; Clark and Sues, 2002; Sues et al., 2003; Clark et al., 2004).
The squamosals of Kayentasuchus (UCMP 131830), Protosuchus richardsoni (UCMP 131827), Protosuchus haughtoni (BP/1/4242), Orthosuchus (SAM-PK-409), and Alligator bear a distinct longitudinal scar on the lateral edge of the squamosal (Clark and Sues, 2002).
54. Squamosal, facet for the paroccipital process on the medial side of the posterior process: (0) mediolaterally thin; (1) rounded and thick (fig. 17) (new).
The facet for the paroccipital process on the medial side of the posterior process of the squamosal is mediolaterally thin in most archosauriforms. In comparison, the same facet is mediolaterally thickened and rounded into a knob in Revueltosaurus (PEFO 34561), Turfanosuchus (IVPP V 3237), and all aetosaurs known from skull material. Some, non-archosaurian archosauriforms cannot be scored for this character because they do not have separate posterior processes of the squamosal (see character 48); the paroccipital fits directly on the medial side of the body of the squamosal.
55. Squamosal, posterodorsal portion: (0) without upper temporal fossa; (1) with upper temporal fossa (fig. 16) (new).
A distinct shelf or upper temporal fossa lies on the dorsal surface of the squamosal surrounding the upper temporal fenestra in the basal crocodylomorphs Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V7907), Litargosuchus (BP/1/5237), and Protosuchus haughtoni (BP/1/4242), and in proterochampsians (e.g., Chanaresuchus, PVL 4586; Tropidosuchus, PVL 4606) a rim surrounds the upper temporal fossa.
56. Squamosal, ventral process: (0) wider than one-quarter of its length; (1) narrower than one-quarter of its length (fig. 20) (Yates, 2003; Langer and Benton, 2006).
Fig. 20
Skulls of avian-line archosaurs in lateral view: A, Lesothosaurus dianosticus in lateral view; B, Plateosaurus engelhardti in lateral view; C, Herrerasaurus ischigualastensis in lateral view; D, Coelophysis bauri in lateral view. Numbers refer to character states. Scale bars = 1 cm.
Yates (2003), followed by Langer and Benton (2006), found that a narrow ventral process of the squamosal is present in sauropodomorphs. I confirm this in Saturnalia (MCP 3845-PV), Plateosaurus (AMNH 6810), and Efraasia (Yates, 2003). However, the distribution of the character outside Sauropodomorpha requires further explanation. Langer and Benton (2006) scored their suprageneric Theropoda as (0). However, the basal theropods Coelophysis bauri (CM 31374) and “Syntarsus” kayentakatae (MNA V2623) have thin ventral processes like that of basal sauropodomorphs, whereas the ventral processes of Dilophosaurus (Welles, 1984), Zupaysaurus (UNLR 076), and Allosaurus (Madsen, 1976) have wide processes. Furthermore, the condition immediately outside Dinosauria remains poorly understood. Silesaurus (Dzik and Sulej, 2007: fig. 18A) has a thin ventral process, whereas nearly all crocodylian-line archosaurs have wide ventral processes.
57. Squamosal, deep pit on the posterodorsal corner of the lateral surface: (0) absent; (1) present (fig. 18) (Brusatte et al., 2008).
As described by Brusatte et al. (2008), the squamosals of Postosuchus kirkpatricki (TTU-P 9000) and Polonosuchus silesiacus (ZPAL Ab III/563) have a deep fossa on the posterodorsal corner of the lateral surface of the squamosal.
58. Parietals, in presumed adults: (0) separate; (1) interparietal suture partially or completely absent (Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
A suture separates the parietals of most basal archosaurs including non-archosaurian archosauriforms, avian-line archosaurs, non-crocodylomorph crocodylian-line archosaurs, and the crocodylomorphs Dromicosuchus, Hesperosuchus, Terrestrisuchus, Pseudhesperosuchus, and Saltoposuchus (Clark et al., 2000, 2004). In Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V 7907), Junggarsuchus (IVPP V 14010), Litargosuchus (BP/1/5237), Protosuchus (MCZ 6727; BP/1/4242), Orthosuchus (SAM-PK-409), and Alligator, the parietals are completely fused.
In previous analyses, Clark et al. (2000, 2004) had a third character state, “interparietal suture partially obliterated (1)” and ordered the character. Additionally, Clark et al. (2000, 2004) scored Gracilisuchus as having an interparietal suture partially obliterated. However, Gracilisuchus has a clear interparietal suture (e.g., MCZ 4117) contrary to what was reported originally (Romer, 1972c); therefore it is scored as a (0) here. Other than Litargosuchus (BP/1/5237), there are no other examples of “interparietal suture partially obliterated.” Thus, the second character state of Clark et al.'s (2000) character 16 is combined with state (2).
59. Parietals, upper temporal fenestrae separated by: (0) broad, flat area; (1) supratemporal fossa separated by a mediolaterally thin strip of flat bone; (2) supratemporal fossa separated by a “sagittal crest” (which may be divided by the interparietal suture) (figs. 17–18) (modified from Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
In most basal archosauriforms, the parietals separate the upper temporal fenestrae by a flat gap. In phytosaurs (e.g., Smilosuchus, USNM 18313), some crocodyliforms (Protosuchus richardsoni, MCZ 6727), Gracilisuchus (MCZ 4117), proterochampsians, and Doswellia (Weems, 1980), the area of the parietals between the upper temporal fenestrae is adorned by pits and ridges. Protosuchus, Orthosuchus (SAM-PK-409), and Alligator are scored as (0). Revueltosaurus (PEFO 34561) and aetosaurs have deep parietal fossae. However, these fossa are located only on the lateral side of the parietal; thus, they do not form a supratemporal fossa. Revueltosaurus and aetosaurs are scored as (0).
The upper temporal fenestrae of Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Batrachotomus (SMNS 52970), and Postosuchus kirkpatricki (TTU-P 9002) are separated by a thin strip of flat bone framed by two thin ridges marking the medial extent of the upper temporal fenestrae. In Dibothrosuchus (IVPP V 7907), Junggarsuchus (IVPP V 14010), Sphenosuchus (SAM 3014), and Effigia (AMNH FR 30587), a sagittal crest separates the upper temporal fenestrae at the midline.
60. Parietals, posteroventral edge: (0) extending more than half the width of the occiput; (1) less than half the width of the occiput (fig. 16) (Clark et al., 2000; Clark and Sues, 2002; Clark et al., 2004).
In nearly all archosauriforms, the posteroventral processes of the parietals are expanded laterally to meet the squamosal and supraoccipital/opisthotic ventrally. In stark contrast, the posteroventral edge of the parietals are highly reduced in Protosuchus (AMNH FR 3024; BP/1/4770), Orthosuchus (SAM-K-409), and other basal crocodyliforms (Clark et al., 2004).
61. Parietals, occipital margin shape: (0) V-shaped in dorsal view; (1) straight in dorsal view (Clark et al., 2000; Clark and Sues, 2002; Clark et al., 2004).
Plesiomorphically in archosauriforms, the lateral processes of the parietals project posterolaterally creating a V-shaped posterior skull table in dorsal view. In contrast, the lateral processes of the parietals of Dibothrosuchus (IVPP V 7907), Sphenosuchus (SAM 3014), Kayentasuchus (UCMP 131830), Protosuchus (UCMP 131827; BP/1/4746), Orthosuchus (SAM-PK-409), and Alligator project laterally creating a straight posterior margin of the skull table.
62. Parietal, posterolateral ( = occipital) process: (0) nearly vertical; (1) anteriorly inclined greater than 45° (fig. 17) (modified from Heckert and Lucas, 1999).
The posterolateral process of the parietal is completely or nearly vertical in nearly all archosauriforms studied here. In Riojasuchus (PVL 3827), Ornithosuchus (BMNH R2409), and aetosaurs (e.g., Aetosaurus, SMNS 5770 S-2) the posterolateral processes of the parietals are anteriorly inclined about 45°. In Aetosaurus (SMNS 5770 S-8), a set of osteoderms lies on the anteriorly inclined parietals. Revueltosaurus (PEFO 34561) has a vertical posterolateral process of the parietal.
63. Parietal foramen: (0) present; (1) absent (fig. 16) (Gauthier, 1984; Benton, 1985; Benton and Clark, 1988; Bennett, 1996; Nesbitt et al., 2009a).
The parietal foramen is absent in Proterosuchus (see Welman, 1998) as well as in some specimens of Prolacerta (Camp, 1945; Modesto and Sues, 2004). However, the presence of a parietal foramen in Prolacerta is variable according to Modesto and Sues (2004); therefore, the character is scored as polymorphic in Prolacerta. A parietal foramen is present in Mesosuchus (SAM 6536).
64. Quadratojugal-postorbital, contact: (0) absent; (1) present (figs. 17, 19) (Parrish, 1991, 1993).
Observation of this character requires exquisite preservation of the quadratojugal and postorbital. Consequently, this cannot be scored for important basal crocodylomorph taxa such as Dibothrosuchus (following Clark et al., 2000), Terrestrisuchus (Crush, 1984), and Dromicosuchus (Sues et al., 2003). The quadratojugal of Sphenosuchus (SAM 3014) is broken as recently interpreted by Clark et al. (2000). However, there is no indication of contact of the quadratojugal on the posterior surface of the well-preserved postorbital. Therefore, Sphenosuchus is scored as (0).
Parrish (1991, 1993) scored both Postosuchus and Gracilisuchus as (1). Repreparation of the holotype of Postosuchus kirkpatricki (TTU-P 9000), as well as the identification of new, well-preserved elements (UCMP 27441; Long and Murry, 1995), indicates that the squamosal excludes the quadratojugal from contacting the postorbital (contra Chatterjee, 1985). As reported by Brinkman (1981), the skulls of Gracilisuchus are dorsoventrally crushed; thus, in the original and subsequent reconstructions of the taxon (Romer, 1972c; Parrish, 1993), the postorbital is shown to contact the squamosal. Here, I agree with Brinkman (1981) that crushing has artificially forced contact between the two elements and that the quadratojugal and postorbital did not contact in life. Brinkman's (1981) reconstruction of the dorsal orientation of the quadratojugal was too extreme given the preserved length of the quadratojugal and the ventral process of the squamosal versus the length of the postorbital bar. Therefore, the orientation of the quadratojugal and the ventral process of the squamosal would resemble that of Riojasuchus (PVL 6827).
Clark et al. (2000) indicated possible quadratojugal-postorbital contact in Hesperosuchus “agilis” (CM 29894). Upon further investigation, however, the quadratojugal is incompletely preserved and slightly displaced in CM 29894. Therefore, it is not clear whether there is quadratojugal-postorbital contact in Hesperosuchus. In a recent paper, redescribing newly prepared material of Aetosaurus, Schoch (2007) described possible quadratojugal-postorbital contact in three of the specimens. These specimens have partially disarticulated skull elements and some are overprepared. However, Aetosaurus is scored as (1) here. Furthermore, there is no contact between the quadratojugal-postorbital in the aetosaurs Desmatosuchus (Small, 2002) and Stagonolepis (Walker, 1961) and in Revueltosaurus (PEFO 34561).
Only Protosuchus richardsoni (UCMP 130860), Orthosuchus (SAM-PK-409), and Alligator are scored as (1).
65. Postorbital, ventral termination of the ventral process: (0) tapered; (1) blunt (figs. 19, 21) (modified from Benton and Clark, 1988; Juul, 1994; Benton, 1999; Alcober, 2000; Benton and Walker, 2002).
Fig. 21
Cranial character states in archosaurs: A, left jugal of Polonosuchus silesiacus (ZPAL Ab III/563) in lateral and medial B, views; C, right postorbital of Batrachotomus kuperferzellensis (SMNS 52970) in lateral view; D, partial left quadrate of Postosuchus (UCMP 27477) in posterior view; E, left nasal of Postosuchus kirkpatricki (TTU-P 9000) in dorsal and lateral F, views; G, left palatine of Polonosuchus silesiacus (ZPAL Ab III/563) in ventral and dorsal H, views. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm in A–D, G–H, and 5 cm in E–F.
The ventral process of the postorbital tapers to a point in most archosauriforms. In contrast, the ventral process of the postorbital of Postosuchus kirkpatricki (TTU-P 9000) and Batrachotomus (SMNS 80260) terminate in a blunt tab. The tab enters the orbit in both taxa, giving the orbital margin a “stepped” shape. The original formulation of this character by Benton and Clark (1988) focused on the postorbital bar; here I focus on the ventral termination of the postorbital to clarify what is accounting for the “stepped” appearance.
A similar character was also used by Rauhut (2003) and Smith et al. (2007) to describe a similar morphology in Theropoda. According to these authors, Ilokelesia, Abelisaurus, Carnotaurus, Majungatholus, Carcharodontosaurus, Giganotosaurus, and Tyrannosaurus have the derived state. The listed theropod taxa share a dorsoventrally elongated orbit with Batrachotomus (Gower, 1999) and Postosuchus (Chatterjee, 1985).
66. Postorbital-squamosal, contact: (0) restricted to the dorsal margin of the elements; (1) continues ventrally for much or most of the ventral length of the squamosal (figs. 17, 19) (new).
Typically in archosauriforms, a posteriorly directed prong of the postorbital fits into a slot into the anterior portion of the squamosal. This articulation is restricted to the dorsal margin of these elements in non-archosaurian archosauriforms, phytosaurs, Riojasuchus (PVL 3827), Turfanosuchus, Gracilisuchus (MCZ 4117), Arizonasaurus (MSM P4590), Effigia (AMNH FR 30587), Batrachotomus (SMNS 80260), Saurosuchus (PVSJ 32), Prestosuchus (UFRGS 0156-T), and most avian-line archosaurs (e.g., Coelophysis bauri, CM 31374).
The articulation between the two elements is expanded ventrally in Revueltosaurus (PEFO 34561), aetosaurs (e.g., Aetosaurus, SMNS 5770 S-5) and crocodylomorphs (e.g., Sphenosuchus, SAM 3014). With the exception of Revueltosaurus, the entire anterior portion of the squamosal terminates on the posterior edge of the postorbital. The condition in Postosuchus kirkpatricki (TTU-P 9000), Rauisuchus (BSP AS XXV-60-121), and Polonosuchus silesiacus (ZPAL Ab III/563) is scored as (1) because the postorbital-squamosal articulation continues ventrally even though part of the squamosal attaches to the posterior edge of the squamosal ventral to the dorsal postorbital-squamosal contact; a fenestra separates the two different articulations (see character 52).
67. Postorbital bar: (0) composed both of the jugal and postorbital in nearly equal proportion; (1) composed mostly by the postorbital (new).
In nearly all archosauriforms, the postorbital bar is composed of both the postorbital and the jugal in somewhat equal proportions. In the aetosaurs Dematosuchus (TTU-P 9024; Small, 2002), Stagonolepis (Walker, 1961), and Aetosaurus (SMNS 5770 S-8; Schoch, 2007), Shuvosaurus (TTU-P 9280) and Effigia (AMNH FR 30587), the postorbital forms nearly the entire posterior border of the orbit. In these taxa the posterodorsal process of the jugal is very short relative to that of other archosauriforms.
68. Jugal, anterior extent of the slot for the quadratojugal: (0) well posterior of the posterior edge of the dorsal process of the jugal; (1) at or anterior to the posterior edge of the dorsal process of the jugal (fig. 19) (new).
The anterior extent of the anterior process of the quadratojugal lies well posterior to the anterior border of the lower temporal fenestra in most archosauriforms. Within Dinosauria, state (0) is present in ornithischians (e.g., Heterodontosaurus, SAM-K-1332), sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810), Herrerasaurus (PVSJ 407), and Eoraptor (PVSJ 512). In Tawa (GR 241), Coelophysis bauri (CM 31374), and Dilophosaurus (UCMP 37302), the anterior process of the quadratojugal stretches anteriorly at or to the anterior border of the lower temporal fenestra.
69. Jugal, anterior process: (0) participates in the posterior edge of antorbital fenestra; (1) excluded from the antorbital fenestra by the lacrimal or maxilla (figs. 17, 20) (Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004; Rauhut, 2003; Langer and Benton, 2006).
This character was used in many phylogenetic analyses of crocodylomorphs, but it is used here for the first time with a more inclusive grouping. Among non-archosaurian archosauriforms, Proterosuchus (NM QR 1484) is the only taxon to have the jugal participate in the antorbital fenestra. The jugal enters the antorbital fenestra in Riojasuchus (PVL 3827), Gracilisuchus (MCZ 4117), and some phytosaurs (e.g., Mystriosuchus planirostris; Hungerbühler, 2002). The jugal is clearly excluded from the antorbital fenestra in crocodylomorphs (Clark et al., 2000), Postosuchus kirkpatricki (TTU-P 9000), Qianosuchus (IVPP V 13899), Turfanosuchus (IVPP V 3237), Saurosuchus (PVL 2062), Revueltosaurus (PEFO 34561), Aetosaurus (SMNS 5770 S-16), and Longosuchus (TMM 31185-98). The condition is unclear in Batrachotomus and Prestosuchus.
Among avian-line archosaurs, the jugal clearly enters the antorbital cavity in Dimorphodon (BMNH R 1035), Eudimorphodon (MCNSB 2888), Herrerasaurus (PVSJ 407), the ornithischians included here, Plateosaurus (Yates, 2003), and Efraasia (Yates, 2003). The jugal does not enter into the antorbital fenestra in Eoraptor (PVSJ 512), Coelophysis bauri (CM 31394), Dilophosaurus (UCMP 37302), and Allosaurus (Madsen, 1976).
70. Jugal-quadratojugal, contact: (0) absent; (1) present (fig. 16) (new).
A jugal-quadratojugal contact is present in all archosauriforms plesiomorphically. In this analysis, the lower temporal bar is incomplete only in the outgroups Mesosuchus (Dilkes, 1998) and Prolacerta (Modesto and Sues, 2004). In these taxa, the posterior process of the jugal tapers, and the quadratojugal is a small bone attached to the lateral side of the quadrate.
71. Jugal, posterior process: (0) lies dorsal to the anterior process of the quadratojugal; (1) lies ventral to the anterior process of the quadratojugal; (2) splits the anterior process of the quadratojugal; (3) is split by the anterior process of the quadratojugal (figs. 17, 19–20) (new formulation).
This character scores how the jugal and the quadratojugal articulate. Within basal archosauriforms (e.g., Erythrosuchus, BP/1/5207; Euparkeria, SAM 5867; Chanaresuchus, PVL 4586), as well as in Riojasuchus (PVL 3827), Batrachotomus (SMNS 52970), Arizonasaurus (MSM P4590), Qianosuchus (IVPP V 13899), Shuvosaurus (TTU-P 9280), and Prestosuchus (UFRGS 0156-T), the jugal lies dorsal to the anterior process of the quadratojugal. In crocodylomorphs (e.g., Dromicosuchus, UNC 15574), Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), and Gracilisuchus (MCZ 4117), the posterior portion of the jugal lies ventral to the quadratojugal. The jugal splits the quadratojugal into two processes (dorsal and ventral) in Revueltosaurus (PEFO 34561) and Aetosaurus (Schoch, 2007). In the previous three states, the jugal terminates in a point posteriorly. In state (3), the posterior portion of the jugal is forked where the anterior process of the quadratojugal splits the jugal into dorsal and ventral portions. This occurs in Proterosuchus (NM QR 1484) and in all dinosaurs examined here (Sereno and Novas, 1994).
72. Jugal, posterior termination: (0) anterior to or at the posterior extent of the lower temporal fenestra; (1) posterior to the lower temporal fenestra (figs. 17, 19) (new).
In most archosauriforms, the jugal terminates anteroventral to or at the posterior extent of the lower temporal fenestra. This includes Euparkeria (SAM 5867), Arizonasaurus (MSM P4590), Batrachotomus (SMNS 52970), Prestosuchus (UFRGS 0156-T), and most dinosaurs as examples. In contrast, the posterior process of the jugal in crocodylomorphs (e.g., Sphenosuchus, SAM 3014; Alligator), Desmatosuchus (TTU-P 9024), Postosuchus kirkpatricki (TTU-P 9000), and Polonosuchus silesiacus (ZPAL Ab III/563) lies well posterior of the lower temporal fenestra. State (1) is also in Erpetosuchus (AMNH FR 29300), Gracilisuchus (MCZ 4117), and Pseudopalatus (UCMP 34249).
73. Jugal, posterior border of the postorbital process: (0) concave; (1) convex (fig. 19) (new).
The posterior border of the postorbital process of the jugal is concave in nearly all basal archosauriforms. However, this portion of the jugal is markedly convex in Sphenosuchus (Walker, 1990), Litargosuchus (BP/1/5237), Junggarsuchus (IVPP V14010), and Protosuchus haughtoni (Gow, 2000; BP/1/4770).
74. Jugal, long axis of the body: (0) nearly horizontal; (1) anterodorsally inclined (fig. 17) (modified from Heckert and Lucas, 1999; Parker, 2007).
This character is rewritten for clarity from “jugal: not downturned (0) or downturned (1)” (Heckert and Lucas, 1999). The original formation is vague and does not provide a strict point of reference. The long axis of the body of the jugal in most archosauriforms is horizontally oriented. In contrast, the aetosaurs Longosuchus (TMM 31185-98), Desmatosuchus (TTU-P 9024), and Stagonolepis (Walker, 1961) all have jugals with the long axis of the jugal significantly inclined. The jugals of Lotosaurus (IVPP V 131827), Plateosaurus (AMNH FR 6810), and Efraasia (Yates, 2003) are also anteriorly inclined.
75. Jugal, longitudinal ridge on the lateral surface of the body: (0) absent; (1) present and sharp; (2) present and rounded; (3) present and rounded, restricted to a bulbous ridge (figs. 16, 17, 19–20) (new).
Some archosauriforms (e.g., Pseudopalatus, UCMP 34249; Riojasuchus, PVL 3827; Silesaurus, Dzik, 2003) lack any kind of ridge on the jugal, whereas others have a distinct ridge. The ridge differs throughout Archosauriformes and is split into three morphologies. In Ornithosuchus (Walker, 1964), proterochampsians (e.g., Chanaresuchus, PVL 4647), Herrerasaurus (Sereno and Novas, 1994), and Coelophysis bauri (CM 31374), the jugal ridge is sharp. In aetosaurs (e.g., Aetosaurus, SMNS 5770), Revueltosaurus (PEFO 34561), Batrachotomus (Gower, 1999), Saurosuchus (PVSJ 32), Hesperosuchus “agilis” (CM 29894), Sphenosuchus (SAM 3014), and Gracilisuchus (MCZ 4117), the jugal has a low, rounded ridge dotted with small foramina. The jugals of Rauisuchus (BSP AS XXV-60-121), Polonosuchus silesiacus (ZPAL Ab III/563), and Postosuchus kirkpatricki (TTU-P 9000) have a bulbous ridge that is distinct from that of other archosaurs.
76. Quadrate: (0) does not contact prootic; (1) contacts prootic (fig. 23) (Walker, 1990; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Gower and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
The head of the quadrate is separated by the squamosal from the prootic in all archosauriforms with the exceptions of crocodylomorphs and avians. In crocodylomorphs, part of the glenoid where the quadrate articulates is formed by the prootic. A small vertical ridge located near the prootic-paroccipital contact separates the quadrate articular surface from the rest of the prootic as demonstrated by Walker (1990) in Sphenosuchus (SAM 3014). An exceptional specimen (e.g., Sphenosuchus) or the complete prootic is needed in order to score this character. Quadrate-prootic contact is found in Hesperosuchus agilis (AMNH FR 6758), Dibothrosuchus (IVPP V 7907), Kayentasuchus (UCMP 131830), Sphenosuchus (SAM 3014), Protosuchus richardsoni (UCMP 130860), and Alligator.
77. Quadrate, dorsal head: (0) does not have a sutural contact with the paroccipital process of the opisthotic; (1) has a sutural contact with the paroccipital process of the opisthotic (new).
The head of the quadrate of most archosaurs articulates with the squamosal, or in some, the prootic. However, in phytosaurs (e.g., Leptosuchus doughtyi; AMNH FR 4919) the head of the quadrate has an interdigitating suture with the anterior surface of the paroccipital process. Part of the proximal portion of the quadrate attaches to the paroccipital process in Alligator, but this condition is not present in any of the other crocodylomorphs included here.
78. Quadrate, head: (0) partially exposed laterally; (1) completely covered by the squamosal (figs. 17–Fig. 1819) (Sereno and Novas, 1994; Juul, 1994; Novas, 1996; Benton, 1999; Langer and Benton, 2006).
As discussed and described by Sereno and Novas (1994) and Langer and Benton (2006), the head of the quadrate is laterally exposed in basal members of Dinosauria and Lewisuchus (UNLR 01). Additionally, the head of the quadrate is exposed in Arizonasaurus (MSM P4590; Nesbitt, 2005a), Qianosuchus (IVPP V 13899), Turfanosuchus (IVPP V 3237), Euparkeria (SAM 5867), Saurosuchus (PVSJ 32), Batrachotomus (Gower, 1999), Revueltosaurus (PEFO 34561), Riojasuchus (PVL 3827), Proterosuchus (NM QR 1484), Chanaresuchus (PVL 4575), Tropidosuchus (PVL 4601), aetosaurs (e.g., Aetosaurus, SMNS 5770 S-16), and in avian-line archosaurs examined in this study. When laterally exposed, the articulation of the head of the quadrate with the squamosal is clearly visible at the posterior end of the articulation. The anterior portion of the articulation is somewhat covered in some of the taxa (e.g., Euparkeria), whereas the posterior portion is clearly visible. The ventrally concave posterolateral margin of the squamosal frames the quadrate head in taxa scored as (0).
The squamosal, either the body or the ventral process, completely covers the head of the quadrate in taxa scored as (1). The squamosal covers the head of the quadrate in Vancleavea (GR 138), Erythrosuchus (Gower, 2003), Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), Rauisuchus (BSP AS XXV-60-121), Crocodylomorpha (Benton and Clark, 1988), and Gracilisuchus (MCZ 4117).
79. Quadratojugal and quadrate, suture between the elements, foramen: (0) present; (1) absent (fig. 16) (modified from Parrish, 1991; Benton and Walker, 2002).
In most basal archosaurs, a foramen is present between the quadratojugal and quadrate. This is the case in non-archosaurian archosauriforms, Postosuchus kirkpatricki (TTU-P 9000), Polonosuchus silesiacus (ZPAL Ab III/563), phytosaurs (e.g., Pseudopalatus, UCMP 34249), Batrachotomus (SMNS 80260), Desmatosuchus (Small, 2002), Stagonolepis (Walker, 1961), Arizonasaurus (MSM P4590), Prestosuchus (UFRGS 0156-T), Saurosuchus (PVSJ 32), and in most avian-line archosaurs (Silesaurus, ZPAL Ab III/361; Herrerasaurus, PVSJ 407). In crocodylomorphs included here and Aetosaurus (Schoch, 2007), the foramen between the quadratojugal and quadrate is absent.
80. Quadrate, body: (0) without fenestrae; (1) fenestrated (fig. 19) (Clark et al., 2004).
In most basal archosauriforms, the quadrate is a solid bone without any fenestrae. The quadrates of Protosuchus richardsoni (UCMP 130860), Protosuchus haughtoni (BP/1/4746), Orthosuchus (SAM-PK-409), Junggarsuchus sloani (Clark et al., 2004), and Alligator have fenestrated quadrates. The circular fenestrations are numerous and small.
81. Quadrate, distal articular margin: (0) largely convex with corresponding concave articular surface of the articular; (1) largely concave with corresponding convex articular surface of the articular (fig. 21) (Nesbitt and Norell, 2006).
The quadrates of nearly all archosauriforms consist of a convex articular surface that fits into a corresponding concave surface in the articular. The opposite is true in Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280). In these taxa, the distal end of the quadrate is mostly concave, whereas the articular is convex (Nesbitt, 2007). Lotosaurus (IVPP V 48013) has an intermediate condition where the quadrate is convex anteroposteriorly but concave mediolaterally in posterior view; given that it was still largely convex, it was scored as (0).
82. Quadrate, angled: (0) posteroventrally or vertical; (1) anteroventrally (fig. 17) (Nesbitt, 2007).
The quadrates of most archosauriforms slope posteroventrally. In Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9280), Aetosaurus (Schoch, 2007), Stagonolepis (Walker, 1961), Desmatosuchus (Small, 2002), spinosaurids, and ornithomimid theropods (Rauhut, 2003), the quadrate slopes anteroventrally (Nesbitt, 2007).
83. Quadrate, dorsoventrally oriented crest located on the posterior side:(0) absent; (1) present (fig. 21) (new).
The posterior surface of the quadrate is smooth in nearly all archosauriforms. Polonosuchus silesiacus (ZPAL Ab III/563) and Postosuchus kirkpatricki (TTU-P 9000) have a rugose, dorsoventral crest located on the posterior side of the quadrate just ventral to the quadrate/quadratojugal foramen. This crest slightly arcs medially and creates a concave surface that opens medially.
84. Pterygoid-ectopterygoid, articulation: (0) ectopterygoid ventral to pterygoid; (1) ectopterygoid dorsal to pterygoid (Sereno and Novas, 1994; Novas, 1996; Benton, 1999; Irmis et al., 2007a).
Sereno and Novas (1994) thoroughly discussed this character and their dichotomy remained robust in subsequent analyses. As stated by Sereno and Novas (1994), all three groups of dinosaurs have state (1) ancestrally. The ectopterygoid articulates on the ventral portion of the pterygoid in non-archosaurian archosauriforms and pseudosuchians.
85. Palatine-pterygoid, fenestra: (0) absent; (1) present (Sereno, 1991a).
Following Sereno (1991a), character state (1) is only present in Ornithosuchus (BMNH R 3143) and Riojasuchus (PVL 3827).
86. Ectopterygoid, ventral recess: (0) absent; (1) present (fig. 22) (Gauthier, 1986; Langer and Benton, 2006).
Fig. 22
Archosaur ectopterygoids: A, the right ectopterygoid of Postosuchus kirkpatricki (TTU-P 9002) in dorsal view; B, the left ectopterygoid of Allosaurus fragilis (AMNH FR 600) in dorsal and C, ventral view; D, the left ectopterygoid of Plateosaurus engelhardti (AMNH FR 6810) in posterodorsal and ventral E, view. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
An ectopterygoid recess was cited as a theropod synapomorphy (Gauthier, 1986), and as a character uniting Eoraptor with theropods (Sereno, 1999; Langer and Benton, 2006). However, I disagree with Sereno (1999) and Langer and Benton (2006) about the scoring of Eoraptor and basal dinosaurs. The ectopterygoid articulates with the dorsal surface of the lateral flange of the pterygoid in dinosaurs (character 84). As a result, the ventral surface of the ectopterygoid has a slight depression for the articulation with the pterygoid. In Allosaurus and other tetanurans, there is a distinct recess (possibly pneumatic) and a depression for the pterygoid. The ectopterygoids of Plateosaurus (AMNH FR 6810), Eoraptor (PVSJ 512), Liliensternus (MB R. 2175; Rauhut, 2003: fig. 19B), and Coelophysis (AMNH FR 7239) possess only a facet for the pterygoid and lack a distinct recess. Yates (2003) described a deep fossa in the ectopterygoid of the basal sauropodomorph Pantydraco. This depression is only the articular face with the pterygoid, not a distinct fossa as in Allosaurus. Furthermore, any slight disarticulation of the ectopterygoid from the pterygoid may look like a distinct recess; I urge caution when scoring this character.
Langer and Benton (2006) stated that a ventral recess is present in Sphenosuchus. However, this slight depression is very different from that of theropods. Therefore, it is scored as (0).
87. Ectopterygoid, body: (0) arcs anteriorly; (1) arcs anterodorsally (fig. 22) (new).
The body of the ectopterygoid connects the jugal with the pterygoid. In non-archosaurian archosauriforms and crocodylian-line archosaurs, the ectopterygoid arches anteriorly between the articulations. In dinosaurs, the ectopterygoid arches anteriorly and dorsally between the articulations. This is true of Heterodontosaurus (SAM-PK-1332), Coelophysis bauri (CM 31374), and Plateosaurus (AMNH FR 6810). This also appears to be the state in Lewisuchus (UNLR 01).
88. Ectopterygoid: (0) does not form or forms some of the lateral edge of the lateral pterygoid flange; (1) forms all of the lateral edge of the lateral pterygoid flange (fig. 22) (Dilkes, 1998; Nesbitt et al., 2009a).
The ectopterygoid attaches to only the anterolateral corner of the lateral pterygoid flange in Mesosuchus (SAM 6536) and in Prolacerta (UCMP 37151), whereas the ectopterygoids of Proterosuchus + Archosauria ( = Archosauriformes) lie along the entire lateral pterygoid flange.
89. Ectopterygoid: (0) single headed; (1) double headed (figs. 21–22) (Weinbaum and Hungerbühler, 2007).
As described by Weinbaum and Hungerbühler (2007), the ectopterygoids of most archosauriforms possess a single lateral head that articulates with the jugal into a single socket. In Postosuchus kirkpatricki (TTU-P 9002), Polonosuchus silesiacus (ZPAL Ab III/563), Batrachotomus (Gower, 1999), Hesperosuchus “agilis” (YPM 41198), and Sphenosuchus (SAM 3014), the head of the ectopterygoid is split into two, a larger ventral head and a smaller posterodorsal head. A well-defined groove splits the two heads; a groove splits two articular facets of the jugal for the ectopterygoid in taxa with state (1). Therefore, this character can be scored if either the medial side of the jugal or the lateral head of the ectopterygoid is visible.
90. Palatine, fossa on the dorsal surface: (0) extends far anteriorly, near the pila postchoanalis; (1) does not extend very far anteriorly along the upper surface of the palatine (fig. 21) (Witmer, 1997; Gower and Walker, 2002).
As discussed by Gower and Walker (2002), Witmer (1997) recognized two basic types of palatine morphology with respect to the extent of the dorsal fossa for attachment of the dorsal part of the M. pterygoideus. The dorsal fossa extends far anteriorly, up to the posterior border of the internal choana in non-archosaurian archosauriforms, phytosaurs (Witmer, 1997b), Ornithosuchus (Walker, 1964; Witmer, 1997), Batrachotomus kupferzellensis (Gower, 1999), and Saurosuchus galilei (Alcober, 2000). The dorsal fossa is shifted more posteriorly, so that there is a flat surface between the posterior edge of the choana and the fossa, and this was cited as present in Sphenosuchus acutus (Walker, 1990; Witmer, 1997) and aetosaurians (e.g., S. robertsoni, Walker, 1961; Witmer, 1997). The condition is aetosaurs deserves more comment. The palatine is foreshortened in aetosaurs making it difficult to compare to other crocodylian-line archosaurs. Nevertheless, the fossa in Stagonolepis is separated from the internal choana by a space much greater than that in Batrachotomus kupferzellensis (Gower, 1999). Not all aetosaurs have state (1), as demonstrated by Longosuchus (TMM 31185–98); there is a large gap between the choana and the dorsal fossa. The space between the fossa and the internal choana in Polonosuchus silesiacus (ZPAL Ab III/543) is great like that of Sphenosuchus (SAM 3014) and Hesperosuchus “agilis” (YPM 41198). The distribution of this character is more complex in Crocodylomorpha than was stated previously. The palatines of Terrestrisuchus (BMNH R7593) and Protosuchus richardsoni (UCMP 130860) have a fossa and posterior border of the internal choana that is separated only by a thin ridge.
91. Palatine, posterior margin of the choana: (0) smooth, no raised rim on ventral surface; (1) raised rim defining a fossa around the choana on the ventral surface ( = spout of Walker, 1990) (fig. 21) (new).
In most archosauriforms, the ventral margin of the choana is unmarked on the ventral surface of the palatine. This is the condition present in Euparkeria (SAM 13664), Saurosuchus (PVSJ 32), Stagonolepis (Walker, 1961: fig. 3A), Batrachotomus (Gower, 1999), and Plateosaurus (AMNH FR 6810). In Polonosuchus silesiacus (ZPAL Ab III/563), Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V 7907), and Kayentasuchus (UCMP 131830), the posterior border of the choana is marked by a distinct raised ridge on the ventral surface.
92. Laterosphenoid: (0) absent; (1) present (fig. 23) (Gauthier et al, 1988; Benton and Clark, 1988; Parrish, 1992; Clark, 1993; Juul, 1994; Bennett, 1996; Nesbitt et al., 2009a).
Fig. 23
The braincase of basal archosaurs in lateral view: A, reconstruction of the braincase of Sphenosuchus acutus; redrawn from Walker (1990); B, close-up of the ear region of Sphenosuchus acutus; redrawn from Walker (1990); C, reconstruction the braincase of Stagonolepis robertsoni; redrawn from Walker (1990); D, reconstruction the braincase of Silesaurus opolensis; redrawn from Dzik (2003); E, reconstruction the braincase of Coelophysis rhodesiensis; redrawn from Raath (1969). Numbers refer to character states. See appendix for anatomical abbreviations.
The laterosphenoid of archosauriforms was well described by Clark (1993). An ossified laterosphenoid is clearly absent in Prolacerta and present in all archosauriforms examined here. A laterosphenoid is clearly present in the proterochampsian Chanaresuchus (PVL 4575).
93. Basipterygoid, processes directed: (0) anteriorly or ventrally at their distal tips; (1) posteriorly at their distal tips (fig. 23) (new).
This character scores the direction of the ventral tips of the basipterygoid processes. The tips point anteriorly in Silesaurus (Dzik, 2003), Marasuchus (PVL 3870), Plateosaurus (AMNH FR 6810), Coelophysis bauri (AMNH FR 7239), Prolacerta (BPI 2675), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9282), Charanesuchus (PVL 4647), and they point posteriorly in Proterosuchus (BPI 3993), Postosuchus kirkpatricki (TTU-P 9002), Batrachotomus (SMNS 80260), Saurosuchus (PVSJ 32), Arizonasaurus (MSM P4590), Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V 7907), Revueltosaurus (PEFO 34561), aetosaurs (e.g., Aetosaurus, SMNS 5770), and phytosaurs (e.g., Pseudopalatus pristinus, UCMP 34249; Smilosuchus, UCMP 27200). The distribution of this character is not straightforward, but it may support small clades.
94. Prootic, ridge on lateral surface of inferior anterior process ventral to the trigeminal foramen: (0) present; (1) absent (Gower and Sennikov, 1996; Nesbitt et al., 2009a).
Prolacerta (Gow, 1975), Mesosuchus (SAM 8552), and Proterosuchus (Gow, 1975) are scored as having a ridge on the lateral surface of the inferior anterior prootic process below the trigeminal foramen. Originally, Gower and Sennikov (1996) scored Euparkeria as having a small ridge (0), but after examining other Euparkeria specimens, Gower and Weber (1998) considered the small ridge a preservational artifact of the specimen originally scored (UMZC T692). Therefore, Euparkeria is scored as (1). The character is scored as absent (1) in Erythrosuchus (Gower, 1997), Chanaresuchus (MCZ 4036), Vancleavea (GR 138), and Archosauria. This character cannot be scored in Tropidosuchus.
95. Parabasisphenoid, foramina for entrance of cerebral branches of internal carotid artery into the braincase positioned on the surface: (0) ventral; (1) posterolateral; (2) anterolateral (fig. 24) (modified from Parrish, 1993; Gower and Sennikov, 1996; Gower, 2002; Nesbitt et al., 2009a).
Fig. 24
The posterior portion of the braincase of basal archosaurs: A, back of the skull of Protosuchus richardsoni (UCMP 131827), posterior view; B, braincase of Postosuchus kirkpatricki (TTU-P 9002), posterior view; C, braincase of Xilousuchus sapingensis (IVPP V 6026), posterior view; D, basioccipital of Effigia okeeffeae (AMNH FR 30587), dorsal view; E, basiocciptial of Sphenosuchus acutus (SAM 3014), ventral view; F, paroccipital process of Postosuchus alisonae (UNC 14475), anterior view; G, braincase of Lewisuchus admixtus (UNLR 001), posterior view; H, braincase of Proterosuchus (NM QR 880), posterior view. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bar = 1 cm.
The internal carotids enter the basisphenoid ventrally in Mesosuchus (SAM 6536; Dilkes, 1998), Prolacerta (BP/1/2675; Evans, 1988), Proterosuchus (BP/1/3993; Gow, 1975), Erythrosuchus (BMNH R3592; Gower, 1997), Euparkeria (UMZC T692; Gower and Weber, 1998), and in the proterochampsians Tropidosuchus (PVL 4604) and Chanaresuchus (PVL 4647). Parrish (1993) reported that Proterochampsa (MCZ 3408) had both a ventral and a lateral entrance, and that is not confirmed nor denied here; however, all other proterochampsian specimens examined by myself have the internal carotids entering from the ventral surface.
Gower (2002) rephrased the original character formation from Parrish (1993) and Gower and Sennikov (1996) in his character set focusing on crocodylian-line archosaur relationships. The wording of Gower (2002) is preferred except for the plesiomorphic entrance of the internal carotids into the braincase. Here, instead of posterior, I use ventral to describe state (0). As explained by Gower (2002), the foramina are located on the lateral surface of the parabasisphenoid just anterior to the notches between the basal tubera and basipterygoid processes in phytosaurs. In phytosaurs, the entrance lies between the notch between the basipterygoid tubera and basitubera (Pseudopalatus pristinus UCMP 137319). Gower (2002) hypothesized that this is intermediate between character state (0) and (1), and therefore ordered the character. However, given that the possible crown-group archosaur Turfanosuchus (Parrish, 1993; Wu and Russell, 2001), the suchian Arizonasaurus (Gower and Nesbitt, 2006), and the dinosauriform Silesaurus (Dzik, 2003; Ab III 364/4) have a ventral entrance of the internal carotids, it is not clear that the difference in the entrances of phytosaurs and most suchians are homologous.
Among the dinosauromorphs, the internal carotids enter from the anterolateral portion of the parabasisphenoid like that of suchians. In Silesaurus (ZPAL Ab III 364/4), the foramina for the entrance of the cerebral branches of the internal carotid artery into the braincase are positioned on the ventral surface.
96. Parabasisphenoid, plate: (0) present and straight; (1) present and arched anteriorly; (2) absent (fig. 24) (modified from Gower and Sennikov, 1996; Nesbitt et al., 2009a).
The basisphenoid plate is an anterodorsally/posteroventrally compressed plate of bone that lies between the basitubera of the parabasisphenoid (Gower and Sennikov, 1996; Gower, 2002). A plate is not present in Mesosuchus (SAM 6536), but it is present in Prolacerta (BP/1/2675), Proterosuchus (BP/1/3993), and Erythrosuchus (Gower and Sennikov, 1996). In these taxa the plate is straight. Here, I score Tropidosuchus (PVL 4604), Chanaresuchus (PVL 4647), and Euparkeria (SAM 5867) as having a parabasisphenoid plate (the basisphenoid/parabasisphenoid plate is scored in Euparkeria as absent in Gower and Weber, 1998). In these taxa, a thin lamina of bone connects the basitubera of the parabasisphenoid like that of taxa scored as (0). However, in proterochampsians and Euparkeria the thin lamina arcs anteriorly at the midline. In phytosaurs, crocodylian-line archosaurs, and dinosauriforms, a distinct basisphenoid/parabasisphenoid plate is not present. In some phytosaur taxa (e.g., AMNH FR 30646) scored as (2), a ridge created by both the parabasisphenoid and the basioccipital connects the basitubera. A low ridge may be present between the basitubera in taxa; however, this ridge differs from taxa scored as (0) and (1), and thus these features are not considered homologous.
A thin, arched plate of bone is present in Arizonasaurus (MSM P4590) and Xilousuchus (IVPP V 6026).
97. Parabasisphenoid, orientation: (0) horizontal; (1) more vertical (fig. 23) (Gower and Sennikov, 1996; Nesbitt et al., 2009a).
Mesosuchus (SAM 6536; Dilkes, 1998), Prolacerta (BP/1/2675; Evans, 1988), and Proterosuchus (BP/1/3993; Gow, 1975) have horizontal basisphenoids; the base of the basitubera and the base of the basipterygoid processes are about the same horizontal level. Verticalized basisphenoids, with the base of the basitubera more dorsal than the base of the basipterygoid processes, are present in Erythrosuchus (BMNH R3592; Gower, 1997), Euparkeria (UMZC T692), Tropidosuchus (PVL 4604), Chanaresuchus (PVL 4647), and most of Archosauria (Gower and Sennikov, 1996).
98. Parabasisphenoid, semilunar depression on the lateral surface of the basal tubera: (0) present; (1) absent (Gower and Sennikov, 1996; Nesbitt et al., 2009a).
This character is present in all non-archosaurian archosauriforms including Chanaresuchus (PVL 4647). As Gower and Sennikov (1996) reported, this character is absent in crown-group Archosauria.
99. Parabasisphenoid, basipterygoid processes: (0) present; (1) absent (figs. 23–24) (Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
Basipterygoid processes are present in most archosauriforms. In crocodyliforms, the basipterygoid processes are absent (Clark et al., 2000).
100. Parabasisphenoid, recess ( = median pharyngeal recess of some authors = hemispherical sulcus = hemispherical fontanelle): (0) absent; (1) present (fig. 24) (modified from Nesbitt and Norell, 2006).
A depression on the ventral surface of the parabasisphenoid is common among basal archosauriforms. Among non-archosaurians, a depression is absent in Prolacerta (BPI 2675), Proterosuchus (BPI 3993), Erythrosuchus (BMNH R3592), Chanaresuchus (PVL 4546), and Euparkeria (SAM 7696). Phytosaurs (e.g., Pseudopalatus pristinus, UCMP 137319) also do not have a recess in the parabasisphenoid. The ornithosuchid Riojasuchus (PVL 3827) possesses a shallow recess, as do the aetosaurs Longosuchus (TMM 31185–98), Desmatosuchus smalli (Small, 2002; Parker, 2005), Tecovasuchus (TTU-P 545 Martz and Small, 2006), Aetosaurus (SMNS 5770 S-16; Schoch, 2007), Typothorax (MCZ 1488), and Coahomasuchus (Desojo and Heckert, 2004). Among other crocodylian-line archosaurs, Arizonasaurus (MSM P4590), Shuvosaurus (TTU-P 9282), and Effigia (AMNH FR 30587) have deep and elongated recesses. Alcober (2000) described the deep recess in Saurosuchus (PVSJ 32) as the “eustachian foramen,” so it is unclear whether there is a true opening here for a eustachian tube and whether the recess is subdivided. The fossa is blind in all non-crocodylomorph pseudosuchians observed in this study. The undivided recess is very deep in Postosuchus kirkpatricki (TTU-P 9002; UCMP 138843) and Tikisuchus (ISI 305). Both Sphenosuchus (SAM 3014; Gower, 2002) and Dibothrosuchus (IVPP V 7907) have a deep trough like that of Postosuchus kirkpatricki (TTU-P 9002), but they also have a deep divided recess entirely within the basioccipital. Only the recess of the parabasisphenoid is scored here. The recess is subdivided in Batrachotomus (SMNS 80260; Gower, 2002). The crocodyliforms used here (e.g., Protosuchus richardsoni, UCMP 131827) lack a depression in the parabasisphenoid. The depression in the parabasisphenoid of taxa scored as (1) is not homologous with that of the median pharyngeal recess of crocodyliforms (see below).
Among dinosauromorphs, the recess is absent in Marasuchus (PVL 3870), but a shallow recess is present in Silesaurus (ZPAL Ab III 364/4). Among dinosaurs, basal ornithischians (e.g., Lesothosaurus, Sereno, 1991b), Herrerasaurus (PVSJ 407), and sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810) lack a recess, whereas a recess is clearly present in theropods (e.g., Coelophysis bauri, AMNH FR 7239). Rauhut (2003) noted that many theropods with parabasisphenoid recesses have midline subdivisions (e.g., Coelophysis rhodesiensis). Witmer (1997) considered this recess pneumatic.
101. Parabasisphenoid, anterior tympanic recess on the lateral side of the braincase: (0) absent; (1) present (fig. 23) (Makovicky and Sues, 1998; Rauhut, 2003).
The presence of an anterior tympanic recess was found as a synapomorphy of Neotheropoda by Rauhut (2003). The recess is located on the lateral side of the basisphenoid just anteroventral to the fenestra ovalis. The recess typically preserves smaller fossae within it suggesting it may be pneumatic (Chure and Madsen, 1996; Witmer, 1997; Makovicky and Sues, 1998; Rauhut, 2003, 2004). Rauhut (2003) scored an anterior tympanic recess absent in dinosauriforms, ornithischians, and Herrerasaurus and present in the basal theropods Dilophosaurus, Coelophysis rhodesiensis, and Piatnitzkysaurus, as well as in other theropods. However, the lateral wall of the braincase of Silesaurus (ZPAL Ab III/361/4), Lewisuchus (Romer, 1972d), Heterodontosaurus (SAM-PK-1332), Eocursor (SAM-PK-0925), and Plateosaurus (AMNH FR 6810) has a feature that I cannot differentiate from that of basal theropods. Therefore, I score these taxa as (1). None of these taxa, though, have smaller “pneumatic” depressions within them.
Among basal archosauriforms and basal crocodylian-line archosaurs, an anterior tympanic recess is not present. As discussed by Gower and Weber (1998), Euparkeria does not have an anterior tympanic recess (contra Welman, 1995). Sphenosuchus has large pneumatic cavities in the same region (pre- and postcarotid recesses) as do other crocodylomorphs (Walker, 1990). Gower and Weber (1998), followed by Rauhut (2003), rightfully showed that the anterior tympanic recess of theropods is not homologous with that of crocodylomorphs. For a similar interpretation of this character for crocodylomorphs see Clark et al. (2000), character 22. Here, crocodylomorphs are scored as inapplicable.
102. Parabasisphenoid: (0) relatively short dorsoventrally; (1) substantially elongated in the region between the basal tubera and the basipterygoid processes, such that the “median pharyngeal recess” is dorsoventrally extended and troughlike (figs. 23–24) (Parrish, 1993; Gower, 2002).
Character state (1) is clearly present in Postosuchus kirkpatricki (TTU-P 9002) and Tikisuchus (ISI 305) but absent in aetosaurs, Batrachotomus (Gower, 2002), Saurosuchus (Alcober, 2000), and Arizonasaurus (MSM P4590). Parrish (1993) used this character to unite Batrachotomus, Gracilisuchus, Postosuchus, and Dibothrosuchus. Gower (2002) clearly showed that Batrachotomus lacks state (1) but explicitly stated that state (1) is not present in any crocodylomorph. Here, I disagree with Gower (2002) and hypothesize that the elongated parabasisphenoid of Sphenosuchus (SAM 3014) and Dibothrosuchus (IVPP V 7907) is homologous with that of Postosuchus kirkpatricki (TTU-P 9002) and Tikisuchus (ISI 305). These taxa all share a blind trough that is anteroventrally elongated in an identical way. The parabasisphenoid of Gracilisuchus (MCZ 4117), although elongated, is not ventrally elongated; therefore, it is scored as (0).
103. Parabasisphenoid, between basal tubera and basipterygoid processes: (0) approximately as wide as long or wider; (1) significantly elongated, at least 1.5 times longer than wide (fig. 23) (Rauhut, 2003; Nesbitt, 2007).
The parabasisphenoids of Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9282) are elongated relative to those of other crocodylian-line archosaurs (Nesbitt, 2007) as well as those of Coelophysis bauri (AMNH FR 7239) and Coelophysis rhodesiensis (QG 195) (Rauhut, 2003). This character is scored as inapplicable for crocodylomorphs and Postosuchus kirkpatricki to avoid the possible correlation with character 102.
104. Basitubera: (0) clearly separated; (1) medially expanded and nearly or completely connected (fig. 24) (new).
The basitubera of nearly all archosauriforms are clearly separated medially by a U-shaped gap. Taxa with a basisphenoid/parabasisphenoid plate are not scored as (1) because the basitubera do not expand medially. In the phytosaurs Smilosuchus (UCMP 27200) and Pseudopalatus (NMMNH P31292) the basitubera merge medially and are scored (1).
105. Prootic-opisthotic, contact: (0) broad overlap; (1) reduced to a small contact (fig. 23) (modified from Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
Although this character is not quantified, it does represent a discrete change within crocodylian-line archosaurs. In non-archosaurian archosauriforms, basal dinosauromorphs, and most crocodylian-line archosaurs, the posterolateral process of the prootic makes broad contact with the opisthotic, thus forming part of the paroccipital process. This is not true of the crocodylomorph Sphenosuchus (fig. 28 of Walker, 1990), where the prootic contacts the opisthotic only on a short surface. Clark et al. (2000) also scored Dibothrosuchus, Protosuchus, and Alligator as (1) for this character, and those scorings are accepted here.
106. Basioccipital, portion of the basal tubera: (0) rounded and anteroposteriorly elongated; (1) bladelike and anteroposteriorly shortened (figs. 23–24) (new).
In most archosauriforms, the basioccipital portion of the basal tubera is rugose, rounded, and anteroposteriorly thick. This is exemplified by Revueltosaurus (PEFO 34561), the phytosaur Smilosuchus (UCMP 27200), the aetosaur Stagonolepis (MCZD 2–4), Erythrosuchus (BMNH R3592), and Herrerasaurus (PVSJ 409). In Saurosuchus (PVSJ 32), Batrachotomus (SMNS 80260), Postosuchus kirkpatricki (TTU-P 9002), Hesperosuchus agilis (AMNH FR 6758), Sphenosuchus (SAM 3014), Dibothrosuchus (IVPP V 7907), Protosuchus richardsoni (UCMP 131827), and Alligator, the basioccipital portion of the basal tubera is bladelike and anteroposteriorly shortened compared with that of the other basal archosaur taxa.
107. Basioccipital, deep recess on the ventral surface: (0) absent; (1) present (fig. 24) (new).
In most archosauriforms, the ventral surface of the basioccipital is smooth and does not bear a fossa. In the basal crocodylomorphs Sphenosuchus (SAM 3014) and Dibothrosuchus (IVPP V 7907), there is a clear recess within the basioccipital that is divided by a lamina located at the midline. This is a different depression than that of the parabasisphenoid recess ( = hemispherical sulcus = hemispherical fontanelle) described above. The depression of the parabasisphenoid is restricted to that element. It is clear that a basioccipital recess is not present in aetosaurs (Aetosaurus, SMNS 5770), Saurosuchus (PVSJ 32), Arizonasaurus (MSM P4590), Batrachotomus (SMNS 80260), Postosuchus kirkpatricki (TTU-P 9002), Tikisuchus (ISI 305; Gower, 2002), or the crocodylomorph Hesperosuchus agilis (AMNH FR 6758). There is a recess in Terrestrisuchus (BMNH P62/20), but it is not clear whether it is subdivided. A foramen in Protosuchus richardsoni (UCMP 131827) and Orthosuchus (SAM-PK-409) is present in the same position as the opening in Sphenosuchus (SAM 3014) and Dibothrosuchus (IVPP V 7907). The foramen in Protosuchus richardsoni (UCMP 131827) and Orthosuchus is mostly within the basioccipital, but the parabasisphenoid creates a sliver of the anteroventral border. In Alligator, a homologous foramen ( = median pharyngeal recess) is almost entirely within the parabasisphenoid.
108. Opisthotic, paroccipital processes: (0) no or slight dorsal and ventral expansion distally; (1) markedly expanded dorsally at the distal ends (figs. 23–24) (character states reversed from Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
In most archosauriforms, the paroccipital processes have nearly parallel dorsal and ventral margins or they gradually expand both dorsally and ventrally at their distal ends. This character state is present in non-archosaurian archosauriforms, phytosaurs, aetosaurs (e.g., Aetosaurus SMNS 5770 S-5), Gracilisuchus (MCZ 4117), Saurosuchus (PVSJ 32), Batrachotomus (SMNS 80260), Arizonasaurus (MSM P4590), Riojasuchus (PVL 3827), and basal avian-line archosaurs. In contrast, the distal ends of the paroccipital processes of Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Tikisuchus (Chatterjee and Majumdar, 1987), and the crocodylomorphs Sphenosuchus (SAM 3014), Terrestrisuchus (Crush, 1984), Litargosuchus (BP/1/5237), Hesperosuchus (CM 29894), Dibothrosuchus (IVPP 7907), Orthosuchus (SAM-PK-409), and Protosuchus richardsoni (UCMP 131827) have marked dorsally expanded distal ends. In Alligator, the paroccipital processes are like those of taxa scored as (0). Clark et al. (2000) used a similar character, but did not differentiate the conditions in Stagonolepis and Gracilisuchus from those in Postosuchus and crocodylomorph taxa such as Sphenosuchus.
109. Opisthotic, extent of the lateral margin of the paroccipital: (0) lateral to the upper temporal fenestra; (1) at the margin or medial to the lateral extent of the upper temporal fenestra (fig. 18) (new).
In most archosauriforms, the paroccipital expands to meet the posteromedial portion of the squamosal lateral to the upper temporal fenestra. This is retained in the basal crocodylomorphs Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), and Dibothrosuchus (IVPP V7907), whereas in Litargosuchus (BP/1/5237), Kayentasuchus (UCMP 131830), Orthosuchus (SAM-K-409), and Protosuchus (AMNH FR 3016), the lateral edge of the paroccipital process is at the margin or medial to the lateral extent of the upper temporal fenestra.
110. Opisthotic, paroccipital processes: (0) directed laterally or dorsolaterally; (1) directed ventrolaterally (fig. 24) (Rauhut, 1997, 2003; Hwang et al., 2004; Smith et al., 2007).
As pointed out by Rauhut (2003), the distal ends of the paroccipital processes of most archosauriforms are laterally or dorsally directed. Among avian-line archosaurs, pterosaurs, Silesaurus, and ornithischians also have paroccipital processes that are either laterally or dorsally directed. In saurischians, such as Herrerasaurus (PVSJ 407), Coelophysis bauri (AMNH FR 7239), and Plateosaurus (AMNH FR 6810), the paroccipital processes are directed ventrally at the distal ends.
111. Opisthotic, ventral ramus ( = crista interfenestralis): (0) extends further laterally than lateralmost edge of exoccipital in posterior view; (1) covered by the lateralmost edge of exoccipital in posterior view (fig. 24) (Gower, 2002).
As explained by Gower (2002), the ventral ramus of the opisthotic extends further laterally than that of the exoccipital plesiomorphically within archosauriforms. In some aetosaurs (e.g., Longosuchus, TMM 31185–97) and crocodylomorphs (Gower, 2002), the ventral ramus of the opisthotic is nearly hidden by the exoccipitals in posterior view. This character cannot be scored for Revueltosaurus at present. Among avian-line archosaurs, the ventral ramus of the opisthotic extends further laterally than the lateralmost edge of the exoccipital in sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810) and Herrerasaurus (PVSJ 407). The ventral ramus of the opisthotic is covered by the lateralmost edge of the exoccipital in posterior view in Silesaurus (ZPAL Ab III 364/4) and theropods (e.g., Dilophosaurus, UCMP 37302).
112. Opisthotic, distal end of the ventral ramus: (0) does not or barely makes contact with prootic anteroventral to fenestra ovalis; (1) has extended contact with prootic (fig. 23) (Gower, 2002).
In non-crocodylomorph crocodylian-line archosaurs, non-archosaurian archosauriforms, and basal avian-line archosaurs, the opisthotic descends ventrally to rest on the basisphenoid/parabasisphenoid and has little if any contact with the prootic. The process separates the fenestra ovalis from the metotic foramen. In crocodylomorphs, the ventral ramus of the opisthotic meets the prootic on its anterior edge and has a ventrally extended contact (Gower, 2002).
113. Exoccipital, relative positions of the exits of the hypoglossal nerve (XII): (0) aligned in a nearly anteroposterior plane; (1) aligned subvertically (fig. 23) (new).
In nearly all basal archosauriforms there are two exits of the hypoglossal nerve (XII) through the exoccipital. In nearly all archosaurs, the two exits of the hypoglossal nerve are aligned in a nearly anteroposterior plane where one foramen is located posterior of the other. In Silesaurus (ZPAL Ab III/364/4) and Lewisuchus (UNLR 01), one foramen lies dorsal to the other.
114. Exoccipital, lateral surface: (0) without subvertical crest ( = metotic strut); (1) with clear crest ( = metotic strut) lying anterior to both external foramina for hypoglossal nerve (XII); (2) with clear crest ( = metotic strut) present anterior to the more posterior external foramina for hypoglossal nerve (XII) (fig. 23) (modified from Gower, 2002).
Here, Gower's (2002) original formulation of this character is modified. All the archosaurs examined in this study have two exits for cranial nerve XII. The lateral ridge on the exoccipital is homologized with that of the metotic strut, a feature commonly referred to in theropod dinosaurs (Nesbitt et al., 2009c). Taxa without a distinct lateral ridge on the exoccipital include non-archosaurian archosauriforms, Arizonasaurus (MSM P4590; Gower and Nesbitt, 2006), and phytosaurs (Pseudopalatus prisintus UCMP 137319 and Paleorhinus UCMP 84810). A small change in the angle of the ventral portion of the exoccipital marking the posterior extent of the metotic opening is present in taxa scored as (0). The more posterior exit of XII is posterior to the angle change whereas the more anterior exit of XII is anterior to the angle change. The lateral ridge in taxa scored as (1) and (2) is present at the same location as the change of angle in taxa scored as (0). Furthermore, without exception, the lateral ridge is continued onto the lateral side of the basioccipital.
In taxa scored as (1), both foramina are posterior to a lateral ridge on the exoccipital. This includes crocodylomorphs (Gower, 2000). Originally, Gower (2000) scored aetosaurs the same as crocodylomorphs. However, I did not observe any aetosaur with both exits of XII posterior to the lateral ridge in this study. In Stagonolepis (MCZD 2–4), Longosuchus (TMM 311085-84b), and Revueltosaurus (PEFO 34561), one opening of XII lies anterior and the other lies posterior to the lateral ridge. An identical arrangement is also present in Batrachotomus (Gower, 2002) and Postosuchus kirkpatricki (Gower, 2002). Gower (2002) reported that Saurosuchus galilei (PVSJ 32) does not have a lateral ridge; however, Saurosuchus galilei is scored as having a lateral ridge. These taxa were scored as (2) here.
Gower (2002) did not consider avian-line archosaurs in his analysis. A lateral ridge on the exoccipital is clearly present in Lewisuchus (UNLR 01), Marasuchus (PVL 3970, 3872), and Silesaurus (ZPAL Ab III/364/4), Heterodontosaurus (SAM-K-337), Coelophysis (AMNH FR 7239), and Plateosaurus (AMNH FR 6810). Both exits of XII are present posterior to the lateral ridge and, thus avian-line taxa are scored as (1).
This character may be correlated with character 111 because a laterally extended lateral ridge on the exoccipital may hide the descending process of the opithotic in posterior view.
115. Exoccipitals: (0) meet along the midline on the floor of the endocranial cavity; (1) do not meet along the midline on the floor of the endocranial cavity (fig. 24) (modified from Gower and Sennikov, 1996; Gower, 2002).
Plesiomorphically among archosauriforms, the exoccipitals meet along the midline preventing the basioccipital from participating in the endocranial cavity (Gower, 2002). As noted by Gower and Sennikov (1996), most taxa scored as (0) have extensive contact along the anteroposterior length of the elements, whereas some non-archosaurian archosauriforms, such as Vjushkovia triplicostata, Fugusuchus, and Xilousuchus, have midline contact anteriorly but diverge posteriorly. Within Archosauria, many taxa have extensive midline contact of the exoccipitals. Among crocodylian-line archosaurs, crocodylomorphs such as Hesperosuchus (AMNH FR 6748), Sphenosuchus (SAM 3014) and Alligator, Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280) have large gaps between the exoccipitals, thus exposing the endocranial cavity.
Gower (2002) scored aetosaurs as having state (1) along with crocodylomorphs. Scoring state (1) for aetosaurs needs further discussion. In Longosuchus (TMM 31185–98), the exoccipitals meet at the midline. Martz and Small (2006) noted that the exoccipitals do not meet at the midline in Tecovasuchus (TTU-P 545). However, the exoccipitals are well ossified to the basioccipital, and the boundary between the elements is unclear. Furthermore, the exoccipitals are very close together whether they are touching or not. In Desmatosuchus smalli, the exoccipitals do not touch (Small, 2002). All 15 aetosaur braincases from the Placerias Quarry (UCMP V A269) have no gap between the exoccipitals on the dorsal surface of the basioccipital. The condition in Aetosaurus and Stagonolepis is unclear. In their potentially close relative, Revueltosaurus (PEFO 34561), the exoccipitals touch on the midline. Furthermore, in aetosaur taxa where the exoccipitals do not meet, the bases of the exoccipitals are near the midline. This condition contrasts with the highly separated exoccipitals of crocodylomorphs (Sphenosuchus, SAM 3014; Hesperosuchus agilis, AMNH FR 6758). Thus, the plesiomorphic state in aetosaurs cannot be summarized as state (1).
Among avian-line archosaurs, the exoccipitals meet at the midline in derived pterosaurs (Pteranodon YPM 2707), Marasuchus (PVL 3872), and Silesaurus (Ab III 364/4). The exoccipitals are well separated in Lesothosaurus (Sereno, 1991b) and Heterodontosaurus (SAM-PK-1332), as well as in Herrerasaurus (PVL 407), Plateosaurus (AMNH FR 6810), and Coelophysis bauri (AMNH FR 7239).
This character can be scored from a disarticulated basioccipital. Taxa that are scored as state (0) have flat exoccipital surfaces that meet at an anteroposteriorly oriented ridge. In contrast, in taxa scored as (1), a U-shaped depression separates the exoccipital articular facets of the basioccipital. Slight disarticulation of the exoccipitals can hinder scoring of this character.
116. Pneumatization of bony elements of the middle ear cavity: (0) absent or restricted; (1) well developed (Gower, 2002).
As described by Gower (2002), only crocodylomorphs have state (1) among suchians. Sphenosuchus (SAM 3014), Hesperosuchus agilis (AMNH FR 6758), Kayentasuchus (UCMP 131830), Dibothrosuchus (IVPP V 7907), Protosuchus richardsoni (UCMP 131827), and Alligator have extensive pneumatization of the middle ear cavity (Gower, 2002).
117. Vestibule, medial wall: (0) incompletely ossified; (1) almost completely ossified (Gower, 2002).
Gower (2002) found that the medial wall of the vestibule is completely ossified in nearly all suchians. Later, Gower and Nesbitt (2006) scored Arizonasaurus as (0). However, after a careful inspection of taxa with completely ossified medial walls of the vestibule, it is clear that the larger specimen of Arizonasaurus (MSM P4647) has a completely ossified medial wall. Therefore, Arizonasaurus is scored as (1) here. Furthermore, the absence of complete ossification of the medial wall in a smaller specimen of Arizonasaurus (MSM P4590) supports Gower and Weber's (1998) hypothesis that this character is somewhat problematic given the absence of ontogenetic data in basal archosaurs.
Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280) both have fully ossified medial walls of the vestibule. This character cannot be scored for Marasuchus, and the medial wall seems to be unossified in Silesaurus (ZPAL Ab III 361/35) as well as in Plateosaurus (AMNH FR 6810).
118. Lagenar/cochlea recess: (0) absent or short and strongly tapered; (1) present and elongated and tubular (Gower, 2002).
The lagenar/cochlea recess is located just anterior to the ventral ramus of the opisthotic and posterior to the opening of cranial nerve VII in a typically unossified gap (Gower, 2002). A well-defined recess for the lagena/cochlea is absent in phytosaurs and non-archosaurian archosauriforms (Gower, 2002). In crocodylian-line archosaurs, the recess is elongated and tubular, terminating in a blind fossa well ventral of the contact between the exoccipital and the basioccipital. In avian-line archosaurs, the character cannot be scored in pterosaurs or Marasuchus, but it is elongated and tubular in Silesaurus (ZPAL Ab III 361/35) and Plateosaurus (AMNH FR 6810).
119. Crista vestibuli: (0) absent; (1) present (Gower, 2002).
As pointed out by Gower (2002), this character is very difficult to score given the paucity of well preserved and described braincase material for basal archosaurs. Nevertheless, it remains a clear synapomorphy within Crocodylomorpha.
120. Lagenar/cochlear prominence: (0) absent; (1) present (fig. 23) (Gower, 2002).
Walker (1990) reported a cochlear prominence, an external feature present on the prootic and opisthotic, in crocodylomorphs. Here, the bone is thickened on the lateral surface covering the lagenar/cochlear recess. State (1) is present in crocodylomorphs (Gower, 2002).
121. Eustachian tubes: (0) not enclosed by bone; (1) partially enclosed by bone; (2) fully enclosed by bone. ORDERED (figs. 23–24) (Gower, 2002).
Eustachian tubes are not enclosed by bone in non-crocodylomorph crocodylian-line archosaurs, non-archosaurian archosauriforms, and basal avian-line archosaurs (Gower, 2002). A small groove/channel at the lateral junction of the basioccipital and the parabasisphenoid in Sphenosuchus (SAM 3014) and Dibothrosuchus was previously hypothesized to house the eustachian tubes (Walker, 1990; Gower, 2002; Wu and Chatterjee, 1993). A similar channel is present in Postosuchus kirkpatricki (TTU-P 9000; UCMP 138842), and given the similarity in position and morphology with that of Sphenosuchus and Dibothrosuchus (IVPP V 7907), Postosuchus is scored as (1). Protosuchus richardsoni (UCMP 131827), P. haughtoni (BP/1/4242), Orthosuchus (SAM-K-409), and Alligator are scored as (2) because the eustachian tubes are fully enclosed by bone.
122. External foramen for abducen nerve: (0) between parabasisphenoid and prootic; (1) within prootic only; (2) within parabasisphenoid only (Gower, 2002).
The abducens nerve exits between the parabasisphenoid and prootic in a number of non-archosaurian archosauromorphs including Prolacerta (BP/1/2675), Proterosuchus (BP/1/3993), and Euparkeria (Gower, 2002). Gower and Sennikov (1996) suggested the external foramen for the abducens nerves passing through the prootic represented a potential synapomorphy only for erythrosuchians (Erythrosuchus, Vjushkovia triplicostata, Shansisuchus) and Xilousuchus. State (1) is also present in Arizonasaurus. In Revueltosaurus (PEFO 34561), and possibly in the aetosaurs Tecovasuchus (TTU-P 9222) and Typothorax coccinarum (TTU-P 9214), the external foramen for the abducens nerves only passes through the parabasisphenoid as in crocodylomorphs (Gower, 2002).
In Plateosaurus (AMNH FR 6810), the external foramen for the abducen nerves passes through the prootic only. This seems to also be the case in the theropod Piatnitzkysaurus (PVL 4073; Rauhut, 2004) and the basal dinosauriform Silesaurus (ZPAL Ab III 364/4).
This character is difficult to score given that archosaurs tend to nearly obliterate the suture between the parabasisphenoid and the prootic and because of the difficulty in examining the anterior side of the dorsal sellum.
123. Parabasisphenoid, external foramina for passage of abducens nerves: (0) on the underside of a horizontal surface; (1) on the anterior of a more vertical, upturned process (Gower, 2002).
In nearly all archosaurs, the external foramina for passage of the abducens nerves lies on the anterior surface of a vertically upturned process ( = dorsum sellae). This is also true of basal dinosauriforms. I disagree with Gower (2002) for scoring Batrachotomus as (0). The condition in Batrachotomus is unknown because the external foramina for passage of the abducens nerve cannot be located with confidence.
124. Basipterygoid processes: (0) of moderate size; (1) markedly enlarged (fig. 23) (Gower, 2002; Clark et al., 2004).
As stated by Gower (2002), the basitubera are usually about the same size if not bigger than the basipterygoid processes within Archosauriformes. In non-crocodyliform crocodylomorphs, the basipterygoid processes are enlarged relative to the plesiomorphic condition. This character is equivalent to “basipterygoid processes simple, without large cavity (0) or greatly expanded, with large cavity (1)” of Clark et al. (2000). Clark et al. (2000) scored all crocodylomorphs except Pseudhesperosuchus as having state (1). Crocodyliforms do not have basipterygoid processes, and so they are scored as inapplicable.
125. Exit of cranial nerve VII: (0) small, only slightly larger than cranial nerve XII; (1) large (new).
In nearly all archosauriforms, the exit of the facial nerve (VII) is a small foramen that pierces the prootic just ventral to the crista prootica (see Gower and Sennikov, 1996; Gower and Weber, 1998; Gower, 2002; Gower and Walker, 2002; Dzik, 2003; Gower and Nesbitt, 2006). However, in Postosuchus alisonae (UNC 15575) and Postosuchus kirkpatricki (TTU-P 9000), the exit of the facial nerve is very large relative to that of closely related taxa. Even though the vague terms small and large are used in these character states, the great size of the opening in Postosuchus relative to that of other closely related taxa is remarkable. The size of the exit of the facial nerve in Postosuchus rivals that in size of the opening for the trigeminal nerve.
126. Supraoccipital: (0) excluded from dorsal border of foramen magnum by mediodorsal midline contact between opposite exoccipitals; (1) contributes to border of foramen magnum (fig. 24) (Gower, 2002).
The supraocciptial is excluded from the dorsal border of the foramen magnum by the exoccipitals in Erythrosuchus (Gower, 1997), Proterosuchus (BP/1/3993), Prolacerta (BP/1/2675), and possibly Mesosuchus (Dilkes, 1998). In Euparkeria and most basal archosaurs, the supraoccipital participates in the foramen magnum (Gower, 2002). Among dinosauromorphs, it is unclear in Marasuchus (PVL 3870); the supraoccipital participates in the foramen magnum in Silesaurus (ZPAL Ab III 364/4), Coelophysis bauri (AMNH FR 7239), Herrerasaurus (PVL 407), and Heterodontosaurus (SAM-PK-1332). This character is difficult to score in many taxa because the suture between the supraoccipital and the exoccipitals is often obliterated.
127. Supraoccipital, rugose ridge on the anterolateral edges: (0) absent; (1) present (fig. 24) (new).
The anterolateral surface of the supraoccipitals of nearly all archosauriforms in this study is smooth. In Lewisuchus (UNLR 01) and Silesaurus (ZPAL Ab III/364/4), the anterolateral surface bears a dorsolateral oriented rugose ridge on each side of the midline.
128. Pila antotica: (0) ossified mainly by prootic and laterosphenoid, such that laterosphenoid-parabasisphenoid contact is absent; (1) ossified largely by laterosphenoid and parabasisphenoid, with contact occurring between these two elements anterior to the trigeminal foramen in the adult braincase (fig. 23) (Gower, 2002).
Here, I keep the character formulation presented by Gower (2002). However, the character essentially scores the presence or absence of laterosphenoid and parabasisphenoid contact. As discussed by Gower (2002), the prootic separates the laterosphenoid from the parabasisphenoid in taxa scored as (0). Therefore, taxa in which the prootics contact at the midline at the anterior portion of the endocranial cavity can be scored as (0) because the prootic separates the laterosphenoid from the parabasisphenoid. Here, only crocodylomorphs are scored as (1).
129. Perilymphatic foramen: (0) with an incompletely ossified border; (1) border entirely ossified such that the ventral ramus of the opisthotic forms a perilymphatic loop incorporating a loop closure suture with itself (fig. 23) (Gower, 2002).
Gower (2002) used the presence of state (1) to suggest a close relationship of aetosaurs and crocodylomorphs to the exclusion of rauisuchians (Batrachotomus was the only “rauisuchian” taxon scored for this character). Poor preservation, absence of access within the metotic foramen, and poor preservation in this delicate region hampers the scoring of this character in most taxa. Even though this character requires extraordinary preservation to score, the presence of state (1) in crocodylian-line archosaurs is a potential synapomorphy (Gower and Walker, 2002). Therefore, I retain this character. In avian-line archosaurs, Silesaurus (ZPAL Ab III/364/4) has an incompletely ossified border, whereas Dilophosaurus (UCMP 16468) apparently has an ossified border.
130. Perilymphatic foramen: (0) in a medial position and oriented so as to transmit the perilymphatic duct out of the otic capsule in a posteromedial or posterior direction; (1) foramen positioned more laterally so that the perilymphatic duct is transmitted posterolaterally/laterally and the foramen is at least partly visible in lateral view (fig. 23) (Gower, 2002).
This character can be scored from the orientation of the opisthotic (Gower, 2002). This character was used by (Gower, 2002) to hypothesize a sister-group relationship between aetosaurs and crocodylomorphs. After examining the braincase of Stagonolepis (MCZD 4-2), I cannot score the taxon as (1) as it appears the braincase is slightly compressed. The orientation of the descending process of the opisthotic (mediolaterally) is like that of other non–crocodylian-line archosaur. Therefore, the foramen cannot be oriented laterally. In Sphenosuchus (SAM 3014) and Alligator, the perilymphatic foramen faces laterally.
131. Foramen for trigeminal nerve and middle cerebral vein: (0) combined and undivided; (1) at least partially subdivided by prootic; (2) fully divided (fig. 22) (modified from Gower and Sennikov, 1996; Gower, 2002).
Partially subdivided openings for the trigeminal nerve and middle cerebral vein are present in Batrachotomus (SMNS 80260) and basal crocodylomorphs (Gower, 2002). In taxa scored as (1) here (e.g., Batrachotomus and Sphenosuchus), the foramen is partially subdivided by bony prongs that penetrate the foramen, but both the trigeminal nerve and middle cerebral vein exit through the same opening. In Stagonolepis (MCZD 4-2; Walker, 1990), Longosuchus (TMM 31185–98), and Desmatosuchus smalli (Small, 2002; Parker, 2005), the trigeminal nerve and middle cerebral vein exit through separate foramina. The posterior border of the foramen for the middle cerebral vein is present on the prootic. Furthermore, a thin bridge of bone in all three aetosaur examples separates the two foramina.
Among dinosauriforms, Silesaurus (ZPAL Ab III/364/4) has a single foramen, as do ornithischians. According to Rauhut (2003), the middle cerebral vein exits through a separate foramen than the trigeminal nerve in Plateosaurus, Massospondylus, Dilophosaurus, and Allosaurus, but not in Coelophysis rhodesiensis, Coelophysis bauri (CM 29894), or ornithischians.
132. Foramen or groove passing above and into the dorsal end of the metotic foramen: (0) absent; (1) present (Gower, 2002).
Gower (2002) presented this character to unite a subset of pseudosuchians including Postosuchus kirkpatricki, Batrachotomus, and extant crocodylians (e.g., Crocodylus). As described by Gower (2002), this opening may be a discrete passage for the posterior cerebral/cephalic vein. Furthermore, this character is difficult to score because the location of the foramen requires exceptional preservation. A groove in Sphenosuchus (SAM 3014) is located in the same position as the foramen in Postosuchus kirkpatricki and Batrachotomus (Gower, 2002), and these features may be homologous. However, Gower (2002) scored Sphenosuchus as absent. Here, I suggest the condition in Dibothrosuchus (IVPP V 7907) is not clear even though it was scored as (0) by Gower (2002). Aetosaurs lack any groove or foramen (Gower, 2002), and I confirmed this with an exquisitely preserved aetosaur braincase from the Placerias Quarry (UCMP 27414). Furthermore, Arizonasaurus (MSM P4590) has a small foramen in the same place as Batrachotomus (scored as [0] in Gower and Nesbitt, 2006).
133. Auricular recess: (0) largely restricted to prootic; (1) extends onto internal surface of epiotic/supraoccipital (Gower, 2002).
The auricular recess is almost exclusively restricted to the prootic in non-archosaurian archosauriforms and phytosaurs (Gower, 2002). In suchians, the auricular recess extends onto the internal surface of the epiotic/supraoccipital (Gower, 2002). This is also the case in dinosauromorphs including Marasuchus (PVL 3872), Lewisuchus (UNLR 01), and Silesaurus (ZPAL Ab III 364/4).
This character cannot be scored in taxa where the prootic and epiotic/supraoccipital have coossified. This is the case in Plateosaurus (AMNH FR 6810). Even though Arizonasaurus was scored as (0) by Gower and Nesbitt (2006), a reexamination of braincases (MSM P4590, P4647) indicates that the suture between the prootic and epiotic/supraoccipital cannot be discerned.
134. Skull length: (0) less than 50% of length of the presacral vertebral column; (1) more than 50% of the length of the presacral vertebral column (Sereno, 1991a; Benton, 1999).
Sereno (1991a) used this character to unite Scleromochlus and pterosaurs. Here it is found in pterosaurs only because Scleromochlus is not included.
135. Skull length: (0) longer than two-thirds of the femoral length; (1) shorter than two-thirds of the femoral length (Gauthier, 1986).
Langer and Benton's (2006) description of this character is detailed and complete; therefore, little can be added. I agree with their interpretations and score sauropodomorphs as (1).
136. Antorbital fenestra: (0) absent; (1) present (Juul, 1994; Gower and Sennikov, 1997; Dilkes, 1998; Nesbitt et al., 2009a).
The presence of an antorbital fenestra supports Proterosuchus + Archosauria in analyses by Benton and Clark (1988), Gauthier et al. (1988), and Juul (1994). The easily recognizable feature is present in all archosauriforms ancestrally. To score this character confidently, the lacrimal should also be present in the taxon being scored.
137. Antorbital fossa: (0) restricted to the lacrimal; (1) restricted to the lacrimal and dorsal process of the maxilla; (2) present on the lacrimal, dorsal process of the maxilla and the dorsal margin of the posterior process of the maxilla (the ventral border of the antorbital fenestra). ORDERED (figs. 15–16) (Nesbitt et al., 2009a).
The antorbital fossa of Proterosuchus (BSP 514) is restricted to the lacrimal. In Erythrosuchus (BP/1/ 5207), the antorbital fossa expands onto the anterodorsal portion of the posterior process of the maxilla. This differs from the condition in archosaurs where the antorbital fossa is located on nearly the entire dorsal margin of the posterior process of the maxilla. A similar fossa is present on the dorsal surface of the maxilla of Shansisuchus (Young, 1964). Therefore, Erythrosuchus is scored as state (1). In the proterochampsians Chanaresuchus (PVL 4575) and Tropidosuchus (PVL 4604), and as observed by Sereno and Arcucci (1990), Gualosuchus (PVL 4576) and Proterochampsa (MCZ 3408), the antorbital fossa expands onto the lacrimal and onto the dorsal process of the maxilla. As with proterochampsians, Euparkeria (SAM 5867) has a similar arrangement of the antorbital fossa. Within phytosaurs, the antorbital fossa is present on the dorsal process of the maxilla and the lacrimal in the primitive phytosaurs Parasuchus (Chatterjee, 1978) and ‘Paleorhinus’ scurriensis (TTU-P 00539; Langston, 1949; Stocker, 2010) but absent on the maxilla and lacrimal in Smilosuchus (UCMP 27200) and Pseudopalatus pristinus (NMMNH 31292). In aetosaurs (Aetosaurus, SMNS 5770), Riojasuchus (PVL 3827), Gracilisuchus (MCZ 4117), Turfanosuchus (IVPP V 3237), Ticinosuchus (PIZ T2817), “rauisuchians,” crocodylomorphs, and dinosauriforms (e.g., Silesaurus ZPAL Ab III/361/26; Herrerasaurus PVSJ 407), the antorbital fossa is located on the lacrimal, the dorsal process of the maxilla, and nearly the entire dorsal margin of the posterior process of the maxilla.
138. Lateral ( = external) mandibular fenestra: (0) absent; (1) present (fig. 16) (Benton and Clark, 1988; Juul, 1994; Bennett, 1996; Nesbitt et al., 2009a).
A lateral mandibular fenestra is present in nearly all archosauriforms plesiomorphically as indicated in the analysis by Juul (1994). A lateral mandibular fenestra has been reported to be small or absent in Proterosuchus (Charig and Reig, 1970; Cruickshank, 1972). Welman and Flemming (1993), confirmed by Juul (1994), and Welman (1998), showed that the well-preserved specimens of Proterosuchus have a small lateral mandibular fenestra. However, given the small size of the opening, the presence of this character in Proterosuchus deserves more discussion. The small fenestra forms at the junction of the dentary, angular, and surangular in Proterosuchus (RC 96, TM 201; Welman, 1998). Here, the mandibular elements do not have a distinct concave region forming an edge as in Erythrosuchus + Archosauria. However, though there are differences, the lateral mandibular fenestra occupies the same area and is composed of the same elements in both Proterosuchus and other archosauriforms. The small gap may be a consequence of the slight disarticulation of the mandibular elements, but a lateral mandibular fenestra is clearly present in QR 1484 (listed as NMC 3014 in fig. 3 of Welman, 1998). Therefore, Proterosuchus is scored as having a lateral mandibular fenestra. The presence or absence of a lateral mandibular fenestra is difficult to determine in isolated mandibular elements of taxa near the base of Archosauriformes. It is unclear whether Vancleavea has a lateral mandibular fenestra. If present, it is very small like that of Proterosuchus.
Pterosaurs have been cited as lacking a lateral (or external) mandibular fenestra (Bennett, 1996). A lateral mandibular fenestra is clearly absent in the holotype of Eudimorphodon (Wild 1978). However, a mandibular fenestra is clearly present in a specimen referred to Eudimorphodon sp. (BPS 1994 I 51; Wild, 1993) and Dimorphodon (BMNH R1034) (S.J.N., personal obs.).
139. External nares, position: (0) terminal (at the anterior part of the skull); (1) nonterminal, posterior rim of nares anterior of anterior rim of antorbital fenestra; (2) nonterminal, posterior rim of nares posterior of anterior rim of antorbital fenestra. ORDERED (fig. 16) (Hungerbühler, 2002).
In nearly all archosauriforms, the nares are positioned on the anterior portion of skull dorsal to the body of the premaxilla. In phytosaurs, the nares open dorsally and are located on the surface of the skull table. Hungerbühler (2002) and Stocker (2008) found that the most basal phytosaurs (e.g., Paleorhinus; TMM 31025–172) have dorsally oriented nares where the posterior rim of nares is in front of the anterior rim of the antorbital fenestra, whereas other phytosaurs (Smilosuchus, USNM 18313; Pseudopalatus, UCMP 34249) have the posterior rim of the nares posterior to the anterior rim of the antorbital fenestra.
140. External nares, directed: (0) laterally; (1) dorsally (fig. 16) (modified from Sereno, 1991a; Nesbitt et al., 2009a).
The external nares of non-archosauriform archosauromorphs, Proterosuchus (NM QR 1484), Erythrosuchus (BP/1/5207), Euparkeria (SAM 5867), and most basal members of the Archosauria opens laterally. Alternatively, the external nares of proterochampsians (e.g., Chanaresuchus, PVL 4586; Tropidosuchus, PVL 4601), Vancleavea (GR 138), and phytosaurs (Camp, 1930) open dorsally. This character was suggested to correlate with an aquatic and semiaquatic lifestyle (Sereno, 1991a).
141. Posttemporal opening, mediolateral width: (0) equal to or greater than half the diameter of the foramen magnum; (1) less than half the diameter of the foramen magnum or absent (modified from Sereno and Novas, 1994; Novas, 1996; Benton, 1999).
This character replaces “post-temporal opening fenestra (0) or foramen (1)” of Benton (1999) because of the arbitrary distinction between a small fenestra and a large foramen. Sereno and Novas (1994) described the relative size of the posttemporal opening compared to the foramen magnum, and that division is used here. Sereno and Novas (1994) reported that the posttemporal opening is larger than half the foramen magnum in pterosaurs, Saurosuchus, Gracilisuchus, aetosaurs, ornithosuchids, and phytosaurs and reduced in dinosaurs, proterochampsians, and Sphenosuchus. A posttemporal opening is absent in crocodylomorphs (Sereno and Novas, 1994).
142. Orbit, shape: (0) circular or elliptical; (1) tall and narrow (the “keyhole-shaped orbit”; maximum width is less than half the maximum height); (2) with distinct ventral point surrounded by V-shaped dorsal processes of jugal (figs. 16–17, 19) (Benton and Clark, 1988; Parrish, 1993; Gower, 2000; Benton and Walker, 2002).
The orbital shape in most archosauriforms is rounded or slightly elliptical. This includes aetosaurs (e.g., Aetosaurus, SMNS 5770), Revueltosaurus (PEFO 34561), Gracilisuchus (MCZ 4117), crocodylomorphs (e.g., Dromicosuchus UNC 15574), and phytosaurs (e.g., Smilosuchus, UCMP 27200). Benton and Clark (1988) diagnosed Rauisuchidae with a “keyhole-shaped orbit.” Although “keyhole-shaped orbit” does describe a subset of suchians, this character is better described by the ventral process of the postorbital invading the orbit in another character described above. However, Saurosuchus (PVL 2062) and Prestosuchus (UFRGS 0156-T) do not have jugals that invade the orbit, but they have a similarly tall and narrow orbit like that of Postosuchus kirkpatricki (TTU-P 9000) and Batrachotomus (Gower, 1999). The taxa scored as (1) have a ventral orbital radius of curvature that is less than that of the dorsal orbital radius of curvature. The shape of the orbit of Erythrosuchus (BP/1/5207) and those of some theropods (Rauhut, 2003; Hwang et al., 2004) is convergent with that of a subset of suchians. The ornithosuchids Riojasuchus (PVL 3827) and Ornithosuchus (BMNH R3142) have unique V-shaped dorsal processes of the jugal that create an inverted teardrop shape for the orbit and are thus scored as (2).
143. Supratemporal fenestra, position: (0) dorsally exposed; (1) lateral exposed (fig. 17) (Long and Murry, 1995; Heckert and Lucas, 1999; Parker, 2007).
In nearly all diapsids, the supratemporal fenestrae are dorsally oriented. In the aetosaurs Longosuchus (TMM 31185–98), Aetosaurus (SMNS 5770), Stagonolepis (Walker, 1961), Neoaetosauroides (Desojo and Baez, 2007), Aetosauroides (Casamiquela, 1961), and Desmatosuchus (Small, 2002), the supratemporal fenestrae open more laterally than dorsally. The supratemporal fenestrae open dorsally in Revueltosaurus (Parker et al., 2005). In some specimens, dorsally oriented supratemporal fenestrae can be observed in lateral view. However, these are still scored as opening dorsally here.
144. Supratemporal fossa: (0) absent anterior to the supratemporal fenestra; (1) present anterior to the supratemporal fenestra (figs. 18–19) (modified from Gauthier, 1986; Novas, 1996).
In its original formulation, Gauthier (1986) used this character to diagnose Dinosauria and focused on those elements the supratemporal fossa was present on. The character is rewritten here in order to test the homology of the extension of the supratemporal fossa anterior to the supratemporal fenestra regardless of which element is anterior to the supratemporal fenestra. Furthermore, in the original form, Gauthier (1986) scored the entire supratemporal fossa as extensive or not extensive. In this vague wording, the presence of the supratemporal fossa in different regions cannot be evaluated. Additionally, it is not clear what constitutes extensive in character state (0) versus state (1).
The supratemporal fossa is present anterior to the supratemporal fenestra in Dinosauria, but absent in the close relative Silesaurus (ZPAL Ab III/361) (contra Langer and Benton, 2006). Among crocodylian-line archosaurs, crocodylomorphs (e.g., Hesperosuchus, Sphenosuchus, Protosuchus) possess a supratemporal fossa anterior to the supratemporal fenestra. Postosuchus kirkpatricki (TTU-P 9000; UCMP 27479) and Batrachotomus (SMNS 52970) also possess a supratemporal fossa anterior to the supratemporal fenestra. Nevertheless the supratemporal fossa of Postosuchus lies entirely on the postfrontal. Conversely, all the other taxa scored as (1) lack a postfrontal; instead, the supratemporal fossa is located on the frontal.
145. Supratemporals: (0) present; (1) absent (fig. 16) (Gauthier, 1984; Benton, 1985a; 311990; Benton and Clark, 1988; Bennett, 1996; Gower and Sennikov, 1997; Dilkes, 1998; Nesbitt et al., 2009a).
The supratemporal element lies between the squamosal and the parietal on the posterior margin of the skull roof. Supratemporals are present in Mesosuchus (Dilkes, 1998), a number of Prolacerta specimens (Modesto and Sues, 2004), and in Proterosuchus. Supratemporals are not present in Erythrosuchus + Archosauria. As noted by Gauthier (1984) and Modesto and Sues (2004), the presence or absence of supratemporals should be based on nearly complete, articulated skull material because the supertemporals may be easily lost during fossilization. For example, Modesto and Sues (2004) listed Prolacerta with or without supratemporals preserved. A facet for the supratemporal on the parietal may indicate the presence of the element in incomplete specimens, but care must be taken when scoring this character.
146. Postparietals: (0) present; (1) absent (fig. 16) (modified from Juul, 1994; Bennett, 1996; Dilkes, 1998; Nesbitt et al., 2009a).
Postparietals are present in Proterosuchus (BSP 514), Erythrosuchus (BP/1/ 5207), Shansisuchus (Young, 1964), and Euparkeria (Ewer, 1965), but they are not in Prolacerta (UCMP 37151), Mesosuchus (Dilkes, 1998), and Archosauria. The condition in Trilophosaurus (various TMM specimens) is difficult to determine given the preservation of the cranial material at the posterior portion of the skull. As explained by Juul (1994), the postparietals of Proterosuchus (BSP 514), Erythrosuchus (BP/1/ 5207), Euparkeria (Ewer, 1965), and Shansisuchus (Young, 1964) are fused into one element, whereas they remain two elements in Youngina (Romer, 1956) and Petrolacosaurus (Carroll, 1988).
Juul (1994) also highlighted the presence of the postparietal in a number of other archosauriform groups including phytosaurs (Westphal, 1976), Longosuchus (originally referred to Typothorax by Sawin, 1947), Gracilisuchus (Romer, 1972c), and Prestosuchus (UFRGS 0156-T; Barberena, 1978). However, I failed to find a separate ossification between the parietals in my examination of these specimens. In his review of phytosaur osteology, Westphal (1976) cited Camp (1930) for the presence of a postparietal in phytosaurs. Camp (1930) reported the postparietal ( = interparietal of Camp, 1930: fig. 29) and made this observation from UCMP 27200 (the holotype of Machaeroprosopus gregorii [ = Smilosuchus gregorii]). However, much of what Camp (1930) identified as the postparietal is actually the supraocciptial; Camp (1930) mistook cracks as sutures deliminating a separate ossification dorsal to the foramen magnum. The posterior dorsal portion of the skull roof of Prestosuchus (UFRGS 0156-T) is poorly preserved, and no clear suture can be found. As with Vancleavea, Gracilisuchus possesses a small posteriorly pointed prong at the midline. In Gracilisuchus (MCZ 4117), this peg is the dorsal exposure of the supraoccipital peg. The posteromedial corners of the parietals dorsally overlap the peg. Therefore, Gracilisuchus does not have a postparietal.
147. Palpebral(s): (0) absent; (1) present (figs. 19–20) (new).
This character may be hard to score given the erratic distribution of the element. Currently, palpebrals are known in suchians and have not been found in the many well-preserved skulls of phytosaurs or the many well-preserved skulls of Euparkeria or proterochampsians. Taxa known from disarticulated and incomplete orbital regions should be scored as unknown. At this point, Aetosaurus (Schoch, 2007), Neoaetosauroides (Desojo and Baez, 2007), Saurosuchus (PVSJ 32), Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Polonosuchus silesiacus (ZPAL Ab III/563), Hesperosuchus “agilis” (Clark et al., 2000), Junggarsuchus sloani (Clark et al., 2004), and crocodyliforms can be scored as having palpebral(s) among non-crocodyliform crocodylian-line archosaurs.
148. Palpebral(s) size: (0) roughly the same; (1) one dominant palpebral that is at least twice the size of the other (fig. 25) (new).
Fig. 25
Photographs of the palepbral of Postosuchus (UCMP 140035) in A, dorsal, B, lateral, and C, ventral views. A line drawing of the ventral view of UCMP 140035 is presented in D, highlighting the complex articular surfaces and the large circular element in the center of the palpebral; E, reconstruction of dorsal view of Postosuchus kirkpatricki by Chatterjee (1985) illustrating the palpebral and its relationship to other skull bones. The gray color highlights the palpebral in the reconstruction. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
The three palpebrals in the orbit of Aetosaurus are all nearly the same size although the anteriormost palpebral is usually 10%–20% larger than the others (Schoch, 2007). In spite of the variation in the size of the anterior palpebral of Aetosaurus, it is never dominant or nearly twice the size of the other palpebrals. In Neoaetosauroides (PVL 5698), one preserved palpebral is located in the anterior portion of the orbit. The single bone is rather small and is proportional to the anteriormost palpebral element of Aetosaurus.
The total number of palpebrals in the orbit of Postosuchus kirkpatricki (TTU-P 9000) and Hesperosuchus is not clear (they both clearly have at least a single palpebral though). However, it is evident that the single palpebrals of Postosuchus and Hesperosuchus are large and dominate in size over other potential palpebrals. If the triangular bone of Postosuchus is composed of three palpebrals, it is clear that the large, circular palpebral dominates in size. Likewise, if Hesperosuchus had a second or third palpebral, it would be overshadowed in size by the large circular palpebral. This character hypothesizes the homology between the large, circular palpebral element of Postosuchus with that of Hesperosuchus, Junggarsuchus, and crocodyliforms.
149. Palpebral(s): (0) separated from the lateral edge of the frontals; (1) extensively sutured to each other and to the lateral margin of the frontals (fig. 25) (Pol and Norell, 2004; Pol et al., 2009).
Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Saurosuchus (PVSJ 32), and Polonosuchus silesiacus (ZPAL Ab III/563), as well as some crocodyliforms, have palpebrals that are fully integrated into the skull table. The palpebral forms a distinct suture with the frontal, prefrontal, and postfrontal. Aetosaurus (SMNS 5770) is scored as (0) because the palpebral elements are not sutured to the frontal even though the palpebral elements are sutured together. Furthermore, taxa that have not been found with palpebrals can be scored because the lateral sides of the frontal, prefrontal, and postfrontal bear distinct articular facets in taxa scored as (1).
150. Separate ossification anterior to the nasals surrounded by the premaxilla: (0) absent; (1) present (fig. 16) (modified from Sereno, 1991a).
This above character is rewritten to incorporate Sereno's (1991a) character “septomaxilla, present or absent.” The term septomaxilla for the structure in phytosaurs is abandoned because it assumes homology among the structure of phytosaurs and other amniotes with septomaxillae (Stocker, 2008). The phylogenetic position of phytosaurs previously found (Gauthier, 1984; Benton and Clark, 1988; Sereno, 1991a; Parrish, 1993; Juul, 1994) within archosauriforms suggests that the “septomaxilla” of non-archosauriform amniotes and the “septomaxilla” of phytosaurs are not homologous. Phytosaurs, Prolacerta (UCMP 37151), and Proterosuchus are scored as (1).
151. Predentary: (0) absent; (1) present (fig. 20) (Sereno, 1986; Butler et al., 2007, 2008b).
The presence of a predentary was long cited as a synapomorphy of Ornithischia (e.g., Norman, 1984; Gauthier, 1986; Sereno, 1986, 1999; Butler, 2005; Butler et al., 2007; 2008b). The predentary is a separate, single ossification that lies anteriorly between the dentaries in ornithischians.
Ferigolo and Langer (2007) argued that the edentulous “beak” of Sacisaurus and Silesaurus is homologous with the predentary of ornithischians. The authors hypothesized that the predentary originated from the paired dentaries of archosaurs and cite examples of an independent origination of the structure on the anterior portion of the dentary in extinct birds and teleosts. Sacisaurus and Silesaurus do have an anterior dentary that is predentary-like, have an anteriorly tapering tip, and anterior portion of the dentary is edentulous. In Sacisaurus and Silesaurus, the anterior portion of the dentary is not a separate ossification separated from the rest of the dentary by a suture. The suture is not present on the exposed medial surfaces (Ferigolo and Langer, 2007: fig. 3I). The suture reported by Ferigolo and Langer (2006) in MCN PV10061 does not extend to the ventral margin. This also is the case in MCN PV10042. In the largest specimen, MCN PV10041 (holotype), there is no trace of the suture (S.J.N., personal obs). Instead, there is a foramen at the ventral extent of the hypothesized suture in the other dentaries. The preceeding evidence suggests that there is no suture between the anterior portion of the dentary and the rest of the dentary. Therefore, Silesaurus and Sacisaurus are not scored as having a predentary. Furthermore, aetosaurs also have a similar anterior end of the dentary and it is clear that the tapering anterior end is composed solely of the dentary (Long and Murry, 1995; Parker, 2007).
152. Anterior half of the dentary, position of the Meckelian groove: (0) dorsoventral center of the dentary; (1) restricted to the ventral border (fig. 27) (new).
In most archosauriforms, the Meckelian groove is located in the dorsoventral center of the medial side of the dentary. Examples of taxa with this morphology include Erythrosuchus (BMNH R2790), Arizonasaurus (MSM P4590), and Longosuchus (TMM 31185–98). In Silesaurus (ZPAL Ab III/437/1), Lewisuchus (UNLR 01), Sacisaurus (MCN PV 10048), and in the ornithschians Eocursor (SAM-PK-0925) and Lesothosaurus (Sereno, 1991a), the Meckelian groove is restricted to the ventral border of the dentary. Taxa with articulated splenials are difficult to score for this character.
153. Dentary, anterior extent of the Meckelian groove: (0) ends well short of the dentary symphysis; (1) present through the dentary symphysis (fig. 27) (new).
In most archosauriforms examined here, the Meckelian groove terminates well short of the dentary symphysis. Examples of taxa with this condition include Erythrosuchus (BMNH R2790), Arizonasaurus (MSM P4590), and Longosuchus (TMM 31185–98). The Meckelian groove continues through the symphysis in Silesaurus (ZPAL Ab III/437/1) and Sacisaurus (MCN PV 10048).
154. Dentary, dorsal margin of the anterior portion compared to the dorsal margin of the posterior portion: (0) horizontal (in the same plane); (1) ventrally deflected; (2) dorsally expanded (figs. 17, 19–20, 27) (new).
The dorsal margin of the anterior portion of the dentary of carnivorous archosauriforms (with recurved, mediolaterally compressed, and serrated teeth) is typically in the same horizontal plain as the posterior portion of the dentary. Exceptions among carnivorous archosauriforms include Hesperosuchus agilis (AMNH FR 6758), Polonosuchus silesiacus (ZPAL Ab III/563), and Postosuchus kirkpatricki (TTU-P 9000) where the anterior portion is dorsally expanded. Additionally, the anterior margin of the dentary is dorsally expanded in aetosaurs (Aetosaurus, SMNS 5770; Longosuchus, TMM 31185–98) as well as in Silesaurus (ZPAL Ab III/437/1). A ventral deflection of the anterior portion of the dentary was used in many phylogenetic analyses of sauropodomorphs (e.g., Sereno, 1999; Yates, 2003). Plateosaurus (AMNH 6810) is the only taxon scored as (1) here.
155. Dentary, anterior extremity: (0) rounded; (1) tapers to a sharp point (figs. 17, 27) (new).
The anterior margin of the dentary of most archosauriforms is rounded. In aetosaurs (e.g., Aetosaurus SMNS 5770), Silesaurus (ZPAL Ab III/437/1), Sacisaurus (MCN PV10041), and Asilisaurus kongwe (NMT RB9), the anterior end of the dentary tapers to a point. In Silesaurus, the tip arcs dorsally (Dzik, 2003).
156. Articular: (0) without dorsomedial projection posterior to the glenoid fossa; (1) with dorsomedial projection separated from glenoid fossa by a clear concave surface; (2) with dorsomedial projection continuous with the glenoid fossa. ORDERED (figs. 24, 26) (Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
Fig. 26
The articular-partial surangular of Postosuchus (UCMP 27485) in A, dorsal, B, medial, and C, posterior views. A foramen passing through the medial process is highlighted. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bar = 1 cm.
The articular of most archosauriforms lacks a dorsomedially projecting process on the medial side of the articular. In these taxa, the area posterior to the glenoid is concave. Postosuchus kirkpatricki (TTU-P 9000; UCMP 27490), Batrachotomus (SMNS 80260; Gower, 1999), Arizonasaurus (MSM P4590; Nesbitt, 2005a) (broken in Polonosuchus silesiacus, ZPAL Ab III/563 and Fasolasuchus, PVL 3851), Stagonolepis (Walker, 1961), and Longosuchus (TMM 31185–98) possess a small dorsomedially projecting process. The dorsomedial process in these taxa is separated from the glenoid by a large concave surface. Gower (1999) termed this process the retroarticular process. In Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), Terrestrisuchus (Crush, 1984), and Protosuchus richardsoni (UCMP 131827), a distinct fingerlike process projects dorsomedially posteromedial to the glenoid. Crocodylomorphs lack a distinct gap between the process and the glenoid. The dorsomedially projecting process is more elongated in crocodylomorphs relative other taxa scored as (1). The process is hypothesized to be homologous between taxa scored as (1) and (2).
157. Articular, ventromedially directed process: (0) absent; (1) present (fig. 26) (new).
In general, the articulars of archosauriforms do not possess medial processes. Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575) Batrachotomus (SMNS 80260), Arizonasaurus (MSM P4590), Polonosuchus silesiacus (ZPAL Ab III/563), Fasolasuchus (PVL 3851), Stagonosuchus (GPIT/RE/3831), Rauisuchus (BSP AS XXV-60-121), Hesperosuchus (UCMP 129470), Sphenosuchus (Walker, 1990) and Dromicosuchus (UNC 15574) all have tonguelike ventromedial processes. In the taxa scored as (1), a foramen passes through the ventromedial process. Gower (2000) noted that phytosaurs also possess ventral processes and they are also scored as (1).
158. Articular, glenoid of the mandible located: (0) level with dorsal margin of the dentary; (1) well ventral of the dorsal margin of the dentary (figs. 17, 20) (modified from Gauthier, 1986; Langer and Benton, 2006).
In most archosauriforms, the articular facet of the mandible is located at the dorsal margin of the mandible (e.g., Riojasuchus, PVL 3827; Euparkeria, SAM 5867; Herrerasaurus, PVSJ 407). Among basal archosauriforms, ornithischians (e.g., Heterodontosaurus, SAM-PK-1332), sauropodomorphs (Plateosaurus, AMNH FR 6810), Silesaurus (Dzik, 2003), and aetosaurs (Longosuchus, TMM 31185–98) have an articular facet of the mandible located near the ventral margin of the mandible. Additionally, taxa scored as (1) generally have surangulars in which the dorsal margin arcs dorsally to meet the dentary. This is, however, not always the case (e.g., Herrerasaurus).
159. Articular, foramen on the medial side: (0) absent; (1) present (fig. 26) (new).
In non-archosaurian archosauriforms, the medial side of the articular lacks a foramen. The foramen is present posteromedial to the glenoid in Euparkeria (Ewer, 1965), Revueltosaurus (PEFO 34561), and paracrocodylomorphs. In Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Sphenosuchus (SAM 3014), Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Batrachotomus (SMNS 80260), and Polonosuchus silesiacus (ZPAL Ab III/563), the foramen has a large diameter relative to that of Arizonasaurus (MSM P4590), Revueltosaurus (PEFO 34561), and phytosaurs (USNM 18313). In basal crocodylomorphs, the foramen was termed the foramen aerum by Walker (1990), thus implying its homology with the pneumatic opening in crocodyliforms. However, as discussed by Gower (1999), the foramen in basal crocodylomorphs is not pneumatic. Therefore, the term foramen aerum should be abandoned for the structure in basal crocodylomorphs. In taxa scored as (1) for character 157, the foramen pierces through a ventromedial process. In phytosaurs, a small foramen is located medial to the glenoid.
160. Dentary-splenial mandibular symphysis, length: (0) distally positioned; (1) present along one-third of lower jaw (Sereno, 1991a).
Sereno (1991a) cited this character as a synapomorphy of Riojasuchus and Ornithosuchus and remarked that phytosaurs have character state (1), but did not score it. An anteroposteriorly expanded suture is also present in both Shuvosaurus and Effigia (Nesbitt, 2007) and in Crocodyliformes (Sereno, 1991a; Nesbitt, 2007).
161. Coronoid process, dorsally expanded: (0) absent; (1) present (fig. 20) (Sereno, 1986, 1999; Butler, 2005; Butler et al., 2008b; Irmis et al., 2007a).
An expanded coronoid process is present on the dorsal margin of the mandible in ornithischians (Sereno, 1986) but not any other taxa scored here.
162. Mandibular fenestra: (0) anteroposterior length more than maximum depth of dentary ramus but less than half the length of the mandible; (1) greater than half the length of the mandible; (2) reduced (anteroposterior length less than maximum depth of dentary ramus) (figs. 17, 20) (Butler, 2005; Nesbitt and Norell, 2006).
The size of the mandibular fenestra is restricted to one-fourth or less than the length of the mandible in every archosauriform in this study except Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280). In the latter two taxa, the extremely long mandibular fenestra is over half the length of the mandible. Furthermore, the surangular and angular are longer than half the length of the dentary, whereas the dentary is restricted to the one-fourth to one-third the length of the mandible. In ornithischians, the mandibular fenestra is highly reduced (Sereno, 1986, 1999) and are scored as (2).
163. Surangular foramen: (0) present and small; (1) present and large; (2) absent (figs. 16–17) (modified from Clark et al., 2004; Nesbitt, 2007).
A foramen is located on the posterior side of the surangular lateral to the articular in most basal archosauriforms including Proterosuchus (BPS 514), Erythrosuchus (Gower, 2003), saurischians, and nearly all non-crocodylomorph suchians. The foramen is present in Revueltosaurus (PEFO various specimens) and the aetosaurs Longosuchus (Parrish, 1994) and Desmatosuchus (Small, 2002), but it is not clear whether it is present in Stagonolepis. In Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9280), the surangular foramen is large (see Nesbitt, 2007). A foramen does not appear to be present in the crocodylomorphs Hesperosuchus, Dromicosuchus, Sphenosuchus, Dibothrosuchus, Terrestrisuchus, but it is present in Junggarsuchus (Clark et al., 2004).
164. Dentary, posteroventral portion: (0) just meets the angular; (1) laterally overlaps the anteroventral portion of the angular (modified from Nesbitt et al., 2009a).
This character was incorrectly identified as the posteroventral portion of the dentary laterally overlaps the anteroventral portion of the surangular in Nesbitt et al. (2009a). However, the posteroventral portion of the dentary laterally overlaps the lateral surface of the angular. The following description corrects the mistake in Nesbitt et al. (2009a). The anteroventral portion of the meets the dentary anterodorsally in Mesosuchus (SAM 6536), Prolacerta (BP/1/471), and Proterosuchus (RC 96). Alternatively, the posteroventral portion of the dentary laterally overlaps the anteroventral portion of the angular in Erythrosuchus (BP/1/5207), Euparkeria (SAM 6050), in the proterochampsians Tropidosuchus (PVL 4601) and Chanaresuchus (UPLR 7), and in members of Archosauria.
165. Splenial, foramen in the ventral part: (0) absent; (1) present (fig. 16) (modified from Rauhut, 2003; Langer and Benton, 2006; Smith et al., 2007).
According to several recent phylogenies of basal dinosaurian relationships, a foramen ( = mylohyoid foramen) is present through the ventral portion of the splenial in saurischians, but not in ornithischians or the closest relatives of dinosaurs (Rauhut, 2003; Langer and Benton, 2006; Smith et al., 2007). Langer and Benton (2006) reported a foramen in a similar location in Postosuchus following Chatterjee (1985) and Long and Murry (1995). A foramen could not be located in the newly reprepared holotype (TTU-P 9000). Therefore, a splenial foramen seems to be restricted to eusaurischians.
166. Dentary teeth: (0) present along entire length of the dentary; (1) absent in the anterior portion; (2) completely absent (figs. 17, 27) (modified from Parrish, 1994; Parker, 2007).
Fig. 27
Left dentary of Silesaurus opolensis (ZPAL Ab III/437/1) in A, lateral, and B, medial views. Numbers refer to character states. Scale bar = 2 cm.
Dentary teeth usually span the entire length of the dentary in archosauriforms. In aetosaurs, (e.g., Longosuchus TMM 31185–98), the Asilisaurus kongwe (NMT RB9), Silesaurus (ZPAL Ab III/361/27), and Sacisaurus (MCN PV10041), the anterior end of the dentary does not bear teeth. This is also the case in one unusual crocodyliform, Macelognathus (Ostrom, 1971; Göhlich et al., 2005). Dentary teeth are completely absent in Lotosaurus (IVPP V 48013), Effigia (AMNH FR 30587), and Shuvosaurus (TTU-P 9280) among basal archosauriforms.
167. Dentition: (0) generally homodont; (1) markedly heterodont (Parrish, 1993).
Parrish (1993) scored both Leptosuchus and Prestosuchus as having heterodont dentition. Additionally, Sereno et al. (1993) listed heterodont dentition as an autapomorphy of Eoraptor. Phytosaurs have markedly heterodont teeth (see Hungerbühler, 2000). However, as noted by Hungerbühler (2000), the subjective term “heterodonty” to describe a set of teeth is highly ambiguous. Here, homodonty describes the general dentition of carnivorous teeth (recurved, serrated) of most archosauriforms and the herbivorous teeth of sauropodomorphs, ornithischians, aetosaurs, and Revueltosaurus. All these taxa have teeth that are generally similar. In contrast, the teeth of phytosaurs differ significantly depending on position (see Hungerbühler, 2000). The two character states are general bins to separate phytosaurs (1) from other basal archosaurs (0). Eoraptor is scored as (0); the difference in the teeth lies in the direction and number of serrations per 5 mm, but the general form of the teeth is very similar.
168. Tooth, serrations: (0) absent; (1) present as small fine knifelike serrations; (2) present and enlarged and coarser (lower density) = denticles. ORDERED (modified from Gauthier et al., 1988; Juul, 1994; Dilkes, 1998; Irmis et al., 2007a).
Tooth serrations are absent in the two non-archosauriform archosauromorphs (Prolacerta, Mesosuchus) used here. Within Archosauriformes, tooth serrations are present in nearly all clades ancestrally. In many forms that are considered carnivorous (e.g., theropods, phytosaurs, basal crocodylomorphs), the serrations form a right angle to the carinae of each tooth edge. Typically, there are 3–7 serrations per millimeter depending on the size of the taxon and tooth position. In contrast, Revueltosaurus (PEFO 34561), aetosaurs (Aetosaurus, SMNS 5770), ornithischians (e.g., Lesothosaurus, BMNH R8501), and sauropodomorphs (Plateosaurus, AMNH FR 6810) have much larger serrations (typically 1–2 units per mm), and they are angled about 45° to the carinae of each tooth edge. As a result, the serrations ( = denticles) are angled dorsally or antero/posterodorsally. A similar description of the differences in the serrations of typically carnivorous taxa versus taxa with denticles was presented by Butler et al. (2008b).
169. Extensive planar wear facets across multiple maxillary/dentary teeth: (0) absent; (1) present (Weishampel and Witmer, 1990).
Typically, most archosauriforms lack extensive planar wear facets across multiple maxillary or dentary teeth. However, within Ornithischia, extensive planar wear facets are present in Pisanosaurus (PVL 3577) and Heterodontosaurus (SAM-PK-1332), whereas they are absent in Lesothosaurus (BMNH R8501) and Scutellosaurus (MNA 175).
170. Medial or lateral overlap of adjacent crowns in maxillary and dentary teeth: (0) absent; (1) present (Sereno, 1986; Butler et al., 2008b).
In most archosauriforms, the tooth crowns do not laterally overlap each other; every tooth crown is well separated from every other tooth crown. In ornithischians (e.g., Lesothosaurus, BMNH R8501) and some sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810), the anterior and posterior margins of the tooth crowns laterally/medially overlap one another.
171. Tooth crown: (0) not mesiodistally expanded; (1) mesiodistally expanded above root in cheek teeth (Sereno, 1986; Butler et al., 2008b).
In most archosauriforms, the base of the crown is mesiodistally the widest between the root and the crown. In contrast, the tooth crowns are mesiodistally expanded in ornithischians (e.g., Lesothosaurus, BMNH R8501), sauropodomorphs (e.g., Plateosaurus, AMNH FR 6810), Revueltosaurus (PEFO 34561), and aetosaurs (e.g., Aetosaurus, SMNS 5770). This character was used to assign Revueltosaurus to Ornithischia (Hunt, 1989), but was shown by Parker et al. (2005) to have a much wider distribution.
172. Moderately developed lingual expansion of crown ( = cingulum) on maxillary/dentary teeth: (0) absent; (1) present (fig. 27) (Sereno, 1986; Butler et al., 2008b).
In most basal archosauriforms, the lingual side is nearly flat. However, in some ornithischians (e.g., Heterodontosaurus, SAM-PK 1332) and in Sacisaurus (Ferigolo and Langer, 2007: fig. 3J), there is a moderately developed lingual expansion of the crown. As explained by Irmis et al. (2007b), the term cingulum should not be used because it is not morphologically homologous with the similarly named structure in mammalian teeth (a distinct ridge), and it is better described as above. The presence of a lingual expansion of the crown was cited by Parker et al. (2005) and Irmis et al. (2007b) as a potential character to assign isolated teeth from the Triassic to ornithischians. However, the presence of a lingual expansion of the crown in the non-ornithischian dinosauriform Sacisaurus (Ferigolo and Langer, 2007) negates that hypothesis. Here, I score Pisanosaurus (PVL 3577) as having a lingual expansion of the crown.
173. Maxillary and dentary crowns, shape: (0) apicobasally tall and bladelike; (1) apicobasally short and subtriangular (Sereno, 1986; Butler et al., 2008b).
In archosauriforms, the maxillary and dentary crowns are typically tall and bladelike. In ornithischians, the teeth are short and typically subtriangular, and the scorings of Sereno (1986) and Butler et al. (2008b) are followed here.
174. Tooth implantation: (0) free at the base of the tooth; (1) teeth fused to the bone of attachment at the base (figs. 16, 27) (modified from Gauthier, 1984; Benton and Clark, 1988; Benton, 1990a; Bennett, 1996; Nesbitt et al., 2009a).
The tooth implantation of basal archosauriforms was discussed in great detail (Romer, 1956; Hughes, 1963; Charig and Sues, 1976; Gauthier et al., 1988; Benton and Clark, 1988). The terms thecodont and subthecodont were confused in the literature in reference to basal archosaur dentition, and both terms were used interchangeably to describe the same taxon and condition. Gauthier et al. (1988) first used the depth of the tooth sockets to score this character for basal archosauriforms. However, as explained by Juul (1994), the depth of the socket is difficult to determine and compare.
The confusion of thecodont versus subthecodont dentition may be related to different authors' interpretations (Bennett, 1996). However, there is a clear difference between the dentition of Prolacerta (BP/1/2675) and Proterosuchus (BP/1/3773), and Erythrosuchus + Archosauria. Here, ideas associated with thecodont versus subthecodont are abandoned. Instead, differences of how the base of each tooth attaches to the tooth-bearing element are explored. The bases of the teeth of Prolacerta (UCMP 37151) and Proterosuchus (BSP 514) are firmly attached to the tooth-bearing element by small ridges of bone that completely surround each tooth. In lateral view, the teeth have flared bases. In contrast, the bases of the teeth of Erythrosuchus + Archosauria are free from a bony attachment. The bases of these teeth are not flared. Furthermore, most members of erythrosuchians + Archosauria also have interdental plates between teeth; interdental plates are not present outside this clade within Archosauromorpha. Silesaurids (e.g., Asilisaurus, Silesaurus) are the only crown-group archosaurs to be scored as (0) (Nesbitt et al., 2010).
175. Palatal teeth present on palatal process of the pterygoid: (0) present; (1) absent (Juul, 1994; Gower and Sennikov, 1997; Nesbitt et al., 2009a).
176. Teeth on transverse processes of pterygoids: (0) present; (1) absent (Gauthier, 1984; Juul, 1994; Bennett, 1996; Gower and Sennikov, 1997; Nesbitt et al., 2009a).
Palatal teeth are in a variety of archosauromorphs and even in members within the archosaur crown group (e.g., Eoraptor; Rauhut, 2003). Palatal teeth are present in Prolacerta (Camp, 1945; Gow, 1975; Modesto and Sues, 2004), Mesosuchus (Dilkes, 1998), Proterosuchus (Haughton, 1924; Welman, 1998), Euparkeria (Ewer, 1965), and all proterochampsians (Romer, 1971b; Arcucci, 1990). Palatal teeth are absent in Erythrosuchus and other erythrosuchians (Parrish, 1992; Gower, 2003) and most members of Archosauria, and the absence of palatal teeth was used to diagnose the clade (Gauthier, 1984; Gauthier et al., 1988; Benton and Clark, 1988; Sereno, 1991a; Juul, 1994). The presence/absence of palatal teeth anywhere on the palate was used previously (Benton and Clark, 1988; Sereno, 1991a; Juul, 1994; Benton, 1999). However, as discovered by Juul (1994), incorporating characters examining the presence/absence of palatal teeth on certain regions of the pterygoid provides phylogenetic information. Prolacerta and Proterosuchus both have a row of palatal teeth on the transverse process of the pterygoids, whereas other taxa closer to Archosauria (and within) do not have palatal teeth here. Palatal teeth on the palatal process are retained by the non-archosaurian archosauromorphs Prolacerta, Proterosuchus, proterochampsians, Doswellia, and Euparkeria, and the crown-group archosaurs Turfanosuchus (Wu and Russell, 2001) and Eoraptor (Rauhut, 2003).
In nearly all of the taxa with palatal teeth, it is unclear whether vomerine teeth are present; in most specimens, this area is poorly preserved or covered by the dentaries. Euparkeria (SAM 6050) has vomerine teeth (Gow, 1970). Examination of the vomer in newly discovered specimens may provide an additional character.
Axial Skeleton
177. Postaxial intercentra: (0) present; (1) absent (Gauthier, 1984; Benton and Clark, 1988; Sereno, 1991a; Parrish, 1993; Juul, 1994; Bennett, 1996; Nesbitt et al., 2009a).
The presence of intercentra was long cited as a character in basal archosaur phylogenies, but the distribution of intercentra within non-archosaurian archosauriforms remains controversial. Sereno (1991a) listed the absence of intercentra as a synapomorphy of proterochampsians + Archosauria because Sereno (1991a) listed Euparkeria as having intercentra in all presacral vertebrae. However, there are only two specimens of Euparkeria (SAM-PK-6047A, SAM-PK-6047B) that preserve intercentra. In these specimens, intercentra are not found between each of the vertebrae, but sporadically throughout the presacral column (Ewer, 1965). Intercentra are apparently absent in all other specimens even though some may have been prepared away. The intercentra of SAM-PK-6047A are very small in comparison with those of Mesosuchus (SAM-PK-6046). Furthermore, the ventral portion of the anterior and posterior articular surfaces of the centra of Euparkeria are not beveled and do not have facets for intercentra as they do in the dorsal vertebrae of Erythrosuchus, Sarmatosuchus (Gower and Sennikov, 1996), and Proterosuchus. The small size and the apparent absence of ossification of some of the intercentra in the column of Euparkeria may suggest that closely related taxa that have been scored as lacking intercentra, may indeed have very small intercentra. Euparkeria is scored as polymorphic for this character. Benton and Clark (1988) used the absence of intercentra to support the clade proterochampsians + Euparkeria + Archosauria (the crown group). Intercentra are present in Prolacerta (Gow 1975), Proterosuchus (NM QR 1484; Cruickshank, 1972), and Erythrosuchus (Gower, 2003).
178. Atlantal articulation facet in axial intercentrum, shape: (0) saddle shaped; (1) concave with upturned lateral borders (fig. 28) (modified from Gauthier, 1986; Langer and Benton, 2006).
Fig. 28
Vertebrae and ribs of archosauriforms: A, anterior cervical vertebra of a coelophysoid (AMNH FR 2701) in anterior (left) and left lateral (right); B, axis centrum of Postosuchus alisonae (UNC 15575) in ventral view; C, anterior cervical vertebra of Postosuchus alisonae (UNC 15575) in lateral view; D, atlas, axis, and anterior cervical vertebrae of Heterodontosaurus tucki (SAM-K-1332) in left lateral view; E, atlas and axis of Lewisuchus admixtus (UNLR 01) in left lateral view; F, posterior cervical vertebra of Riojasuchus tenuisceps (PVL 3827) in left lateral view; G, posterior cervical neural spine of Postosuchus alisonae (UNC 15575) in dorsal view; H, posterior cervical neural spine of Arizonasaurus babbitti (MSM 4590) in left lateral view; I, dorsal vertebra of Fasolasuchus tenax (PVL 3850) in posterior view; J, cervical rib of Smilosuchus gregorii (USNM 18313) in dorsolateral view; K, middle caudal vertebrae of Ticinosuchus ferox (PIZ T2817) in left lateral view. Anterior direction to the left. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
In non-archosaurian archosauriforms, crocodylian-line archosaurs, and ornithischians, the anterior articulation of the axial intercentrum is saddle shaped, concave anteroposteriorly, and convex mediolaterally (Langer and Benton, 2006). In contrast, saurischians have a concave articulation facet, with upturned lateral borders. Thus, saurischians are scored as (1) following Langer and Benton (2006).
179. Axis, dorsal margin of the neural spine: (0) expanded posterodorsally; (1) arcs dorsally, where the anterior portion height is equivalent to the posterior height (fig. 28) (new).
In most archosauriform taxa, the neural spine of the axis is anteroventrally slanted where the anterodorsal corner is much lower than the posterodorsal height. In some ornithodiran taxa (e.g., Marasuchus, PVL 3870; Lewisuchus, UNLR 01; Coelophysis bauri, AMNH FR 7224), the neural spine of the axis arcs dorsally, where the anterodorsal height is or nearly is equivalent to the posterior height.
180. Axis, ventral surface: (0) possesses a midline keel; (1) possesses two paramedian keels (fig. 28) (new).
Plesiomorphically, most archosauriforms bear a midline keel on the ventral side of the axis. In Postosuchus kirkpatricki (TTU-P 9002), Postosuchus alisonae (UNC 15575), Polonosuchus silesiacus (ZPAL Ab III/563), Rauisuchus (BSP AS XXV-60-121), the axis vertebra has two paramedian keels separated by a shallow fossa. Saurosuchus (PVSJ 32), Batrachotomus (SMNS 80322), Ticinosuchus (PIZ T2817), Fasolasuchus (PVL 3850), UFRGS 156-T, and Arizonasaurus (MSM P4590) as well as the crocodylomorphs Hesperosuchus (AMNH FR 6758) and Protosuchus richardsoni (AMNH FR 3024) possess a single midline ventral keel on the axis.
181. Cervical 3–5 centrum length: (0) shorter or the same length as the middorsal; (1) longer than middorsal (Sereno, 1991a; Nesbitt et al., 2009a).
The anterior cervical vertebrae of many basal archosaurs are marginally the same length as the middorsal vertebrae, whereas the length of the anterior cervicals is longer than that of the middorsal vertebrae in dinosauromorphs (Sereno, 1991a). However, given the increased taxon sampling since Sereno's (1991a) discussion, this character needs further discussion.
Among basal archosauriforms, and their closest relatives, the anterior cervical vertebrae are longer in Prolacerta (BP/1/2675), Proterosuchus (NM QR 1484), and Vancleavea (PEFO 33978). Within Proterochampsia, Chanaresuchus (PVL 4647), and Gualosuchus (PVL 4576) have short anterior cervicals, whereas Tropidosuchus (PVL 4601) has anteroposteriorly elongated cervicals relative to the middorsal vertebrae. Phytosaurs (e.g., Parasuchus; Chatterjee, 1978), Riojasuchus (PVL 3827), Revueltosaurus (PEFO 34561), aetosaurs (e.g., Longosuchus; Sawin, 1947), Postosuchus kirkpatricki (TTU-P 9000), Saurosuchus (PVSJ 32), Ticinosuchus (PIZ T2817), and Prestosuchus (UFRGS 0156-T) have cervicals that are shorter or the same length as the middorsals. In Gracilisuchus (UNLR 08), Arizonasaurus (MSM P4590), Effigia (AMNH FR 30587), and basal crocodylomorphs (e.g., Hesperosuchus agilis, AMNH FR 6758; Dromicosuchus, UNC 15574), the anterior cervicals are elongated relative to the middorsals.
182. Cervical vertebrae, deep recesses on the anterior face of the neural arch, lateral to the neural canal ( = prechonos of Welles, 1984): (0) absent; (1) present (fig. 28) (new).
There are deep recesses on the anterior face of the neural arch just lateral to the neural canal in basal neotheropods such as Coelophysis bauri (AMNH 7224) and Dilophosaurus (UCMP 37302). The deep fossae are framed by the centroprezygapophyseal lamina (sensu Wilson, 1999) and a unnamed lamina that borders the neural canal laterally. In theropods more closely related to avians (e.g., Allosaurus, Madsen, 1976), the recess transforms into a group of small foramina, the hallmark of pneumaticity (Britt, 1993). Similar depressions are present in sauropods (Wilson, 1999) and the ?erythrosuchid Guchengosuchus (IVPP V 8808; Peng, 1991).
183. Third cervical vertebra, centrum length: (0) subequal to the axis centrum; (1) longer than the axis centrum (Gauthier, 1986; Langer and Benton, 2006).
According to Langer and Benton (2006), the third cervical vertebra is longer than the axis in saurischians (e.g., Herrerasaurus, PVSJ 407; Coelophysis bauri, AMNH FR 7224) and in Lewisuchus (UNLR 01). Their scorings are followed here with the exception of Eoraptor. I could not get an accurate measurement of the axis and the third cervical. Similarly, the third cervical vertebra of Dimorphodon (Sereno, 1991a), Asilisaurus kongwe (e.g., NMT RB21), Prolacerta (BP/1/2675), Qianosuchus (IVPP V 14399), and Xilousuchus (IVPP V 6026) is longer than the axis.
184. Anterior to middle cervical vertebrae, diapophysis and parapophysis: (0) well separated; (1) nearly touching (new).
The diapophysis and parapophysis in anterior cervical vertebrae in most archosauriforms are well separated. In a few taxa with elongated cervicals (e.g., Xilousuchus, IVPP V 6026), the diapophysis and parapophysis in the anterior cervical vertebrae are very close together; however, the diapophysis and parapophysis quickly diverge from each other in the midcervical vertebrae. Within Dinosauria, the diapophysis and parapophysis in anterior to midcervical vertebrae are nearly touching in neotheropods such as Dilophosaurus (UCMP 37302) and Coelophysis bauri (AMNH FR 7224).
185. Anterior cervical vertebrae, neural arch, posterior portion ventral to the postzygapophysis: (0) smooth posteriorly or has a shallow fossa; (1) with a deep excavation with a thin lamina covering the anterior extent on the posterolateral surface (fig. 28) (modified from Langer and Benton, 2006).
As described by Langer and Benton (2006), the neural arches of the anterior cervical vertebrae are smooth plesiomorphically whereas the same region has a deep excavation on the posterolateral surface ( = caudal chonos of Welles, 1984) in some dinosaurs. Here, I do not agree with all the scoring of this character by Langer and Benton (2006). They score nearly all saurischians as having state (1). The anterior corner of the excavation tapers to a point anteriorly and is covered laterally by a lamina in only neotheropods (e.g., Coelophysis bauri, AMNH FR 7224; Dilophosaurus, UCMP 37302), whereas the posterolateral excavation is shallow in all other archosauriforms (e.g., Vancleavea, PEFO 33978; Arizonasaurus, MSM P4590; Nesbitt, 2005a). The anterior cervical vertebrae of Effigia (AMNH FR 30587; Nesbitt, 2007: fig. 28C) is the only exception among nontheropod archosauriforms to be scored as (1). A clear, anteriorly tapering excavation is hidden in lateral view in this specimen.
186. Epipophyses: (0) absent in postaxial anterior cervical vertebrae; (1) present in postaxial anterior cervical vertebrae (fig. 28) (Gauthier, 1986; Novas, 1996; Langer and Benton, 2006).
187. Epipophyses: (0) absent in posterior cervical vertebrae (6–9); (1) present in posterior cervical vertebrae (6–9) (Sereno et al., 1993; Langer and Benton, 2006).
Epipophysis are posterodorsally directed processes that lie on the dorsal surface of the postzygapophyses (Gauthier, 1986; Sereno and Novas, 1994). In some taxa, the posterior termination of the structure is expanded well posterior to the postzygapophyses (e.g., Heterodontosaurus, SAM-PK-1332; Herrerasaurus, PVL 407; Dilophosaurus, UCMP 37302) whereas the epipophyses in other taxa are just dorsal expansions that do not project more posteriorly than the postzygapophyses (Liliensternus, MBR. 1275) (Langer and Benton, 2006). The presence of epipophyses was recently reviewed by Langer and Benton (2006) and they concluded that the presence of epipophyses are synapomorphic for dinosaurs because they are clearly absent in Silesaurus, Lewisuchus, and Marasuchus. Furthermore, Langer and Benton (2006) follow Sereno et al. (1993) and noticed that only saurischians (including Eoraptor and Herrerasaurus) had epipophyses in the posterior cervical vertebrae.
Langer and Benton (2006) state that epipophyses are present in non-dinosaurian archosaurs such as Batrachotomus. In fact, they are more common among basal archosauriforms than discussed by Langer and Benton (2006). Epipophyses are present in Batrachotomus (Langer and Benton, 2006; Gower and Schoch, 2009), Revueltosaurus (Parker et al., in prep.), Vancleavea (PEFO 33978), Mesosuchus (SAM 8552), Xilousuchus (IVPP V 6026), and on the atlantal neural arch of Effigia (AMNH FR 30587; Nesbitt, 2007) and Hesperosuchus (AMNH FR 6758).
188. Cervical vertebrae, pneumatic features ( = pleurocoels) in the anterior portion of the centrum: (0) absent; (1) present as deep fossae; (2) present as foramina. ORDERED (fig. 28) (modified from Holtz, 1994; Rauhut, 2003; Smith et al., 2007).
The presence of pneumatic features have been discussed at length elsewhere (see Britt, 1993; Rauhut, 2003; O'Connor, 2004; O'Connor and Claessens, 2005; Wedel, 2003, 2007; Sereno et al., 2008). In nonsaurischian archosauriforms, clear pneumatic structures are absent in the cervical vertebrae. In contrast, theropods possess pneumatic excavation(s) on the lateral sides of cervical vertebrae (Holtz, 1994; Rauhut, 2003). In coelophysoids (e.g., Coelophysis bauri, AMNH FR 7224), blind and deep fossae lie medially to the parapophyses on the anterior part of the centrum. In nearly all other theropods, pneumatic features are present as foramina (Rauhut, 2003).
It was reported by Nesbitt (2007) that Effigia and Shuvosaurus possess deep excavations ( = pleurocoels) on the lateral portion of the cervical centra. However, it is not clear whether the cervical vertebrae assigned to Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9001 = holotype of “Chatterjeea elegans”) unambiguously belong to those taxa. Both specimens had disarticulated cervical regions when discovered and both were found among the remains of coelophysoids (Nesbitt, 2007; Nesbitt and Chatterjee, 2008), taxa with excavations on the posterior portion of the cervical centra. Sillosuchus (PVSJ 85), a taxon closely related to Effigia and Shuvosaurus (Nesbitt, 2007), has deep excavations in the cervical and dorsal vertebrae that are very similar to that sauropods and theropod dinosaurs (Alcober and Parrish, 1997; Nesbitt, 2007). In all likelihood, the anterior cervical originally assigned to Effigia and Shuvosaurus belong to those taxa; it is not unambiguous at this point in time.
189. Cervical vertebrae, rimmed depression on the posterior part of the centrum: (0) absent; (1) present (fig. 28) (modified from Gauthier, 1986; Rauhut, 2003).
The posterior portion of the centrum of the cervical vertebrae of most archosauriforms is free from any depression. A rimmed depression is present in the theropod Coelophysis bauri (AMNH FR 7224), Sillosuchus (PVSJ 85), and apparently some specimens of Shuvosaurus (TTU-P 9001). It is not clear if these depressions are pneumatic in origin even though this has been argued previously (see Britt, 1993).
190. Cervical vertebrae, middle portion of the ventral keel: (0) dorsal to the ventralmost extent of the centrum rim; (1) extends ventral to the centrum rims (fig. 28) (new).
Nearly all basal archosauriforms have keels on the midline of the cervical centra on the ventral surface. In Riojasuchus (PVL 3827) and Ornithosuchus (BMNH R 3916), the keels are expanded ventral to the centrum rims. Vancleavea is also scored as (1).
191. Cervical vertebrae, distal end of neural spines: (0) expansion absent; (1) laterally expanded in the middle of the anteroposterior length; (2) expanded anteriorly, so that the spine table is triangular or heart shaped in dorsal view (fig. 28) (modified from Gauthier, 1984; Juul, 1994; Nesbitt et al., 2009a).
The distal ends of the neural spines of the cervical vertebrae are expanded ( = spine tables) in Euparkeria (SAM 6047A), Postosuchus kirkpatricki (TTU-P 9002), Riojasuchus (PVL 3827; Bonaparte, 1971), Revueltosaurus (PEFO 34561), aetosaurs (Desmatosuchus, MNA V9300), phytosaurs (Pseudopalatus, UCMP 34260), but absent in some crocodylian-line archosaurs (e.g., “Clade X” of 302Nesbitt, 2005, 2007). In taxa scored as (1) or (2), the dorsal surface of the neural spine is flat. Distal expansions of the cervical neural spines are not present in proterochampsians, Erythrosuchus (Gower, 2003), Vancleavea, Proterosuchus (BP/1/3993), or Mesosuchus (SAM 8552). The presence of osteoderms does not coincide with the presence of distal expansions of the neural spines as demonstrated by Chanaresuchus (PVL 4575) and other proterochampsians; they have osteoderms, but no distal expansion. Furthermore, distal expansions are not present in some taxa where the dorsal region bears dorsal osteoderms over the entire presacral column (see below). Therefore, the absence of distal expansions does not determine whether a taxon has dorsal osteoderms.
In Euparkeria (SAM 6047A), Riojasuchus (PVL 3827), and phytosaurs the distal expansions are laterally expanded in the middle of the anteroposterior length of the spine. In dorsal view, the neural spine is oval where the long axis is oriented anteroposteriorly. In Postosuchus kirkpatricki (TTU-P 9002), Postosuchus alisonae (UNC 15575), Revueltosaurus (PEFO 34561), Batrachotomus (SMNS 80285), and Saurosuchus (PVSJ 32), the neural spine is anteriorly expanded relative to the posterior end. In dorsal view, the neural spine is triangular or heart shaped in dorsal view.
Novas (1994) and Langer and Benton (2006) reported distal expansions in the neural arches of Herrerasaurus and Eoraptor in the dorsal, caudal, and sacral vertebrae. The features in Eoraptor and Herrerasaurus contrast with those in Euparkeria and Postosuchus in that the dorsal and lateral surfaces of the distal expansions of Eoraptor and Herrerasaurus are rounded and the lateral sides have longitudinal striations. Some of the “spine tables” of the dorsal vertebrae of Herrerasaurus also expand anteriorly and posteriorly beyond the anterior and posterior edges of the shaft of the neural spine. A similar pattern occurs in theropods (e.g., Tyrannosaurus rex; Brochu, 2003) and avians (S.J.N., personal obs.) as well as the suchian Effigia (Nesbitt, 2007). Therefore, dinosaurs are scored as (0) for this character.
192. Middle cervical vertebrae, hypapophyses: (0) absent; (1) present (fig. 28) (new).
Small, ventral projections at the anterior and posterior ends of the ventral keel ( = hypapophyses) of the cervical vertebrae are present in Postosuchus kirkpatricki (TTU-P 9000, 9002), Postosuchus alisonae (UNC 15575; Peyer et al., 2008), and Rauisuchus (BSP AS XXV-60-121). This character cannot be scored for Polonosuchus silesiacus (ZPAL Ab III/563).
193. Posterior cervical vertebrae, divided parapophyses: (0) absent; (1) present (modified from Weinbaum and Hungerbühler, 2007).
In nearly all basal archosaurs, the parapophysis of the posterior cervical vertebrae is a single unit that attaches to the capitulum of a rib. In the posterior cervicals of Arizonasaurus (MSM P4590) and Poposaurus (TTU-P 10419 and TMM 31025–177), the parapophysis is divided by a non-articulating gap (Nesbitt, 2005a). It is unclear whether the rib that attaches here is triple headed like that of some of the dorsal ribs erythrosuchians (Parrish, 1992; Gower, 2003).
194. Posterior cervical vertebrae, neural spines: (0) directed dorsally, straight; (1) arc anteriorly (fig. 28) (new).
The neural spines of the posterior cervical vertebrae of nearly all archosauriforms are directed dorsally at their distal margins. In Arizonasaurus (MSM P4590), Xilousuchus (IVPP V 6036), Lotosaurus (IVPP V 48013), and Ctenosauriscus (GZG 419-1), the neural spines arc anteriorly at their distal tips.
195. Posterior cervical and/or dorsal vertebrae, hyposphene-hypantrum accessory intervertebral articulations: (0) absent; (1) present (fig. 28) (Gauthier, 1986; Juul, 1994; Benton, 1999; Rauhut, 2003; Langer and Benton, 2006; Weinbaum and Hungerbühler, 2007).
Accessory intervertebral articulations between the dorsal vertebrae (hyposphene-hypantrum) are present in Dinosauria and a number of crocodylian-line archosaurs (Gauthier, 1986; Langer and Benton, 2006). Within Dinosauria, hyposphene-hypantrum articulations are present in Herrerasaurus, Guaibasaurus, theropods (e.g., Dilophosaurus, UCMP 37302), and sauropodomorphs (Rauhut, 2003; Langer and Benton, 2006). Among crocodylian-line archosaurs, Arizonasaurus (MSM P4590; Nesbitt, 2007; Weinbaum and Hungerbühler, 2007), Effigia (AMNH FR 30587), Batrachotomus (SMNS 80296; Gower and Schoch, 2009), Xilousuchus (IVPP V 6036), Poposaurus (YPM 57100), Postosuchus alisonae (UNC 15575), and Postosuchus kirkpatricki (TTU-P 9000) as well as in the aetosaur Desmatosuchus (MNA V9300; Parker, 2008). In these taxa the hyposphene is a simple, dorsoventrally oriented lamina that is continued ventrally from the ventromedial bases of the postzygapophyses. Furthermore, the hypantrum is just a simple gap between the prezygapophyses.
Eoraptor is scored as unknown here because the intervertebral articulations are not visible in the specimen (PVSJ 512).
196. Cervical ribs: (0) slender and elongated; (1) short and stout (fig. 28) (Gauthier, 1986; Benton and Clark, 1988; Juul, 1994; Benton, 1999).
Plesiomorphically, archosauriforms possess elongated cervical ribs that parallel the cervical vertebrae (Gauthier, 1986). In basal pterosaurs (e.g., Eudimorphodon ranzii, MCSNB 2888), phytosaurs (e.g., Smilosuchus, USNM 18313), Postosuchus alisonae (UNC 15575), Gracilisuchus (UNLR 08), in aetosaurs preserving cervical ribs (Stagonolepis BMNH R 4789; Aetosaurus, SMNS 5770 S-21), and in crocodylomorphs (e.g., Hesperosuchus, AMNH FR 6758; Dromicosuchus, UNC 15574; Sues et al., 2003) and crocodyliforms (e.g., Protosuchus richardsoni, AMNH FR 3024; Alligator) the cervicals ribs are very short. Here, a short cervical rib is defined as a rib with an anteroposterior length shorter than the posterior edge of the following centrum. Arizonasaurus (MSM P4590), Qianosuchus (IVPP V13899) and Poposaurus (YPM 57100) have elongated cervical ribs like that of taxa scored as (0).
Langer and Benton (2006) use a similar character (states reversed) to describe the cervical ribs of dinosauromorphs. However, the plesiomorphic state of Langer and Benton (2006), short and directed posteroventrally, is not equivalent to state (0) employed here. Nearly all the taxa scored in Langer and Benton (2006) as (0) and (1) would be scored as (0) here. As described by Langer and Benton (2006), the cervical ribs of ornithischians are short and are scored as (1). Juul (1994) incorrectly states that Lagerpeton has slender cervical ribs; the cervical region of Lagerpeton is unknown.
197. Dorsal vertebrae, neural spine distal expansion: (0) absent; (1) present with a flat dorsal margin; (2) present with a rounded dorsal margin (fig. 28) (new).
The neural spines of the dorsal vertebrae of Euparkeria (SAM 6047B), phytosaurs (e.g., Smilosuchus, USNM 18313), Riojasuchus (PVL 3827), Revueltosaurus (PEFO 34561), aetosaurs (e.g., Longosuchus, TMM 31185–98), Saurosuchus (PVSJ 32), Batrachotomus (Gower and Schoch, 2009), and Fasolasuchus (Bonaparte, 1981) expand laterally at the distal end and form a flat surface. This morphology is periodically referred to as a spine table. In Herrerasaurus (Novas, 1994), Eoraptor (PVSJ 512), and Heterodontosaurus (SAM-K-1332) the distal end of the neural spines of the dorsal vertebrae expand, but do not form a flat dorsal surface. Here, the condition in dinosaurs is not considered homologous to that of state (1), but is instead scored as (2).
198. Dorsal vertebrae, neural spines: (0) about the same height as the posterior cervical vertebrae neural spines; (1) 2–5 times taller as the posterior cervical vertebrae neural spines (fig. 28) (new).
The neural spines of the dorsal vertebrae of nearly all archosauriforms are about the same height as the neural spines of the posterior cervical vertebrae. However, the neural spines of the dorsal vertebrae of Arizonasaurus (MSM P4590), Lotosaurus (IVPP V4880 or V4881), Ctenosauriscus (GZG 419-1), and Hypselorhachis (Butler et al., 2009) are greatly elongated (Nesbitt, 2003). In these taxa the dorsal vertebrae form a distinctive “sail” (Nesbitt, 2003, 2005a).
199. Middle dorsal vertebrae, diapophyses and parapophyses: (0) close to the body of the midline; (1) expanded on stalks (new).
In most basal archosauriforms the diapophyses and parapophyses are not significantly laterally expanded beyond the neural arch. In contrast, the diapophyses and parapophyses lie together on a laterally expanded transverse process in aetosaurs (e.g., Desmatosuchus, MNA V9300), Revueltosaurus (PEFO 34561), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P assorted specimens), and in dinosaurs.
200. Sacral centra: (0) separate; (1) coossified at the ventral edge (new).
The coossification of the sacral centra is common with Archosauria. Most non-archosaurian archosauriforms and many crocodylian-line archosaurs have separate sacral vertebrae. Within crocodylian-line archosaurs, Arizonasaurus (MSM P4590), Poposaurus (TMM 43683-1), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9280), Sillosuchus (PVSJ 85), and Desmatosuchus (MNA V9300; Parker, 2008) have coossified sacral centra. Pterosaurs also have coossified sacral centra (Langer and Benton, 2006). Within Dinosauria, the sacral centra are coossified in ornithischians (e.g., Heterodontosaurus, SAM-PK-1332; Santa Luca, 1980), some sauropodomorphs (Sellosaurus gracilis, SAM 12684; Yates, 2003) and all neotheropods (e.g., Coelophysis bauri, AMNH FR 7224; Colbert, 1989) whereas the sacral centra of Herrerasaurus (PVL 2566), Staurikosaurus (MCZ 1669), and Saturnalia (MCP 3944-PV) are not coossified. The sacral centra of Silesaurus (ZPAL unnumbered) are not coossified on the ventral margins in any examples. The sacral ribs are shared between vertebrae and as a result, the centra appear to be coossified. However, without the sacral ribs, the centra would not be coossified at all. Thus, Silesaurus is scored as (0).
201. Sacral vertebrae, prezygapophyses and complimentary postzygapophyses: (0) separate; (1) coossified (fig. 29) (new).
Fig. 29
The sacrum of archosauriforms: A, right ilium of Massospondylus (BP/1/4934) in medial view; B, left sacral ribs of Massospondylus (BP/1/4934) in lateral view (reversed); C, sacrum of Massospondylus (BP/1/4934) in ventral view (reversed); D, right sacral ribs of Arizonasaurus babbitti (MSM 4590) in lateral view; E, coelophysoid sacrum (NMMNH 31661) in left lateral view; F, sacrum of Arizonasaurus babbitti (MSM 4590) in right lateral view; G, sacrum of Proterosuchus fergusi (NM QR 1484) in dorsolateral view. Abbreviations: 1′, 1st primary sacral vertebra/rib; 2′, 2nd primary sacral vertebra/rib; i, insertion. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
The prezygapophyses and postzygapophyses of sacral vertebrae remain separated in most basal archosauriforms even if the sacral centra are coossified. However, the prezygapophyses and postzygapophyses are coossified in the sacra of Arizonasaurus (MSM P4590), Poposaurus (TMM 43683-1), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9001), and Sillosuchus (PVSJ 85). Coossification of the prezygapophyses and postzygapophyses also occurs in pterosaurs and neotheropods.
In Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9001) the neural spines are completely coossified into a sheet of bone (Nesbitt, 2007). This is also true of some of the sacral series of Silesaurus (ZPAL unnumbered) and in Coelophysis bauri (Colbert, 1989).
202. Primordial sacral one, sacral rib: (0) does not or weakly articulates with anteriorly directed process ( = preacetabular process) of the ilium; (1) an anterior process of the rib articulates with the anteriorly directed process of the ilium (fig. 29) (Nesbitt, 2005a, 2007).
The sacral rib of primordial sacral one doesn't or weakly articulates with anteriorly directed process ( = preacetabular process) of the ilium in most basal archosaurs including dinosaurs. In non-archosaurian archosauriforms as well as most crocodylian-line archosaurs, the first sacral rib is massive and circular in lateral view (e.g., Erythrosuchus BMNH R3592; Gower, 2003). The first sacral rib articulates with the ilium on the dorsal portion of the pubic peduncle in taxa scored as (0). Conversely, the anterodorsal portion of the first primordial sacral rib extends anteriorly to articulate with the anteriorly directed process of the ilium in Arizonasaurus (MSM P4590), Poposaurus (TMM 43683-1), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9001), Sillosuchus (PVSJ 85), and possibly Lotosaurus (IVPP V4880 or V4881). This character can be scored from just an ilium because the sacral rib of the primordial sacral leaves a distinct scar on the medial side of the ilium.
203. Second primordial sacral, rib: (0) bifurcated; (1) a single unit (fig. 29) (Dilkes, 1998).
In Mesosuchus (SAM-PK-6046), Prolacerta (BP/1/2675), and Proterosuchus (NM QR 1484) the second sacral rib is bifurcated in dorsal view into a larger anterior portion and a smaller posterior portion. All archosauriforms in this analysis besides Proterosuchus have a second sacral rib that is not bifurcated.
204. Sacral vertebrae, centra articular rims: (0) present in sacrum; (1) nearly obliterated (fig. 29) (Nesbitt, 2007).
The centra rims of sacral vertebrae normally expand well ventral of the ventral surface of the body of the centrum in the sacrum of most archosauriforms whether the centra are coossified or not. In Effigia (AMNH FR 30587) and Shuvosaurus (TTU-P 9001) the centra rims within the sacrum are nearly obliterated; the four sacral vertebrae have been fused into rod similarly in theropods (e.g., Coelophysis bauri; Colbert, 1989).
205. Trunk vertebrae: (0) free from the sacrum; (1) incorporated into the sacrum, with their ribs/transverse processes articulating with the pelvis (Sereno et al., 1993; Langer and Benton, 2006).
206. Caudal vertebrae: (0) free from the sacrum; (1) incorporated into the sacrum, with their ribs/transverse processes articulating with the pelvis (Galton, 1976; Langer and Benton, 2006).
207. “Insertion” of a sacral vertebra between the first and second primordial sacral vertebrae: (0) absent; (1) present (new).
These three characters describe the possible homologies of the sacral vertebrae of taxa with more than the primordial two found in basal archosauriforms. As discussed by Langer and Benton (2006), there are two scoring strategies when describing the number and homology of sacral vertebrae: one scores the absolute number of sacral vertebrae (e.g., Gauthier, 1986; Benton, 1990a; Juul, 1994; Novas, 1996; Rauhut, 2003; Irmis et al., 2007b; Smith et al., 2007) and the second parses out the origin of the sacral vertebrae (Sereno et al., 1993; Sereno, 1999; Langer, 2004; Langer and Benton, 2006). Given that sacral vertebrae increased in a number of archosaurian taxa independently (Juul, 1994; Novas, 1996; Rauhut, 2003; Irmis et al., 2007a), the latter strategy is further explored here.
As explained by Langer and Benton (2006), identifying the origin of the sacral vertebrae requires the identification of primordial sacrals one and two. Within archosauriforms, the plesiomorphic state is the presence of two sacral vertebrae (Gauthier, 1984) as evidenced by only two sacral vertebrae in all non-archosaurian archosauriforms, basal dinosauromorphs, and most crocodylian-line archosaurs. These taxa are the key to identifying the primordial sacral vertebrae. Even though Langer and Benton (2006) stated the importance of identifying the primordial sacrals, they did not explicitly state their criteria for identifying the primordial sacral vertebrae in taxa with additional sacral vertebrae.
Langer and Benton (2006) extensively discussed the possible homologies of additional sacral vertebrae other than the primordial two in Dinosauria and close relatives. Nevertheless, Langer and Benton (2006) only suggested that sacral vertebrae are added anterior to or posterior to the two primordial sacrals, and they always assumed the primordial sacral vertebrae are adjacent. Here, I argue that the primordial sacrals do not always have to be adjacent and an additional sacral vertebra is present between primordial one and two in a number of archosaurs.
The morphology of the primordial sacral ribs and their attachment sites on the ilium is essential in identifying the two primordial sacral vertebrae. Sacral vertebrae are defined as vertebrae that contact the ilium by means of a sacral rib or transverse process. The first primordial sacral rib extends laterally to meet the anterior portion of the ilium at the junction of the anterior process ( = preacetabular process) and the pubic peduncle in taxa that retain the plesiomorphic condition (e.g., Euparkeria). The articular surface of the first primordial sacral rib and the corresponding scar on the ilium are rounded. A small anterior process may articulate with the anterior process of the ilium (see character 202). The second primordial sacral rib is more massive than the first sacral rib and is posterolaterally expanded. In lateral view, the articular surface is teardrop shaped, and this corresponds to a scar on the posteromedial side of the ilium. The more massive anterior portion attaches to the medial side of the ischial peduncle. The posterior portion of the second primordial sacral rib lies ventral to an anteroposteriorly trending ridge located on the medial portion of the posterior process ( = posterior ilia wing = postacetabular process) of the ilium. Additionally, the articular surface of the second sacral rib is greater than that of the first sacral rib. The posterior edge of the first primordial sacral rib meets the anterior edge of the second primordial sacral rib. The identification of the two primordial sacral vertebrae can be made from the sacral ribs alone and their corresponding attachment scars on the medial side of the ilium. Ideally, the sacral centra, sacral ribs, and the medial side of the ilium should be used to identify the primordial sacrals.
It appears that the morphology of the first and second primordial ribs remain rather conserved when there are three or more sacral vertebrae in the sacrum. However, any vertebra that lies between the primordial sacral vertebrae is an “insertion”; a sacral vertebra anterior to the first primordial sacral is considered a dorsosacral, and a sacral vertebra posterior to the second sacral vertebrae is considered a caudosacral.
The sacrum of Arizonasaurus illustrates a clear example among crocodylian-line archosaurs and the sacra of Silesaurus and Allosaurus illustrate examples among dinosauromorphs of taxa with an insertion between the primordial sacral vertebrae.
The well-preserved sacrum of Arizonasaurus (MSM P4590) consists of three sacral vertebrae and all of the sacral ribs (Nesbitt, 2003, 2005a). Sacral rib one of Arizonasaurus corresponds to the first primordial rib of Euparkeria and other taxa based on the location of articulation with the ilium and the morphology of the articular facet of the sacral rib. However, sacral rib one of Arizonasaurus does not expand proportionally as much posteriorly as that of primordial sacral rib one of Euparkeria. Sacral rib three of Arizonasaurus expands posteriorly and attaches ventrally to the anteroposteriorly trending medial ridge of the posterior portion of the ilium like that of the second primordial rib of Euparkeria (SAM 6049B). Therefore, the third sacral of Arizonasaurus is identified as primordial sacral two. Sacral rib three of Arizonasaurus does not expand as much anteriorly as that of primordial sacral rib two of Euparkeria. Sacral rib two of Arizonasaurus is different than both the primordial sacral ribs of Euparkeria and other taxa with only the two primordial sacral vertebrae in both shape and connectivity; it does not have the posteriorly elongated flange of primordial sacral rib two and does not articulate at the junction of the anterior process and the pubic peduncle of the ilium. It is clear that sacral rib two of Arizonasaurus lies between the posteriorly foreshortened primordial sacral rib one and the anteriorly foreshortened primordial sacral rib two (third sacral rib of Arizonasaurus). Furthermore, the articular surface area of the three sacral ribs of Arizonasaurus is similar to the articular surface area of the two sacrals of phytosaurs, but the ilia of the two taxa are relatively the same length. Therefore, a sacral vertebra is “inserted” between primordial one and two in Arizonasaurus, and this is a reasonable explanation of the shortening of sacral ribs one and three. Among putative close relatives of Arizonasaurus, a similar “insertion” of sacral vertebra between primordial one and three is found in Effigia (AMNH FR 30587), Poposaurus (TMM 43683-1), Shuvosaurus (TTU-P 9001), and in Sillosuchus (PVSJ 85).
Among other crocodylian-line archosaurs, a similar “insertion” of a third sacral vertebra is possibly present in Batrachotomus. There are three sacral vertebrae in Batrachotomus (Gower and Schoch, 2009). Even though all sacrals known from Batrachotomus are disarticulated, the well-preserved ilia have three distinct sacral rib scars. These scars are very similar to those in the ilia of Arizonasaurus in that a circular sacral rib scar lies between the primordial sacral rib scars.
Silesaurus (ZPAL unnumbered) was reported as having four sacral vertebrae (Dzik, 2003; Dzik and Sulej, 2007). However, only three sacral ribs attach to the ilium. Silesaurus is unique in that four centra are coossified together by sharing three sacral ribs (see character 208). The three sacral ribs attach in a similar location as the sacral ribs of Arizonasaurus and Batrachotomus. Conversely, the sacral ribs are much more delicate than those of Euparkeria and crocodylian-line archosaurs (see Dzik, 2003; Dzik and Sulej, 2007). Nonetheless, the first sacral rib of Silesaurus attaches in the same location as primordial sacral rib one in plesiomorphic forms, whereas the third sacral rib of Silesaurus attaches with the anteroposteriorly trending ridge on the medial side of the ilium and is posteriorly elongated as with primordial sacral two of plesiomorphic forms. Therefore, the third sacral of Silesaurus is homologous with that of primordial sacral two. Sacral rib two, like that of Arizonasaurus, is squeezed between the first and second primordial sacral ribs. Therefore, Silesaurus also has an “insertion” in the sacrum.
It is clear from the ilium and sacral ribs of Allosaurus (Madsen, 1976) that the taxon has a similar arrangement as Silesaurus and Arizonasaurus; an additional sacral is present between primordial sacrals one and two. The medial surface of the ilium preserves five sacral rib scars. The position and shape of what Madsen (1976) labeled as sacral rib two corresponds to primordial sacral rib one, whereas what Madsen (1976) labeled as sacral rib four corresponds to primordial sacral rib two. Therefore, the scar labeled as sacral rib three corresponds to an insertion.
As more sacral vertebrae are added to the sacral series, the identification of primordial sacral vertebrae becomes more difficult because the sacral ribs become smaller. As they become smaller, the primordial sacrals lose their identifying characteristics. In the sacrum of the abelisaurid Carnotaurus sastrei (MACN 894; Bonaparte et al., 1990), sacral two of Bonaparte et al. (1990) possibly corresponds to primordial sacral one, whereas sacral five of Bonaparte et al. (1990) possibly corresponds to primordial sacral two. Therefore, two vertebrae were inserted between the primordial sacrals.
The identification of dorsosacral and caudosacral vertebrae is possible assuming that my identification of primordial sacrals is correct. Among crocodylian-line archosaurs, Shuvosaurus (TTU-P 9001), Poposaurus (TMM 43683-1), Sillosuchus (PVL 85), Effigia (AMNH FR 30587), and Desmatosuchus (MNA V9300) have a dorsosacral added into the sacrum. There do not appear to be any crocodylian-line archosaurs that add a caudosacral.
Within Dinosauria, a dorsosacral and a caudosacral appear to be added to the sacra of Lesothosaurus (Sereno, 1991b) and Heterodontosaurus (SAM-PK-1332), as well as in neotheropods (e.g., Allosaurus; Madsen, 1976). The origination of sacral vertebrae in sauropodomorphs has received much attention (Galton, 1976; Yates, 2003; Langer, 2003; Langer and Benton, 2006). It is now clear that some sauropodomorphs add an “insertion” (e.g., Massospondylus), whereas others add a caudosacral (e.g., Saturnalia).
This new method for identifying the origin of sacral vertebrae is not fully tested. However, I outline a repeatable methodology for identifying the two primordial sacrals. The identification of caudo- and dorsosacrals and insertions is based strictly on the identification of the two primordial sacrals in taxa with more than two sacral vertebrae.
208. Sacral ribs: (0) almost entirely restricted to a single sacral vertebra; (1) shared between two sacral vertebrae (fig. 29) (new).
In most archosauriforms, sacral ribs are almost entirely restricted to a single sacral vertebra. This includes some taxa with at least three sacral vertebrae (e.g., Batrachotomus, inferred from the ilium of SMNS 80270; Desmatosuchus, MNA V9300; Arizonasaurus, MSM P4590). In Poposaurus (TMM 43683-1), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9001), Sillosuchus (PVSJ 85), Allosaurus (Madsen, 1976) and Silesaurus (ZPAL unnumbered), the sacral ribs lie between two sacral vertebrae and are shared. The sacral ribs coossify the lateral sides of the articular ends of the sacral centra in Silesaurus (ZPAL unnumbered).
209. First primordial sacral, articular surface of sacral rib: (0) circular; (1) C-shaped in lateral view (modified from Langer and Benton, 2006).
The articular surface of the first primordial sacral rib of non-archosaurian archosauriforms and crocodylian-line archosaurs are generally circular. A corresponding circular scar is present on the medial side of the ilium in these taxa. The articular surface of first primordial sacral rib is also circular in Marasuchus (PVL 3871), Pseudolagosuchus (UNLR 53), and Silesaurus (ZPAL unnumbered). Lesothosaurus (BMNH RU B.17) and Eocursor (SAM-PK-0925) also have a similar arrangement to that of basal dinosauromorphs. In contrast, the anterior margin of the rib of first primordial sacral expands dorsally in saurischians (Langer and Benton, 2006). All together, the sacral rib is C-shaped in lateral view where the posterior side is open. A corresponding C-shaped scar is present on the medial side of the ilium. The simple correspondence between the articular surface of the sacral rib and the scar on the medial side of the ilium allows this character to be scored with just the medial side of the ilium.
210. Middle caudal vertebrae, accessory laminar process on anterior face of neural spine: (0) absent; (1) present (fig. 28) (Benton and Clark, 1988; Juul, 1994; Benton, 1999; Benton and Walker, 2002; Rauhut, 2003; Irmis et al., 2007a).
In most archosauriforms, the anterior edge of the neural spine of the caudal vertebrae is continuous ventrally without interruption. Accessory laminar projections sit on the anteroventral portion of the ventral half of the neural spines in phytosaurs (e.g., Smilosuchus, USNM 18313), Ornithosuchus (BMNH R3561), Polonosuchus silesiacus (ZPAL Ab III/563), Prestosuchus (UFRGS 0152-T), Rauisuchus (BSP AS XXV-60-121), Qianosuchus (IVPP V 14300), Ticinosuchus (PIZ T2817), CM 73372, Saltoposuchus (SMNS 12596), Batrachotomus (SMNS 80339; Gower and Schoch, 2009), and Terrestrisuchus (Crush, 1984). As observed in taxa with largely articulated tails, the morphology and location of the anterior laminar process depends on the position within the caudal series. For example, in both Ticinosuchus (PIZ T2817) and Qianosuchus (IVPP V 14300), the projections lie more distally on the neural spine of the anterior caudal vertebrae than they do on the more posterior caudal vertebrae. Furthermore, the projections on the anterior caudal vertebrae are kinks where the dorsal part of the anterior margin is more strongly inclined posteriorly than the ventral part whereas the anterior projections are located at the base of the neural spine and are triangular in lateral view in the more posterior caudal vertebrae. Anterior projections are present in far more caudal vertebrae in Ticinosuchus (caudal 5 to at least caudal 35) compared to Qianosuchus (roughly in only 10 midcaudal vertebrae). Given this disparity, only taxa with relatively complete tails can be scored as (0) whereas taxa with any caudal vertebrae with any form of anterior laminar projections are scored as (1). Here, I am the first to record anterior projections in the phytosaur Smilosuchus (USNM 18313).
Makovicky (1995) and Rauhut (2003) recognized similar features in the caudal vertebrae of Theropoda. Rauhut (2003) has two characters describing the anterior laminar projections among basal theropods, one describing the kink on the anterior edge of the anterior midcaudal vertebrae (character 123) and one describing an anterior process anterior to the neural spine in midcaudal vertebrae (character 125). As in Ticinosuchus (PIZ T2817) and Qianosuchus (IVPP V 14300), both of Rauhut's (2003) two distinct characters occur in the same tail in some crocodylian-line archosaurs. This also may be the case in theropods given that both derived states are present or completely absent for both characters in all taxa in Rauhut's (2003) analysis.
211. Distal caudal vertebrae, prezygapophyses: (0) not elongated; (1) elongated more than a quarter of the adjacent centrum (Gauthier, 1986; Rauhut, 2003; Nesbitt, 2007).
The prezygapophyses of the distal caudal vertebrae are short in non-archosaurian archosauriforms, most crocodylian-line archosaurs, and many basal dinosauromorphs. Effigia (AMNH FR 30588) and Shuvosaurus (TTU-P 9001) have elongated prezygapophyses in the distal caudal vertebrae among crocodylian-line archosaurs. Pterosaurs have elongated caudal prezygapophyses also (Wellnhoffer, 1978). In Dinosauria, ornithischians have short prezygapophyses in the distal caudal vertebrae (e.g., Heterodontosaurus; SAM 1332). This is also true of sauropodomorphs (Gauthier, 1986; Sereno, 1999; Rauhut, 2003; Langer and Benton, 2006). Both Herrerasaurus (PVL 373) and Staurikosaurus (MCZ 1669) have elongated prezygapophyses in the distal caudal vertebrae. Character state (1) is widespread within Theropoda (Rauhut, 2003); elongated caudal prezygapophyses are present in Coelophysis rhodesiensis (Raath 1969), Coelophysis bauri (AMNH FR 7234), Dilophosaurus (Tykoski, 2005a), and most tetanurans, whereas it is absent and Ceratosaurus (Madsen and Welles, 2000) and Coelurus (YPM 2010).
Pectoral Girdle
212. Forelimb–hind limb, length ratio: (0) more than 0.55; (1) less than 0.55 (Gauthier, 1984; Sereno, 1991a; Juul, 1994; Benton, 1999).
= Humerus + radius: Femur + tibia
The ratio of the length of the forelimb to that of the hind limb has been used repeatedly in most phylogenetic analyses of basal archosaurs. Gauthier (1984) first used this character as an ornithodiran synapomorphy, but stated that avian-line archosaurs have a forelimb–hind limb ratio of 0.5. Gauthier (1984) considered pterosaurs as having a ratio less than 0.5 and thus excluded the elongated manus whereas Sereno (1991a) scored pterosaurs as having a ratio greater than 0.5. Benton (1999) later changed the ratio to greater or less than 0.55.
None of the authors ever defined what was measured in both the hind limb and the forelimb. It was assumed by later authors that the humerus to the distal end of the longest ungual was measured for the forelimb and the femur to the ungual of the longest pedal digit for the hind limb. However, many of the taxa that were scored lack manus material. Here, I include only the total length of the humerus + radius for the forelimb and the femur + tibia for the hind limb. This formulation allows most taxa to be scored and the scoring technique employed here agrees well with the scoring of taxa previously.
213. Clavicles: (0) present and unfused; (1) fused into a furcula (modified from Gauthier, 1986; Sereno, 1991a; Benton, 1999; Benton and Walker, 2002).
Clavicles are present in non-archosaurian archosauriforms and basal crocodylian-line archosaurs. Clavicles are not present in crocodylomorphs (e.g., Hesperosuchus “agilis,” CM 29894; Protosuchus richardsoni, AMNH FR 3024) and, therefore, they are scored as inapplicable. Like the interclavicle, the clavicles of the pterosaur Eudimorphodon are separate ossifications in a small specimen and incorporated into the sternum (Wild, 1993). All other pterosaurs seem to lack distinct ossifications of the clavicles. Within Dinosauria, clavicles are present, but do not contact in some ornithischians (e.g., Psittacosaurus) and are unossified in others (Butler et al., 2008a). The clavicles of some nonsauropod sauropodomorphs (e.g., Massospondylus) may contact each other at the midline, but do not fuse (Yates and Vasconcelos, 2005). A furcula ( = fused clavicles) is present in nearly all theropods known from complete skeletons including Coelophysis bauri (AMNH FR 30647; Rinehart et al., 2007; Nesbitt et al., 2009d) and Allosaurus fragilis (UUVP 6102; Chure and Madsen, 1996). This character has been employed by various datasets exploring theropod relationships (e.g., Norell et al., 2001; Clarke, 2004).
214. Interclavicle: (0) present; (1) absent (fig. 30) (Gauthier, 1986; Sereno, 1991a; Juul, 1994; Benton, 1999).
Fig. 30
Examples of pectoral girdle character states of archosauriforms: A, right scapulocoracoid of Smilosuchus gregorii (USMN 18313) in lateral view; B, right scapula of Batrachotomus kuperferzellensis (SMNS 80271) in lateral view; C, left portion of the pectoral girdle of Proterosuchus fergusi (NM QR 1484) in lateral view; D, partial left scapulocoracoid of Postosuchus alisonae (UNC 14475) in lateral view; E, partial left coracoid of Hesperosuchus agilis (AMNH FR 6758) in lateral view F, right coracoid and scapula of Lewisuchus admixtus (UNLR 01) in lateral view; G, ?right clavicle of Postosuchus alisonae (UNC 14475) in lateral view; H, intercalvicle of Smilosuchus gregorii (USMN 18313) in dorsal view. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 5 cm in A–B, D, H and 1 cm in C, E, F.
The interclavicle is present in archosauriforms plesiomorphically (Sereno, 1991a) and persists through Pseudosuchia. In Pterosauria, an interclavicle appears to be present in young individuals of Eudimorphodon (MCSNB 8950), but fuse to the pectoral elements in larger individuals (Wild, 1993). A distinct interclavicle is not present in all other pterosaurs. Ornithischians and saurischians lack an interclavicle. However, the pectoral girdles in the successive sister taxa to Dinosauria (Silesaurus, Marasuchus, Lagerpeton) do not have the pectoral region completely preserved. As a result, the optimization of this character within Dinosauromorpha is not clear.
215. Interclavicle: (0) T-shaped; (1) anterolateral processes reduced or absent (fig. 30) (modified from Gauthier, 1984; Sereno, 1991a; Gower and Sennikov, 1997; Nesbitt et al., 2009a).
The well-preserved, articulated interclavicle of Proterosuchus (NM QR 1484) has long tapering lateral processes. Gower and Sennikov (1997) report that the interclavicle of one erythrosuchian, Vjushkovia triplicostata has an interclavicle with reduced lateral processes. To date, no other interclavicle is known in erythrosuchians. As pointed out by Sereno (1991a) the holotype of Euparkeria capensis (SAM 5867) possesses short lateral processes as with members of the Archosauria. Although not completely preserved in any proterochampsian, the pectoral girdle of Tropidosuchus (PVL 4606) bears two thin clavicles in articulation with short processes of the fragmentary interclavicle (Arcucci, 1990). All archosaurs with an interclavicle are scored as (1).
216. Scapula, length: (0) more than 75% of humerus length; (1) less than 75% of humerus length (Sereno, 1991a).
Sereno (1991a) used this character to unite Scleromochlus and Pterosauria. Even though the scapula of pterosaurs is short relative to the humerus, other taxa such as crocodylomorphs (e.g., Hesperosuchus, AMNH FR 6758) have short scapulae relative to the humerus.
217. Scapula, entire anterior margin: (0) straight/convex or partially concave; (1) markedly concave (fig. 30) (modified from Gower and Sennikov, 1997; Nesbitt et al., 2009a).
The scapulae of Mesosuchus (SAM 6536; Dilkes, 1998), Prolacerta (BP/1/2675; Gow 1975), and Proterosuchus (NM QR 1484) have wide scapulae that have a partly concave partly convex anterior margin. In contrast, Erythrosuchus (BMNH R3267a), Vancleavea (GR 138), Euparkeria (SAM 5867), Tropidosuchus (PVL 4604), Chanaresuchus (PVL 4575), and Archosauria have scapulae that have markedly concave anterior margins. Gauthier (1984) had a similar character, scapula 50% taller than wide, to describe erythrosuchians + Archosauria. These two characters cover the same basic observation and both are not used here.
218. Scapula, blade height versus distal width: (0) less than 3 times distal width; (1) more than 3 times distal width (fig. 30) (Sereno, 1999).
In most basal archosauriforms, the distal width of the scapula is about half the height of the blade. In both Silesaurus-like taxa and Neotheropoda, the scapula is tall relative to the distal width. Sereno (1999) listed this asa character of Herrerasauridae + Neotheropoda. However, a complete scapula of Herrerasaurus is not known and the referred scapula to Herrerasaurus ( = PVL 53; “Frenguellisaurus”) is missing the distal extremity. Therefore, state (1) cannot support Herrerasauridae + Neotheropoda at present. Furthermore, Eoraptor is scored as (0).
219. Scapula, teardrop-shaped tuber on the posterior edge, just dorsal of the glenoid fossa: (0) absent; (1) present (fig. 30) (new).
In most archosauriforms, the lateral surface of the scapula bears a small scar on the posterior edge of the element just dorsal to glenoid (e.g., Erythrosuchus, BMNH R3762a). This scar corresponds to the origin of the scapular head of the M. triceps (Gower, 2003; Gower and Schoch, 2009). In Prestosuchus (BSP XXV 1-3/5-11/ 28-41/49), Batrachotomus (SMNS 80271), and Riojasuchus (PVL 3827) there is a large, distinct a tuber on the anterior edge just dorsal to the glenoid.
220. Scapula, acromion process: (0) in the about the same plane as ventral edge of the scapula; (1) distinctly raised above the ventral edge of the scapula (fig. 30) (new).
In the archosauriforms, Proterosuchus (NM QR 1484) and Prolacerta (BP/1/2675), Euparkeria (SAM 6758) and phytosaurs (e.g., Smilosuchus, USNM 18313), the anteroventral portion of the scapula is flat. In the scapula of Erythrosuchus (Gower, 2003), Chanaresuchus (PVL 4575), and nearly all crown-group archosaurs, the acromion process is distinctly raised above the ventral edge of the scapula.
221. Scapulocoracoid, anterior margin: (0) distinct notch between the two elements; (1) uninterrupted edge between the two elements (Parrish, 1993; Benton, 1999).
In many basal archosauriforms, the anterior margin of the junction of the coracoid has a distinct notch. This is usually a consequence of a rounded anterior margin of both the coracoid and the scapula. A scapulocoracoid notch is present in phytosaurs (Smilosuchus, USNM 18313), Euparkeria (SAM 5867), Chanaresuchus (PVL 4575), Tropidosuchus (PVL 4604), aetosaurs (e.g., Aetosaurus, SMNS 5770 S-2), Ornithosuchus (BMNH R 3916), Revueltosaurus (PEFO 34561), Riojasuchus (PVL 3827), Prestosuchus (BSP XXV 1-3/5-11/ 28-41/49), Saturnalia (MCP 3845-PV), Plateosaurus (AMNH FR 6810), Silesaurus (Dzik and Sulej, 2007: fig. 18), and Allosaurus (Madsen, 1976). There is no notch in Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Hesperosuchus agilis (AMNH FR 6758), Dromicosuchus (UNC 15574), Dibothrosuchus (IVPP V 7907), Sphenosuchus (SAM 3014), and Protosuchus richardsoni (AMNH FR 3024). The anterodorsal corner of the coracoid is “squared-off” in taxa scored as (1). The condition in Batrachotomus is not known because all the anterior portions of the coracoids are broken and reconstructed (contra Parrish, 1993).
222. Coracoid: (0) subcircular in lateral view; (1) with postglenoid process (notch ventral to glenoid).
223. Coracoid, postglenoid process: (0) short; (1) elongate and expanded posteriorly only; (2) elongate and expanded anteriorly and posteriorly. ORDERED (fig. 30) (modified from Clark et al., 2004).
The coracoids of Proterosuchus (NM QR 1484), Erythrosuchus (BMNH R3592), Chanaresuchus (PVL 4575), Euparkeria (SAM 6049), and phytosaurs (e.g., Smilosuchus, USNM 18313) are subcircular in lateral view. In Revueltosaurus (PEFO 34561), aetosaurs (e.g., Longosuchus TMM 31185-84a), Riojasuchus (PVL 3827), rauisuchians (e.g., Batrachotomus, SMNS 80271; Postosuchus alisonae, UNC 15575), Effigia (AMNH FR 30587), and crocodylomorphs (e.g., Hesperosuchus agilis, AMNH FR 6758, Protosuchus richardsoni, AMNH FR 3024), there is a distinct notch ventral to the glenoid, thus creating a posteriorly projecting process. Furthermore, the postglenoid process is rather short (not expanded much posterior to the posterior lip edge of the glenoid) in the taxa listed except in crocodylomorphs. In crocodylomorphs, the posteromedial edge of the coracoid expands posteromedially to meet the interclavicle at the midline.
Clark et al. (2004) added a forth state “with extremely elongate posteromedial process” from Clark et al. (2000) and scored Dromicosuchus, Sphenosuchus, Dibothrosuchus, Junggarsuchus for this character state. Pseudhesperosuchus, Hesperosuchus, and Terrestrisuchus are scored as (1). However, the differences between the length of the postglenoid process between Hesperosuchus and Pseudhesperosuchus and Dromicosuchus cannot be substantiated. The postglenoid processes of Dibothrosuchus (Wu and Chatterjee, 1993) and Sphenosuchus (Walker, 1990), however, are marginally longer than that of other non-crocodyliform crocodylomorphs. State (3) of Clark et al. (2004) is not used here.
Here, I have modified the third character state to account for the condition in the crocodyliforms Protosuchus, Orthosuchus, and Alligator. These taxa have an elongated postglenoid process that expands posteromedially and the medial extent of the element expands both anteriorly and posteriorly to form a pendulum shape. Litargosuchus (BP/1/5237) is scored as (2).
224. Coracoid, posteroventral edge, deep groove: (0) absent; (1) present (fig. 30) (new).
The posteroventral edge of the coracoid of most archosauriforms tapers to a thin edge. In Postosuchus kirkpatricki (TTU-P 9000) and the crocodylomorphs Hesperosuchus (AMNH FR 6758), Dromicosuchus (UNC 15574), Terrestrisuchus (Crush, 1984), Pseudhesperosuchus (PVL 3830) and Sphenosuchus (SAM 3014), a deep groove is present on the posteroventral edge of the coracoid. In Postosuchus and crocodylomorphs, the interclavicle fits into this groove.
225. Coracoid, posteroventral portion: (0) smooth; (1) possesses a “swollen” tuber ( = biceps tubercle) (fig. 30) (new).
Among non-archosaurian archosauriforms (Chanaresuchus, PVL 4575; Euparkeria, SAM 5867) and phytosaurs (Smilosuchus, USNM 18313), the coracoid is smooth medially to the glenoid on the lateral surface. In contrast, the coracoid of suchians such as Revueltosaurus (PEFO 34561), Arizonasaurus (MSM P4590), Batrachotomus (SMNS 80271), Hesperosuchus (AMNH FR 6758) and dinosauriforms such as Silesaurus (ZPAL Ab III/361), Saturnalia ( = acrocoracoid tubercle of Langer et al., 2007; MCP 3844-PV), and Heterodontosaurus (SAM-K-1332; Santa Luca, 1980) have a “swollen” tuber on the posterolateral surface of the coracoid medial to the glenoid. Taxa scored as (1) have a shallow fossa that lies between the tuber and the glenoid. Additionally, a notch is present between the glenoid and the “swollen” tuber on the posterior edge in all taxa scored as (1). The poorly preserved coracoid of Marasuchus (PLV 3871) does not preserve the area where the “swollen” tuber would be found; however, a clear notch is present on the posterior edge. Therefore, Marasuchus is scored as (1). Furthermore, in the original description of Lewisuchus, Romer (1972d: fig. 7) illustrates the coracoid with a broken posterior border. Nevertheless, the posterior border is not broken and it possesses a clear notch and a “swollen” tuber.
226. Coracoid, anterior portion: (0) rounded; (1) distinctly hooked (fig. 30) (modified from Sereno, 1991a).
The anterior portion of the coracoid of most archosauriforms is rounded whereas the anterior portion of the coracoid of phytosaurs is hooked (e.g., Smilosuchus USNM V18313). Additionally, phytosaurs lack a coracoid foramen (Sereno, 1991a).
227. Glenoid, orientation: (0) posterolaterally; (1) directed posteroventrally (fig. 30) (Fraser et al., 2002).
The glenoid faces posterolaterally in non-archosaurian archosauriforms (e.g., Erythrosuchus, BMNH R3592; Chanaresuchus, PVL 4575) as well as phytosaurs (Smilosuchus, USNM 18313), Revueltosaurus (PEFO 34561), aetosaurs (e.g., Typothorax, MCZ 1488), and some rauisuchians (Batrachotomus, SMNS 80271). In these taxa, the humerus is oriented more laterally then posteriorly. The glenoid is directed posteroventrally in Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), and basal crocodylomorphs (e.g., Hesperosuchus agilis, AMNH FR 6758).
Fraser et al. (2002) cited a posteroventrally directed glenoid as a synapomorphy of Dinosauria. However, a posteroventrally directed glenoid is present in Marasuchus (PVL 3870) and Silesaurus (ZPAL Ab III/362). Thus, the distribution of character state (1) has a wider distribution among avian-line archosaurs.
228. Coracoid, deep fossa on the posterodorsal edge: (0) absent; (1) present (new).
The posterior edge of the coracoid is either rounded or notched (see character 223). The coracoids of Sillosuchus (PVSJ 85), Effigia (AMNH FR 30587), and Shuvosaurus (TTU-P 9001) have an elongated postglenoid process. A deep fossa is located on the dorsal side of the postglenoid process in these forms (Nesbitt, 2007).
229. Coracoid, sharp ridge leading from the glenoid to anteroventral corner: (0) absent; (1) present (new).
The lateral surface of the coracoid of most archosauriforms is smooth. In Prestosuchus (UFRGS 0156-T), and a new taxon ( = Tanzanian pseudosuchian) there is a sharp ridge leading from the glenoid to anteroventral corner of the coracoid.
Forelimb
230. Humerus, apex of deltopectoral crest situated at a point corresponding to: (0) less than 30% down the length of the humerus; (1) more than 30% down the length of the humerus (fig. 31) (modified from Bakker and Galton, 1974; Benton, 1990a; Juul, 1994; Novas, 1996; Benton, 1999).
Fig. 31
Forelimb pro- and epipodials of archosauriforms: A, left humerus of Shuvosaurus inexpectatus (TTU-P unnumbered) in posterior view; B, right humerus of Eocursor parvus (SAM-PK-0925) in anterior view; C, left humerus of Postosuchus alisonae (UNC 15575) in posterior (left) and anterior (right) views; D, right humerus of Batrachotomus kuperferzellensis (SMNS 80275) in anterior (left) and posterior (right) view; E, left ulna of Smilosuchus gregorii (USNM 18313) in proximal (top), lateral (middle), and distal (bottom) views; F, right ulna of Batrachotomus kuperferzellensis (SMNS 80275) in lateral view; G, left ulna of Fasolasuchus tenax (PVL 3850) in proximal (top), lateral (middle), and distal (bottom) views; H, left ulna and radius of Hesperosuchus agilis (AMNH FR 6758) in medial (top) and distal (bottom) views; I, left radius of Postosuchus alisonae (UNC 15575) in posterior view. J, right forelimb of Euparkeria capensis (SAM 5867). Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 5 cm in C–G, I, and 1 cm in A, B, H, J.
Langer and Benton (2006) thoroughly discussed the distribution of the character states of this character and find that state (1) is restricted to dinosaurs within Archosauria. Here, I follow the conclusions and scorings of Langer and Benton (2006). Furthermore, the commonly used character “deltopectoral crest on humerus: (0) rounded or pointed (1) subrectangular” is redundant with the character discussed here; a subrectangular deltopectoral crest is a consequence of having a distally elongated crest. Erythrosuchus is scored as (1).
231. Humerus, length: (0) longer than or subequal to 0.6 of the length of the femur; (1) shorter than 0.6 of the length of the femur (modified from Novas, 1996; Langer and Benton, 2006).
Langer and Benton (2006) thoroughly discussed the distribution of the character states and find that state (1) is restricted to Herrerasaurus (PVSJ 373), Eoraptor (PVSJ 512), and neotheropods.
232. Humerus, proximal head: (0) confined to the proximal surface; (1) posteriorly expanded and hooked (fig. 31) (new).
In nearly all archosauriforms, the articular surface of the head of the humerus is confined to the proximal surface of the element. In Postosuchus kirkpatricki (TTU-P 9002), P. alisonae (UNC 15575), and the crocodylomorphs Hesperosuchus (AMNH FR 6758), Terrestrisuchus (BMNH R7591b), Litargosuchus (BP/1/5237), and Sphenosuchus (SAM 3014), the head of the humerus expands posteriorly. The posterior expansion is concave ventral to the articular surface, thus creating a hooked shape.
233. Humerus, proximal articular surface: (0) continuous with the deltopectoral crest; (1) separated by a gap from the deltopectoral crest (fig. 31) (new).
In most archosauriforms, the proximal articular surface is continuous with the dorsal portion of the deltopectoral crest. In most dinosaurs (e.g., Tawa), the dorsal portion of the deltopectoral and the proximal surface of the humerus are separated usually by a thin ridge of bone.
234. Humerus, ectepicondylar flange: (0) present; (1) absent (fig. 31) (Benton and Clark, 1988; Gauthier et al., 1988).
Benton and Clark (1988) listed the absence of an ectepicondylar groove as a synapomorphy of archosauriforms whereas in Gauthier et al. (1988) the absence of an ectepicondylar groove is a synapomorphy of Erythrosuchus + Archosauria. Even though a groove is not present in Euparkeria or proterochampsians, a clear groove is present in phytosaurs (Smilosuchus, USNM 18313), aetosaurs (Stagonolepis, BMNH R4784; Aetosaurus, SMNS 5770 S-5), Batrachotomus (SMNS 80275), Postosuchus kirkpatricki (TTU-P 9000), Stagonosuchus (GPIT/RE/3831), and Poposaurus (YPM 57100). In a humerus assigned to the aetosaur Desmatosuchus (UCMP A269/32184), the ectepicondylar groove is folded over to create a foramen. An ectepicondylar groove is absent in the crocodylomorphs observed here. Among avian-line archosaurs, an ectepicondylar groove is also present in a humerus (TTM-31000-1329) assigned to the non-dinosauriform dinosauromorph Dromomeron gregorii but unknown in any other member.
235. Humerus, distal end width: (0) narrower or equal to 30% of humerus length; (1) greater than 30% of humerus length (fig. 31) (Langer and Benton, 2006).
As explained by Langer and Benton (2006), the distal width of the humerus is greater than 30% the length of the element in sauropodomorphs.
236. Humerus, proximal portion: (0) expanded more than twice the width of the midshaft of the humerus; (1) expanded less than twice the width of the midshaft of the humerus (fig. 31) (Nesbitt, 2007).
Among archosauriforms, the proximal portion of the humerus is greatly expanded relative to the width of the midshaft. In contrast, the poorly expanded proximal end of the humeri of Effigia (AMNH 30587), Shuvosaurus (TTU-P 9001) and possibly in Sillosuchus (PVSJ 85) are not expanded more than twice that of the midshaft.
237. Ulna, lateral tuber ( = radius tuber) on the proximal portion: (0) absent; (1) present (fig. 31) (new).
The proximal portion of the ulna is mediolaterally compressed without a lateral tuber in Proterosuchus (NM QR 1484), Vancleavea (GR 138), Euparkeria (SAM 5867), and phytosaurs (e.g., Smilosuchus, USNM 18313). In aetosaurs, Revueltosaurus (PEFO 34561), most paracrocodylomorphs, and basal dinosauriforms (e.g., Marasuchus, PVL 3870, Dinosauria), a distinct tuber is present on the lateral side of the proximal portion of the ulna.
238. Ulna, distal end in posterolateral view: (0) rounded and convex; (1) squared off where the distal surface is nearly flat (fig. 31) (new).
Among basal archosauriforms, the distal end of the ulna is rounded and convex in posterolateral view in Proterosuchus (NM QR 1484), Vancleavea (GR 138), Euparkeria (SAM 5867), phytosaurs (e.g., Smilosuchus, USNM 18313), Revueltosaurus (PEFO 34561), and aetosaurs (e.g., Aetosaurus, SMNS 5770 S-6). Likewise, all avian-line archosaurs have distal surfaces that are well rounded. The distal end of the ulna is squared off where the distal surface is at nearly a right angles to the shaft surfaces, in Ticinosuchus (PIZ T2817), Fasolasuchus (PVL 3851), Batrachotomus (SMNS 80275), Postosuchus kirkpatricki (TTU-P 9002), P. alisonae (UNC 15575), Hesperosuchus agilis (AMNH FR 6758), Dromicosuchus (UNC 15574), Terrestrisuchus (BMNH R7562), and Protosuchus richardsoni (AMNH FR 3024).
239. Ulna, distal end: (0) anteroposteriorly compressed or rounded; (1) with anterior expansion (fig. 31) (new).
In archosauriforms, the distal end of the ulna typically is anteroposteriorly compressed or rounded. This includes Proterosuchus (NM QR 1484), Erythrosuchus (SAM 905), Euparkeria (SAM 5853), Revueltosaurus (PEFO 34561), Riojasuchus (PVL 3827), and basal avian-line archosaurs as examples. In contrast, the distal end of the ulna has an expansion on the anterior surface in Fasolasuchus (PVL 3851), Batrachotomus (SMNS 80275), Postosuchus kirkpatricki (TTU-P 9002), P. alisonae (UNC 15575), Hesperosuchus agilis (AMNH FR 6758), Dromicosuchus (UNC 15574), Terrestrisuchus (BMNH R7562), Protosuchus richardsoni (AMNH FR 3024), and Alligator. The anterior expansion tapers to a ridge that extends proximally along the shaft.
240. Radius, distal end: (0) convex; (1) shallow longitudinal groove on the posterior side (fig. 31) (new).
The posterior side of the radius in most archosauriforms is convex and rounded. In Postosuchus kirkpatricki (TTU-P 9002), Postosuchus alisonae (UNC 15575), Hesperosuchus agilis (AMNH FR 6758), and Revueltosaurus (PEFO 34561), there is a groove on the posterior side of the radius.
241. Radius, length: (0) longer than 80% of humerus length; (1) shorter than 80% of humerus length (Langer and Benton, 2006).
In most archosauriforms, the humerus and the radius are nearly the same length. The radius is shorter than 80% of humerus in ornithischians, Saturnalia, sauropodomorphs, and theropods as detailed by Langer and Benton (2006). However, the radius and the humerus are about the same length in the Tawa and Herrerasaurus (estimated from PVSJ 407).
242. Proximal carpals (radiale, ulnare): (0) equidimensional; (1) elongate (fig. 32) (Benton and Clark, 1988; Parrish, 1993; Benton and Walker, 2002; Clark et al., 2004).
Fig. 32
The right manus of archosauriforms: A, Erythrosuchus africanus (redrawn from Gower, 2003); B, Vancleavea campi (based from GR 138); C, Euparkeria capensis (left manus) (SAM 13666); D, Pseudopalatus (based from UCMP 27235); E, Dibothrosuchus elaphros (IVPP V 7907); F, Terrestrisuchus gracilis (redrawn from Crush, 1984); G, Postosuchus alisonae (based from UNC 15575); H, Longosuchus meadei (redrawn from Sawin, 1947); I, Heterodontosaurus tucki (redrawn from Santa Luca, 1980); J, Plateosaurus engelhardti (based from AMNH FR 6810); K, Herrerasaurus ischigualastensis (redrawn from Sereno, 1994). Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 1 cm.
In most archosauriforms with ossified carpals, the proximal carpals are rounded or cubic. Examples of short proximal carpals include Proterosuchus (SAM 160), Typothorax (MCZ 1488), Riojasuchus (PVL 3827), Postosuchus alisonae (UNC 15575), and Heterodontosaurus (SAM-PK-1332). In crocodylomorphs, the proximal carpals are highly elongated and the shafts of the elements resemble those of the limb bones (Benton and Clark, 1988). Elongated proximal carpals are present in Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Dibothrosuchus (IVPP V 7907), Hesperosuchus agilis (AMNH FR 6758), Terrestrisuchus (BMNH R7557), Protosuchus richardsoni (MCZ 6727), and Orthosuchus (SAM-K-409).
243. Ulnare, length: (0) shorter than the longest metacarpal; (1) longer than the longest metacarpal (fig. 32) (new).
In crocodylomorphs, the ulnare and radiale are elongated (Benton and Clark, 1988). Within Crocodylomorpha the ulnare and radiale are shorter than the longest metacarpal in Hesperosuchus “agilis” (CM 29894), Dromicosuchus (UNC 15574), Terrestrisuchus (Crush, 1984), and Alligator. In contrast, the ulnare and radiale are longer than the longest metacarpal in Dibothrosuchus (IVPP V 7907), Orthosuchus (SAM-K-409), and Protosuchus richardsoni (AMNH FR 3024).
244. Pteroid bone: (0) absent; (1) present (Bennett, 1996).
A pteroid bone is a specialized carpal element in pterosaurs. The element articulates with the preaxial carpal ( = lateral distal carpal), is directed medially, and controlled the propatagium (Bennett, 2007). A pteroid is present in nearly all pterosaurs including the basal pterosaurs Eudimorphodon ranzii (MCSNB 2888) and Peteinosaurus zambellii (MCSNB 3359). A pteroid has not been found thus far outside Pterosauria.
245. Longest metacarpal: Longest metatarsal: (0) >0.5; (1) <0.5 (new).
This character attempts to compare the size of the pes with that of the manus. The size of the manus is rather small relative to the pes in most basal archosaurs. Even though the ability to score this character relies on presence of both a complete manus and pes, it can be scored in a variety of basal archosauriforms. Among non-archosaurian archosauromorphs, Prolacerta (BP/1/2674), Proterosuchus (SAM 140), Vancleavea (GR 138) and Euparkeria (pes from SAM 5867 scaled to the manus of SAM 13666), the longest metacarpal is longer than 50% of the longest metatarsal. The same is true in phytosaurs (e.g., Pseudopalatus, UCMP 27235). The longest metacarpal is longer than 50% the length of the longest metatarsal in most members of Archosauria. Pterosaurs are not scored because of the greatly modified manus. Nevertheless, if the second longest metacarpal is compared to the longest metatarsal, they would be scored as (0) given the proportions in a variety of basal pterosaurs. Among most members of the Archosauria, metacarpal three is usually the longest in the manus whereas metatarsal three is the longest in the pes.
246. Metacarpals, proximal ends: (0) overlap; (1) abut one another without overlapping (fig. 32) (Sereno and Wild, 1992; Clark et al., 2000; Olsen et al., 2000; Benton and Walker, 2002; Sues et al., 2003; Clark et al., 2004).
In most basal archosauriforms, the proximal portions of the metacarpals overlap each other. In this configuration, the contacting surfaces of the metacarpals are imbricated laterally where metacarpal I lies on the anterior/dorsal surface of metacarpal II. Clark et al. (2000), followed by later studies using the same dataset (Clark et al., 2004), scored CM 29894, Saltoposuchus, and Dibothrosuchus as having abutting metacarpals. However, in these taxa, the metacarpals are imbricated as in Alligator.
In avian-line archosaurs, the proximal portions of the metacarpals abut one another in Herrerasaurus (PVSJ 373), Heterodontosaurus (SAM-PK-1332), and neotheropods examined in this study. Sereno (1999) used a similar character “metacarpals I–III, intermetacarpal articular facets” to unite Herrerasaurus and neotheropods.
247. Manual length (measured as the average length of digits I–III): (0) accounts for less than 0.3 of the total length of humerus plus radius; (1) more than 0.3 but less than 0.4 of the total length of humerus plus radius; (2) more than 0.4 of the total length of humerus plus radius. ORDERED (modified from Gauthier, 1986; Langer and Benton, 2006).
248. Medialmost distal carpal: (0) subequal other distal carpals; (1) significantly larger than other distal carpals (Gauthier, 1986; Langer and Benton, 2006).
The distal carpals are proportionate to the size of the proximal of portion of its metacarpal. Langer and Benton (2006) argue that the enlarged carpal 1 of sauropodomorphs and theropods is homologous. Sauropodomorphs (e.g., Massospondylus BP/1/4934) do have an enlarged carpal 1, but this is proportionate with the enlargement of metacarpal I relative to basal members of Dinosauria. In theropods (Coelophysis, AMNH FR unnumbered and Allosaurus DINO 11541), the carpal capping metacarpal one also caps metacarpal II. This is not the case in sauropodomorphs were a fully formed carpal 2 lies on top of metacarpal II. Here, I suggest that only theropods can be scored as (1). Langer and Benton's (2006) scoring of this is character should not be a eusaurichian synapomorphy and, as a result, this character would not be a character excluding Herrerasaurus from Eusaurischia.
249. Distal carpal V: (0) present; (1) absent (fig. 32) (Sereno, 1999; Langer and Benton, 2006).
Langer and Benton (2006) thoroughly described this character and, following Sereno (1999), find that the absence of distal carpal V as a saurischian synapomorphy. However, a juvenile specimen of Tawa has distal carpal V, suggesting that (1) either the taxa autapomorphically reevolved a distal carpal V or (2) the distal carpal V may be present in juvenile saurischians, but fused to other distal carpals in more mature individuals.
250. Extensor pits on the proximodorsal portion of metacarpals I–III: (0) absent or shallow and symmetrical; (1) deep and asymmetrical (fig. 32) (modified from Sereno et al., 1993; Langer and Benton, 2006).
Originally, Sereno et al. (1993) used deep extensor pits to unite in Herrerasaurus (PVSJ 373) and neotheropods. Later, Sereno (1999) scored Eoraptor as (1) and modified the character by including symmetry versus asymmetry to the character. As discussed by Langer and Benton (2006), deep extensor pits are present in Heterodontosaurus and basal sauropodomorphs as well. Furthermore, the relative depth and asymmetry of basal dinosaur taxa are difficult to assess as pointed out by Langer and Benton (2006). Nevertheless, Sereno's (1999) character states and scorings are retained here even though this character needs further revision.
251. Metacarpal I, width at the middle of the shaft accounts for: (0) less than 0.35 of the total length of the bone; (1) more than 0.35 of the total length of the bone (fig. 32) (modified from Bakker and Galton, 1974; Langer and Benton, 2006).
252. Digit I with metacarpal: (0) longer than the ungual; (1) subequal or shorter than the ungual (Sereno, 1999; Langer and Benton, 2006).
253. Manual digit I, first phalanx: (0) is not the longest nonungual phalanx of the manus; (1) is the longest nonungual phalanx of the manus (fig. 32) (Gauthier, 1986; Langer and Benton, 2006).
254. Metacarpal I, distal condyles: (0) approximately aligned or slightly offset; (1) lateral condyle strongly distally expanded relative to medial condyle (fig. 32) (modified from Bakker and Galton, 1974, Langer and Benton, 2006; Irmis et al., 2007a).
255. Manual digit II, second phalanx ( = 2.2): (0) shorter than first phalanx; (1) longer than first phalanx (fig. 32) (modified from Gauthier, 1986; Langer and Benton, 2006).
256. Metacarpal II: (0) shorter than metacarpal III; (1) equal to or longer than metacarpal III (fig. 32) (Gauthier, 1986; Langer and Benton, 2006; Irmis et al., 2007a).
257. Manual digits I–III: (0) blunt unguals on at least digits II and III; (1) trenchant unguals on digits I–III (fig. 32) (Gauthier, 1986; Juul, 1994; Benton, 1999; Irmis et al., 2007a).
258. Manual digit IV: (0) five phalanges; (1) four phalanges; (2) one phalanx (fig. 32) (Gauthier, 1986; Benton and Clark, 1988; Sereno et al., 1993; Novas, 1996; Benton, 1999; Irmis et al., 2007a).
259. Metacarpal IV: (0) present; (1) reduced to a nubbin or absent (fig. 32) (Gauthier, 1986).
Metacarpal IV is present in all non-archosaurian archosauriforms, crocodylian-line archosaurs, and nontetanuran avian-line archosaurs. As described by Gauthier (1986) and Rauhut (2003), metacarpal IV is either highly reduced or absent in tetanurans (see Xu et al., 2009, for a different interpretation).
260. Metacarpal IV, length: (0) longest of the metacarpals; (1) about the same length or shorter than metacarpal III (fig. 32) (new).
In Mesosuchus (SAM 6046), Prolacerta (BP/1/2675), and Proterosuchus (SAM 140), metacarpal IV is the longest of the metacarpals in the manus. Alternatively, metacarpal III is longer than or about the same length as metacarpal IV in Vancleavea (GR 138), Euparkeria (SAM 13666), Pseudopalatus (UCMP 27235), Aetosaurus (SMNS 5770 S-10), Ticinosuchus (PIZ T2817), Postosuchus kirkpatricki (TTU-P 9000), Postosuchus alisonae (UNC 15575), Poposaurus gracilis (YPM 57100), Hesperosuchus “agilis” (CM 29894) and all avian-line archosaurs sampled in this analysis.
261. Metacarpal IV, shaft width: (0) about the same width as that of metacarpals I–III; (1) significantly narrower than that of metacarpals I–III (fig. 32) (modified from Sereno et al., 1993; Langer and Benton, 2006).
262. Manual digit IV length: (0) less than or equal to 50% of total forelimb length; (1) more than 50% of total forelimb length (Bennett, 1996; Irmis et al., 2007a).
In pterosaurs, manual digit IV is greatly elongated (Bennett, 1996) and only pterosaurs are scored as (1).
263. Manual digit V: (0) possesses one or more phalanges; (1) absent or reduced to a tiny nubbin (fig. 32) (modified from Bakker and Galton, 1974; Langer and Benton, 2006; Irmis et al., 2007a).
Pelvic Girdle
264. Ilium, supraacetabular crest ( = supraacetabular rim): (0) projects laterally or ventrolaterally; (1) projects ventrally (fig. 34) (Gauthier, 1986).
A supraacetabular crest roofs the acetabulum in all archosauriforms. In nearly all non-archosaurian archosauriforms, crocodylian-line archosaurs, and avian-line archosaurs, the supraacetabular crest projects laterally or ventrolaterally. In Poposaurus (FMNH UR 357; YPM 57100), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9001), and Sillosuchus (PVSJ 85), the supraacetabular crest projects ventrally at its distal margin. A similar condition is also found in Coelophysis (AMNH FR 7224) and Dilophosaurus (UCMP 37302). The acetabulum covers the lateral portion of the proximal portion of the femur (Gauthier, 1986) in taxa scored as (1).
265. Ilium, crest dorsal to the supraacetabular crest/rim: (0) absent; (1) present and divides the anterior ( = preacetabular) process from the posterior ( = postacetabular) process; (2) confluent with anterior extent of the anterior ( = preacetabular) process of the ilium (figs. 33–34).
Fig. 33
The pelvic girdle of archosauriforms in lateral view: A, Proterosuchus fergusi (redrawn from Cruickshank, 1972); B, Chanaresuchus bonapartei (redrawn from Romer, 1972b); C, Batrachotomus kuperferzellensis (redrawn from Gower and Schoch, 2009); D, Arizonasaurus babbitti (redrawn from Nesbitt, 2005a); E, Shuvosaurus inexpectatus (modified from Long and Murry, 1995); F, Terrestrisuchus gracilis (redrawn from Crush, 1984); G, Protosuchus richardsoni (modified from Colbert and Mook, 1951); H, Lagerpeton canarensis (redrawn from Sereno and Arcucci, 1994a); I, Marasuchus lilloensis (redrawn from Sereno and Arcucci, 1994b); J, Silesaurus opolensis (redrawn from Dzik, 2003); K, Lesothosaurus dianosticus (redrawn from from Sereno, 1991b); L, Coelophysis bauri (based on AMNH FR 7224). Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 5 cm in C–E, J–L, and 1 cm in A, B, F–I.
Fig. 34
Archosauriform ilia: A, left ilium of Phytosauria (SMNS 52971) in lateral view; B, left ilium of Batrachotomus kuperferzellensis (SMNS unnumbered) in lateral view; C, left ilium fragments of Hesperosuchus agilis (AMNH FR 6758) in dorsal (top) and lateral (bottom) views; D, left ilium of Dromicosuchus grallator (UNC 15574) in lateral view; E, left ilium of Aetosauria (UCMP 32422) in lateral view; F, right ilium of Lesothosaurus dianosticus (SAM 401) in lateral view; G, right ilium of Poposaurus gracilis (TTU-P 10419) in lateral view. Arrow indicates anterior direction. Numbers refer to character states. See appendix for anatomical abbreviations. Scale bars = 5 cm in A–B, E, G, and 1 cm in C–D, F.
266. Ilium, crest dorsal to the supraacetabular crest/rim: (0) vertical; (1) anterodorsally inclined (figs. 33–34).
267. Ilium, crest dorsal to the supraacetabular crest/rim: (0) thick; (1) thin ridge (fig. 34) (new formulations).
The presence of a crest dorsal to the supraacetabular crest ( = rim) has been repeatedly cited as a character uniting various suchian taxa (see review of Gower, 2000) especially taxa traditionally regarded as rauisuchians. The various descriptors (buttress, swelling, supraacetabular crest, rugose ridge) of this feature have led to confusion because they (1) are never described using specific taxa, (2) are vague and later authors have confused the terms when scoring matrices, and (3) only one of them incorporated a wide range of variation. Gower (2000) provided a thorough discussion of the problem and suggested that the feature must be thoroughly described. A dorsal crest has also been reported in dinosauriforms. Here, the morphology and orientation of the crest dorsal to the supraacetabular crest is discussed and divided into three characters.
Among suchians, a crest dorsal to the supraacetabular crest is present in various forms in Arizonasaurus (MSM P4590), Lotosaurus (IVPP V V4880 or V4881), Bromsgroveia (WARMS G.3), a poposauroid from the Middle Triassic Moenkopi Formation (Nesbitt, 2005b; Schoch et al., 2010), CM 73372, Postosuchus kirkpatricki (TTU-P 9002), Sillosuchus (PVSJ 85), Effigia (AMNH FR 30587), Shuvosaurus (TTU-P 9001), Poposaurus), Rauisuchus (BSP AS XXV-60-121), Saurosuchus (PVL 2198), Batrachotomus (SMNS 80269), Dromicosuchus (UNC 15574), and Hesperosuchus agilis (AMNH FR 6758). In these taxa, the dorsal crest separates the anterior ( = preacetabular) process from the posterior ( = postacetabular) process. The crest of Dromicosuchus (UNC 15574) and Hesperosuchus agilis (AMNH FR 6758) expands laterally only at the dorsal margin, which is not as distinct as the vertical, laterally expanded crest in Postosuchus kirkpatricki (TTU-P 9002). The crests in Dromicosuchus (UNC 15574) and Hesperosuchus agilis (AMNH FR 6758) are rugose like that of the dorsal margin of the dorsal crests of Postosuchus kirkp