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17 August 2012 A Review of Dromaeosaurid Systematics and Paravian Phylogeny
Alan H. Turner, Peter J. Makovicky, Mark A. Norell
Author Affiliations +
Abstract

Coelurosauria is the most diverse clade of theropod dinosaurs. Much of this diversity is present in Paraves—the clade of dinosaurs containing dromaeosaurids, troodontids, and avialans. Paraves has over 160 million years of evolutionary history that continues to the present day. The clade represents the most diverse living tetrapod group (there are over 9000 extant species of Aves—a word used here as synonomous with “bird”), and it is at the root of the paravian radiation, when dromaeosaurids, troodontids, and avialans were diverging from one another, that we find the morphology and soft tissue changes associated with the origin of modern avian flight. Within the first 15 million years of known paravian evolutionary history members of this clade exhibited a difference of nearly four orders of magnitude in body size, a value that is similar to the extreme body size disparity present today in mammalian carnivorans, avians, and varanoid squamates. In this respect, Paraves is an important case study in characterizing the patterns, processes, and dynamics of evolutionary size change. This last point is of particular interest because of the historical significance placed on the role of body size reduction in the origin of powered avian flight.

Our study reviews and revises the membership of Dromaeosauridae and provides an apomorphy-based diagnosis for all valid taxa. Of the currently 31 named dromaeosaurid species, we found 26 to be valid. We provide the most detailed and comprehensive phylogenetic analysis of paravians to date in order to explore the phylogenetic history of dromaeosaurid taxa. The general pattern of paravian relationships is explored within the broader context of Coelurosauria with an emphasis on sampling basal avialans, because of their importance for character optimizations at the base of Paraves.

A large dataset was constructed by merging two datasets, one examining coelurosaur relationships broadly (based on previous TWiG datasets) and the other examining avialan relationships specifically (Clarke et al., 2006). This merged dataset was then significantly revised and supplemented with novel character analysis focusing on paravian taxa. During character analysis, particular attention was given to basal members of Dromaeosauridae, enigmatic basal paravians such as Jinfengopteryx elegans and Anchiornis huxleyi, and the incorporation of new morphological information from two undescribed troodontid species from the Late Cretaceous of Mongolia. A final dataset of 474 characters scored for 111 taxa was used to address paravian evolution. This dataset is important in that it bridges a phylogenetic gap that had persisted between studies on birds and studies on all other coelurosaurs. Most scorings in this matrix were based on the direct observation of specimens.

All most parsimonious trees recovered in the cladistic analysis support the monophyly of Paraves, Troodontidae, Dromaeosauridae, and Deinonychosauria. A new clade of basal troodontids is discovered including two undescribed Mongolian troodontids and Jinfengopteryx elegans. Xiaotingia and Anchiornis form a clade at the base of Troodontidae. Recently proposed relationships within Dromaeosauridae are further supported and a succession of clades from Gondwana and Asia form sister taxa to a clade of Laurasian dromaeosaurids. Avialan monophyly is strongly supported with Archaeopteryx, Sapeornis, Jeholornis, and Jixiangornis forming the successive sister taxa to the Confuciusornis node.

INTRODUCTION

Rationale

Dromaeosaurid remains are rare. Yet in the past several years we have seen an increase in dromaeosaurid diversity and geographic extent. Even the distribution and evolution of complex featherlike integumentary structures (Allain and Taquet, 2000; Makovicky et al., 2005; Norell and Xu, 2005; Norell et al., 2006; Turner et al., 2007a; Longrich and Currie, 2009) is now well understood in the clade. Numerous species-level taxa have been described, including several fragmentary forms that are ambiguously dromaeosaurids (see in part Norell and Makovicky, 2004). Until recently the group was best known from the Upper Cretaceous of Asia and North America. In the last few years many new taxa have been discovered from the Lower and Upper Cretaceous of Asia, Europe, North America, and South America (Norell and Makovicky, 2004; Makovicky et al., 2005; Novas et al., 2009; Longrich and Currie, 2009). Among the most intriguing of these finds are specimens from the Lower Cretaceous Yixian Formation of Liaoning, China, many of which preserve epidermal stuctures including feathers (Norell and Xu, 2005). Some of these even provide evidence of coloration (Li et al., 2010a, 2010b, 2012 ). Dromaeosaurids have been a focus of many studies because of their close relationship to Avialae (Gauthier, 1986; Sereno, 1997; Makovicky and Sues, 1998; Holtz, 1998; Norell et al., 2001). The primary aim of this study is to clarify the phylogenetic history of dromaeosaurid theropods as well as broader deinonychosaur and paravian relationships.

Deinonychosauria is the group of coelurosaurian theropod dinosaurs that consist of the sickle-clawed Dromaeosauridae and Troodontidae. This is generally regarded as the group most closely related to birds. Chinese, Mongolian, and South American dromaeosaurids and Chinese and Mongolian troodontids display an interesting mosaic of derived and primitive characters that raise issues concerning dromaeosaurid phylogeny, the relationships of these taxa to other deinonychosaurs and birds, and even the monophyly of Deinonychosauria (Norell et al., 2006).

With the rapid increase in dromaeosaurid diversity, many of the historically diagnostic characters of Dromaeosauridae become smeared farther down the coelurosaur tree or re-sorted, so they arise convergently. The same is true for many so-called “avian” or avialan features. As basal members of Dromaeosauridae and Troodontidae have been described many of these features have been reinterpreted at a more general level as paravian synapomorphies. Moreover, the fact that most detailed avialan phylogenetic studies have been conducted outside the context of a species-level coelurosaur dataset (Chiappe and Calvo, 1994; Chiappe, 1995; Norell and Clarke, 2001; Clarke and Norell, 2002; Clarke, 2004; Clarke et al., 2006) complicates a clear understanding of the character changes occurring across the nonavialan/avialan transition. These studies present detailed and repeatedly corroborated hypotheses of avialan evolution but do not provide the more comprehensive framework required to explore three of the main questions we seek to answer through rigorous tests of: (1) the sister-group status of Deinonychosauria to Avialae, (2) the monophyly of Deinonychosauria in the context of newly described basal troodontids, dromaeosaurids, and avialans, and (3) the placement of Rahonavis ostromi, which was considered a basal avialan but later shown to be a basal dromaeosaurid (Makovicky et al., 2005).

Providing answers to these questions is critical. The presence of filamentous integumentary structures (Xu et al., 1999, 2000, 2001; Ji et al., 2001) and feathers of modern aspect in some dromaeosaurids (Norell et al., 2002; Norell and Xu, 2005), an avianlike sleeping posture in the basal troodontid Mei long (Xu and Norell, 2004), the troodontid affinities of the feathered Jinfengopteryx (Turner et al., 2007b), and the small body size of many basal paravians underscore the avianlike features of deinonychosaurs and indicate that characteristic “bird” morphology and behavior evolved in nonavialan dinosaurs prior to the origin of powered flight. It is, therefore, important that investigations of characters associated with flight origins within dinosaurs be based, not on speculative scenarios, but on testable hypotheses of character optimization that our phylogeny allows.

As of this writing, the number of dromaeosaurid taxa continues to increase almost weekly. Most of these specimens are found in incredibly rich northern Chinese rocks referred to collectively as the Jehol Group, but continued work in other localities has led to the discovery of additional dromaeosaurids in Djadokhta Formation and equivalent deposits in Inner Mongolia (Xu et al., 2010) and in undersampled regions in southeastern Europe (Csiki et al., 2010). As a result of this, and the ever-expanding toolkit for exploring theropod systematics and biology, it is impossible to provide a truly synthetic treatment of a single diverse clade like Dromaeosauridae. Nevertheless, we are as comprehensive and up-to-date as possible in this review of dromaeosaurid systematics and evolution.

Systematic Framework

Precladistic studies and their resultant classifications are hampered by plesiomorphic diagnoses or overreliance on particular typological morphologies (e.g., the body size–related Carnosauria/Coelurosauria division within Theropoda). This resulted in an oversimplified but ambiguous taxonomy and rampant paraphyly. Application of cladistic methodology to dinosaurian taxa, beginning in the early 1980s, led to the resolution of large-scale patterns of dinosaur phylogeny (Gauthier, 1984, 1986; Sereno, 1986, 1997, 1999; Holtz, 1994, 1998, 2001; Wilson and Sereno, 1998; Makovicky and Sues, 1998; Norell et al., 2001; Makovicky et al., 2005). Since the seminal work of Gauthier (1986) that established the framework for many of the major relationships among theropod dinosaurs, subsequent authors have made important strides in clarifying fine-scale relationships among constituent lower clade levels (Russell and Dong, 1993; Forster et al., 1998; Holtz, 1998; Makovicky and Sues, 1998; Sereno, 1999).

Theropoda

Gauthier (1986) applied a stem-based definition to Theropoda, which consists of the last common ancestor of birds (Aves) and all descendents closer to birds than to sauropodomorph dinosaurs. This definition combined the traditional usage of Marsh's (1881) Theropoda with the realization that Aves (Linnaeus, 1758) was deeply nested within Theropoda as originally proposed by Thomas Huxley (1868). Gauthier (1986) also broke the Carnosauria1 versus Coelurosauria convention for size-related classifications by specifying an explicit definition of Coelurosauria, tying it to a less inclusive clade near the avian origination within Theropoda.

Subsequent analyses to Gauthier's (1986) have rejected Carnosauria in favor of tyrannosaurids as coelurosaurs and Allosaurus and Acrocanthosaurus as more basal taxa (Holtz, 1994; Sereno, 1997). Unlike Crocodylomorpha, which has only a few large clades, showing just a few levels of gross morphological organization (i.e., “sphenosuchians,” “protosuchians,” Thalattosuchia, Notosuchia, Neosuchia), Theropoda is comprised of numerous “family”-level clades that form a ladderlike, pectinate organization of relationships. The monophyly of these theropod clades are well established, although among basal theropods a few large questions remain. These include ceratosaur monophyly and Coelophysoidea monophyly (Carrano et al., 2002; Carrano and Sampson, 2008; Rauhut, 2003; Smith et al., 2007).

Coelurosauria

Coelurosauria includes several clades with both small- and large-bodied taxa such as Tyrannosauroidea, Ornithomimosauria, Therizinosauria, Oviraptorosauria, Compsognathidae, Alvarezsauroidea, Dromaeosauridae, Troodontidae, and Avialae (fig. 1). Both some of the largest (Tyrannosaurus rex) and smallest (bumble bee hummingbird) known theropods belong to this clade. Additionally, a number of small-bodied basal taxa are known that are not readily assignable to any one of the established subclades of Coelurosauria: Ornitholestes hermanni, Coelurus fragilis, Tugulusaurus faciles, Xinjiangovenator parvus, Tanycolagreus topwilsoni, Aniksosaurus darwini, Zuolong salleei, and Nqwebasaurus thawazi. Major clade level relationships within Coelurosauria were first addressed by Gauthier (1986), Holtz (1994, 1998), Makovicky and Sues (1998), and Sereno (1997, 1999). While these analyses did much to establish the general framework of relationships among coelurosaurs it was not within a species-level framework. The study of Norell et al. (2001) was the first to analyze at the species level and corroborated much of the underlying topology found in previous supraspecific analyses. Nevertheless, the position of compsognathids and the placement of alvarezsauroids within Coelurosauria remain extremely contentious, although the discovery and description of several new taxa may ameliorate some of this uncertainty shortly.

Fig. 1

Cladogram showing interrelationships of the major clades of Coelurosauria.

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Paraves

As coined and defined by Sereno (1997, then 1998 respectively), Paraves is a stem-based clade of derived coelurosaurs named to set apart the closest relatives of birds (Avialae) from more distinct coelurosaur relatives (i.e., Oviraptor and more basal taxa).2 Dromaeosaurids, troodontids, and avialans currently constitute this clade.

Deinonychosauria was erected by Colbert and Russell (1969) to include the few dromaeosaurids then known, with the name derived from the best-known dromaeosaurid at the time, Deinonychus. Deinonychosaurs are characterized by a distinctive foot morphology comprised of a modified, raptorial digit II (fig. 2), exhibited in both dromaeosaurids and troodontids. Both Colbert and Russell (1969) and Ostrom (1969a) discussed the great similarity between dromaeosaurids (which were expressly included in Deinonychosauria) and the two troodontids known at the time (Saurornithoides and Stenonychosaurus ( =  Troodon)), but neither explicitly included the latter two taxa in Deinonychosauria. In Gauthier (1986) Deinonychosauria was a terminal taxon in the analysis and he explicitly discussed it in terms of including both Dromaeosauridae and Troodontidae. However, he expressed reservations about the monophyly of the clade in the addendum included at the end of the paper (Gauthier, 1986: 47). At the time that this study was undertaken, few of the relevant taxa were well described or illustrated, and access was limited.

Fig. 2

Illustration of the foot of Deinonychus antirrhopus showing the derived pedal morphology characteristic of Deinonychosauria. Adapted from Ostrom (1969a).

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Barsbold (1983a), Currie (1987), Osmólska (1981), Osmólska and Barsbold (1990), Currie and Zhao (1993), and Currie (1995) have all questioned the monophyly of Deinonychosauria. This skepticism was generally based on the argument that the modified second digit in these two taxa are morphologically dissimilar and therefore nonhomologous and that Dromaeosauridae lacked many of the derived pneumatic morphologies present in the braincase of troodontids. As a result, these studies favored a close relationship between troodontids and ornithomimosaurs. The analyses of Holtz (1994, 1998, 2001) and Senter et al. (2004) similarly did not support deinonychosaur monophyly. With the discovery of basal taxa in both Ornithomimosauria and Troodontidae it has been well established that the derived similarity between the two clades was convergently acquired as basal members in each lack those derived morphologies (Makovicky et al., 2003; Hwang et al., 2004a; and see discussion in Xu et al., 2002a). Forster et al. (1998) recovered a paraphyletic Deinonychosauria with Troodontidae the sister taxon of Avialae and Dromaeosauridae the sister taxon to that clade.

In all the cases mentioned above (except for Senter et al., 2004), Dromaeosauridae and Troodontidae were analyzed at the supraspecific level, greatly limiting the ability to reliably estimate the basal conditions in each clade and therefore reconstruct the interrelationships of the two clades. Sereno (1997, 1999) recovered a monophyletic Deinonychosauria even though Dromaeosauridae and Troodontidae were included as supraspecific composite terminal taxa. Beginning with the publication of Norell et al. (2001), all subsequent analyses employing expanded and modified versions of that dataset (Xu et al., 2002a; Makovicky et al., 2003; Xu and Norell, 2004; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b) or analyzed at a species level (Senter, 2007, although not Senter et al., 2004) have recovered a monophyletic Deinonychosauria as the sister taxon of avialans (but see Xu et al., 2011).

Historically these two groups were known by only a handful of taxa and characterized by exemplar taxa like Deinonychus antirrhopus and Troodon formosus. Only recently has the diversity of dromaeosaurid and troodontid dinosaurs begun to expand and become appreciated.

Dromaeosauridae

In just the past decade, the number of described dromaeosaurids has increased from six species to more than 31. Hypotheses of a dromaeosaurid species-level phylogeny have generally been unresolved or conflicting (figs. 3, 4). Most descriptive work remains preliminary with detailed, well-illustrated descriptions provided for only a handful of taxa (Currie, 1995; Norell and Makovicky, 1997, 1999; Barsbold and Osmólska, 1999; Hwang et al., 2002; Turner et al., 2007a, 2011). Consequently, morphological variation among and between dromaeosaurid taxa is poorly understood. Adding to the issues of the underappreciation of morphological variation in this clade is the notion that the first described dromaeosaurids (Deinonychus, Velociraptor, Dromaeosaurus) are morphologically similar. The recently discovered small, avialanlike taxa Microraptor zhaoianus, Sinornithosaurus millenii, and Buitreraptor gonzalezorum, along with dromaeosaurids previously thought to be true avialans (Unenlagia comahuensis and Rahonavis ostromi) and their kin (Austroraptor), greatly increase the range of morphological variation within the Dromaeosauridae (Novas et al., 2009) and alter what is reconstructed as the prototypical basal conditions for dromaeosaurids.

Fig. 3

Strict consensus topologies from previous versions of the Theropod Working Group (TWiG) matrix. A, Norell et al. (2001); B, Hwang et al. (2002); C, Makovicky et al. (2005); D, Norell et al. (2006).

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Fig. 4

Strict consensus topologies from recent phylogenetic analyses of coelurosaur relationships that used a version of the TWiG matrix as a backbone. A, Zanno (2010); B, Choiniere et al. (2010); C, Senter (2007).

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Troodontidae

Although still less diverse taxonomically than Dromaeosauridae, a similar increase in the number of named troodontid species has occurred. Four of the basalmost taxa were described within the past decade and two undescribed Mongolian taxa represent new additions to the base of Troodontidae. The phylogeny of troodontids is better resolved and more stable than that of dromaeosaurids (Makovicky et al., 2003; Xu and Norell, 2004),3 but this may be due to a lack of focus on this problem. Mei long from the lower Yixian Formation of China and Sinornithoides youngi from the Ejinhoro Formation of Inner Mongolia are important for being preserved in the stereotypical sleeping posture present in living birds—that posture further suggests that avianlike behavior and not just morphology occurs earlier in theropod history than the origin of birds, something that is also suggested by fossils that show brooding behavior in the oviraptorosaur Citipati osmolskae (Norell et al., 1995). The exact phylogenetic position of the recently described four-winged paravian Anchiornis, which has variably been interpreted as either an avialan or a troodontid (Xu et al., 2009) has great significance for understanding the evolution of aerial locomotion in paravians and ultimately for the origin of avian flight. Furthermore, the discovery of Jinfengopteryx and Anchiornis clearly shows that like dromaeosaurids, at least some troodontids had a complete component of feathers of modern aspect.

Avialae

Basal avialan relationships are fairly well resolved (Chiappe, 2002; Clarke, 2004; Clarke et al., 2006; You et al., 2006) (fig. 5). However, current analyses including paravian taxa usually include few avialans; often only a few exemplar taxa such as Archaeopteryx and Confuciusornis. This has resulted in spurious relationships at times (e.g., Mayr et al., 2005), poor control on which characters optimize as avialan synapomorphies, and the absence of a phylogenetic framework for inferring ancestral conditions for Avialae and in turn Paraves.

Fig. 5

Summary cladogram showing the broad pattern of relationships within Avialae. Adapted from Clarke (2004) and Chiappe (2002).

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Clade Names and Taxonomic Conventions

All taxonomic systems are ultimately subjective. Traditionally, archosaur systematists have largely developed and defended the idea of ancestry-based taxonomy (e.g., phylogenetic taxonomy) (Clark, 1986; Gauthier, 1986; Gauthier and Padian, 1985; de Queiroz and Gauthier, 1990, 1992, Brochu, 1999; Brochu and Sumrall, 2001). Theropod workers in particular have been prolific in applying phylogenetically defined clade names to nodes in the theropod tree (see Sereno et al., 2005, for a list of clade names).

The application of a phylogenetic taxonomy remains contentious. A complete discussion of this issue would be lengthy and is certainly beyond our scope (see de Queiroz and Gauthier, 1990, 1992, 1994; Moore, 1998; Nixon and Carpenter, 2000; Benton, 2000; Brochu and Sumrall, 2001; Dyke, 2002). Although a few comprehensive taxonomic schemes have been proposed for Theropoda (Sereno, 1997,1998; Padian et al., 1997a, 1997b, 1999), changing relationships among theropod clades and an explosion of the number of known paravian taxa has resulted in new clade names and changes in the use of existing clade names. The clade names applied in the present study (fig. 6) were selected to maximize and facilitate tree discussions and conform most closely to contemporary use among basal coelurosaur and avialan workers. However, in no way are they rank based in a classical sense.

Fig. 6

Generalized cladograms showing the clade names and taxonomic conventions employed in the current study. A, clade names within nonavialan Coelurosauria; B, clade names within Avialae. Open circles denote node-based names; curves, stem-based names.

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Comment on the definition of Aves

Gauthier (1986) applied a crown-group definition to Aves, which consists of the last common ancestor of Ratiti, Tinami, and Neognathae and all of its descendants. This restricted the name Aves to the least inclusive monophyletic group containing all living birds. This definition excluded Archaeopteryx lithographica and numerous fossil taxa commonly referred to as “birds” from Aves. These taxa and Aves proper are subsumed by the more inclusive stem-based lineage Avialae, which Gauthier (1986) coined to encompass fossil taxa historically thought to be birds because of the presence of feathers and presumed flight abilities. However, nearly every single character that at one time was thought to make something a “bird” is now known to occur progressively earlier in theropod evolution. Therefore, “bird” is a colloquial term that lacks a meaningful taxonomic or scientific basis as it has no precise phylogenetic meaning.

Although accepted by many theropod systematists, there are some who still prefer the traditional (noncrown group) use of Aves (Sereno, 1997, 1999; Chiappe, 2002; Senter, 2007). However, crown groups reflect the notion that living taxa preserve a larger suite of characters (including behavioral and molecular data) and therefore their relationships may be more highly corroborated than fossil-only taxa (Jefferies, 1979). Additionally, crown-group definitions ensure that paleontologists and neontologists refer to the same taxon when issues in phylogeny (or those based on phylogeny such as diversity, evolutionary rate or biogeography) span the fossil record to the Recent. We find these justifications for the crown-group definition of Aves and use of Avialae compelling, and furthermore see no reason to ascribe special status to Archaeopteryx as the earliest member of Aves, and as such will employ the Gauthier (1986) definitions herein.

PART 1: THE DROMAEOSAURIDAE, MATTHEW AND BROWN, 1922

This section is intended to provide a detailed treatment of all taxa that have been referred to Dromaeosauridae. At times this may include descriptions of all or portions of holotype material in places where such material was not initially described, or when available descriptions were found lacking in sufficient detail. Unfortunately, we do not provide as many detailed descriptions and images of many of these taxa as we would like. These are points of focus for future work. More often this section will simply discuss and comment on dromaeosaurid species and review our current understanding of the validity of these taxa (table 1). For each taxon, the temporal and stratigraphic occurrence will be noted along with the original diagnosis provided by the author(s). This will be followed by a discussion and a revised diagnosis relying on unique combinations of characters and specific references to autapomorphies (marked by asterisks) when available for each taxon based on firsthand revision of the holotype and referred material as well as the phylogenetic analysis presented in Part 2.

TABLE 1

Valid Species of Dromaeosauridae

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ASIAN DROMAEOSAURIDS

Achillobator giganticus Perle et al., 1999

Holotype

MNUFR 15 (figs. 7, 8).

Fig. 7

Maxilla of Achillobator giganticus (MNUFR 15) in lateral (A) and medial (B) views.

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Fig. 8

Select postcranial remains of Achillobator giganticus (MNUFR 15). A, femur in posterior view; B, tibia in anterior view; C, illustration of pubis and ischium.

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Distribution

Cenomanian-Santonian, Late Cretaceous, Baynshiree Formation (Dornogov), Mongolia (fig. 9; table 2).

Fig. 9

Geographic distribution of Laurasian dromaeosaurids illustrated on a paleogeographic globe of the mid-Cretaceous (adapted from Smith et al., 1994).

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TABLE 2

Temporal and Geographical Distributions of Laurasian Dromaeosaurids

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Original Diagnosis

The diagnosis for the taxon given by Perle et al. (1999) included a large number of symplesiomorphies with coelurosaurs and more derived coelurosaurian clades. Following Perle et al. (1999: 8), these included: “hindlimbs very stout, massive, and comparable short; forelimbs probably elongated as well, based on the preserved radius, pes of medium (shortened) length; penultimate and ungual phalanges of the second digit robust, particularly the proximoventral process of the penultimate phalanx; metacarpal III long and irregular with a slender shaft; skull with large semicircular to oval vertically placed antorbital fenestra, second and third accessory fenestrae oriented subvertically; 11 maxillary teeth, all teeth with anterior and posterior serrated margins; denticles on posterior margin bigger than anterior ones; cervical vertebrae short, massive and sharply angled; caudal vertebrae long and platycoelus; caudals with extremely long rodlike prezygapophyseal processes and chevrons also very much elongated into long, paired double bony rods extending forward; ischium with large triangular obturator process situated on proximal half of the ischial shaft; pubis is long, very stout with anteroposterior directed large distal expansion; in general, the pubis is propubic.”

Revised Diagnosis

A large dromaeosaurid diagnosed by the following combination of characters and autapomorphies: promaxillary fenestra completely exposed; promaxillary and maxillary fenestra elongate and vertically oriented at same level in maxilla*; metatarsal III wide proximally; femur longer than tibia (estimated mass >300 kg); pelvis propubic; large triangular obturator process on ischium situated on proximal half of ischial shaft; and boot at distal symphysis of pubis both cranially and caudally developed.

Discussion

This is the second largest of the described dromaeosaurid taxa with a tibial length of 490 mm (fig. 8). The taxon is based on a left maxilla, left femur and tibia, left metatarsals III and IV, right ilium, pubis, and ischium, fragmentary teeth, several isolated caudal vertebrae, and rib fragments.

The only description of this taxon is nefarious in that it was published without the knowledge of any of the junior authors based on a preliminary draft left in Mongolia in 1997. Most of Perle et al.'s (1999) original diagnosis is uninformative and consists of plesiomorphic characteristics. “Short, stout, and massive hindlimbs” is uninformative but when more precisely stated, as “femur longer than tibia,” this feature is revealed as unique with respect to other dromaeosaurids when both elements are known. Elongate forelimbs are entirely speculative based on the preserved radius, whereas the short pes and robust, modified penultimate and ungual phalanges of the second digit are more widely characteristic of deinonychosaurs (Norell and Makovicky, 2004). The cervical vertebrae are short and massive, and the articular facets are sharply angled anteriorly—a feature not unique to Achillobator giganticus but instead common to members of Oviraptorosauria and Paraves. Caudals with extremely long, rodlike prezygapophyses and chevrons are not diagnostic for Achillobator giganticus. Extremely elongate prezygapophyses and chevrons are synapomorphic for a subclade of dromaeosaurids, and as such are grounds for referring this taxon to this clade, but not to diagnose it.

The large oval-shaped and vertically oriented antorbital fenestrae are diagnostic for this taxon, whereas teeth with anterior and posterior serrated margins are present in Dromaeosaurus albertensis, Atrociraptor marshalli, and Utahraptor ostrommaysorum. The large triangular obturator process on the ischium is situated on the proximal half of the ischial shaft (fig. 8). This is a reversal relative to other dromaeosaurids and in this context it is autapomorphic for Achillobator giganticus.

The propubic nature of the pubis and the morphology of the pubic boot are the most diagnostic characters of this taxon. The ilium is similar to Adasaurus mongoliensis and to a lesser extent Velociraptor and Deinonychus by having a squared-off anterior iliac process and a small flange for the m. iliotibialis. The distal symphysis of the pubis is both cranially and caudally developed into a “boot” in Achillobator giganticus, as is the case in Deinonychus antirrhopus (Norell and Makovicky, 2004).

In the phylogenetic analyses of Xu et al. (2002a), Hwang et al. (2002) and Xu and Norell (2004), Achillobator is recovered in a clade with Dromaeosaurus, or Deinonychus + Dromaeosaurus when Adasaurus is removed from the analysis. This clade is unambiguously supported by the presence of D-shaped premaxillary teeth and a loss of opisthopuby (Norell and Makovicky, 2004). Novas and Pol (2005) recover Achillobator in an unresolved clade with Dromaeosaurus, Adasaurus, and Utahraptor when Saurornitholestes langstoni is removed from the analysis. Senter et al. (2004) found Achillobator the sister taxon of a Dromaeosaurus + Utahraptor clade. This was based on the presence of a dorsoventrally deep jugal process on the maxilla ventral to the antorbital fenestra, posterior dorsal vertebrae with a pair of pneumatopores on each side, an obturator process that is not distally displaced, and a femur that is longer than the tibia (fig. 8). More recent analyses (Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b) find Achillobator in a clade with Utahraptor ostrommaysorum, Dromaeosaurus albertensis and sometimes Adasaurus mongoliensis.

It has been suggested that Achillobator was a chimera (Burnham et al., 2000). However, since the specimen was found in semiarticulation (A. Perle, personal commun.) and all of the elements are the same color and preservation, assignment of these elements to a single individual is justified (Norell and Makovicky, 2004). Furthermore, even though Achillobator is an unusual dromaeosaurid with seemingly aberrant features it continues to be supported as a dromaeosaurid in empirical analyses (e.g., Norell and Makovicky, 2004; this study).

Adasaurus mongoliensis Barsbold, 1983

Holotype

IGM 100/20 (figs. 10Fig. 11Fig. 1213).

Fig. 10

Skull of Adasaurus mongoliensis (IGM 100/20). A, right lateral view; B, left lateral view; C, posterior view; D, dorsal view. 1, expanded maxillary process of jugal (char. 238.1); 2, dorsally displaced anterior process of quadrate; 3, large surangular foramen (char. 74.1); 4, anteriorly inclined lacrimal.

i0003-0090-371-1-1-f10.tif

Fig. 11

Interpretive line drawing of the skull of Adasaurus mongoliensis.

i0003-0090-371-1-1-f11.tif

Fig. 12

Right ilium, ischium, and pubis of Adasaurus mongoliensis (IGM 100/20). Arrow indicates the notched anterior margin characteristic of Adasaurus.

i0003-0090-371-1-1-f12.tif

Fig. 13

Right pes of Adasaurus mongoliensis (IGM 100/20). A, anterolateral view; B, detail of digit II. The “reduced” ungual phalanx on digit II is autapomorphic for Adasaurus.

i0003-0090-371-1-1-f13.tif

Distribution

Campanian or Maastrichtian, Late Cretaceous, Nemegt Formation (Bayankhongor) of southwestern Mongolia.

Original Diagnosis

Reduced second pedal ungual.

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters and autapomorphies: dorsoventrally expanded maxillary process of jugal; descending process of lacrimal strongly curved anteriorly; large surangular foramen on mandible; dorsally displaced triangular process along lateral edge of quadrate shaft*; pleurocoels only on anterior sacrals; “notched” anterior margin of preacetabular blade of ilium; and reduced pedal ungual of digit II*.

Discussion

Although partially figured and labeled as “Adasaurus” in a paper by Barsbold (1977), this taxon was not formally described until 1983. Barsbold (1983a) based this taxon on an incomplete skull, ischium, and pubis (IGM 100/20), as well as a partial right hindlimb including a complete pes (IGM 100/21). Barsold (1983a) refrained from giving a detailed description of this taxon, instead referring to its “great similarity…with other representatives of the [Dromaeosauridae].” The only diagnostic trait listed was the reduction of the usually enlarged, trenchant second pedal ungual to a size comparable to that of the other pedal digits. Additional references to the morphology of Adasaurus mongoliensis appear in Barsbold (1983b), Perle et al. (1999), and Norell and Makovicky (2004). Contrary to Norell and Makovicky (2004) the holotype of Adasaurus mongoliensis does not lack a skull and the pelvis is not pathological.

Currie and Varricchio (2004) reference additional cranial and postcranial material (IGM 100/22 and IGM 100/23) that remains undescribed. As it turns out, this additional material appears to represent a new taxon, a brief description of which was given in an abstract (Kubota and Barsbold, 2007). IGM 100/23 and IGM 100/224 are from the Shine Us Khuduk and Teel Ulaan Jalzai localities, respectively, at which the Baynshiree Formation crops out (Kubota and Barsbold, 2007). Therefore, these specimens are older (Cenomanian) than Adasaurus, which is known from the Maastrichtian age Nemegt Formation that overlies the Djadokhta Formation. This new taxon is reported to be distinguishable from all other dromaeosaurids in having two shallow subalveolar grooves on the labial surface of the dentary, a convex posterodorsal edge of ilium with a posteriorly curved distal end, and a transversely wide distal end on phalanx II-1 (Kubota and Barsbold, 2007). We have not been able to examine the postcranial remains of IGM 100/23, but the cranial remains indicate that the two shallow subalveolar grooves are not unique to this taxon. The paired rows of subalveolar nutrient foramina are prominent in other dromaeosaurid taxa like Velociraptor mongoliensis (AMNH FARB 6515) and Dromaeosaurus albertensis (AMNH FARB 5356). Since we have not seen the postcranial material we cannot comment on its similarity or dissimilarity with Adasaurus mongoliensis. Kubota and Barsbold (2007) recover IGM 100/23 + IGM 100/22 in a Dromaeosaurinae clade with Dromaeosaurus and Achillobator.

The skull of Adasaurus mongoliensis (IGM 100/20) is incomplete anterior to the preorbital bar (fig. 10). The right side is well preserved, whereas the left side is largely incomplete and insufficiently prepared and conserved. Across many parts of the skull, a thin layer of matrix remains on the bone partially obscuring the underlying morphology. On the right side it is apparent that the suborbital portion of the jugal is dorsoventrally expanded. The quadratojugal and the descending quadratojugal process of the squamosal are not preserved. The quadrate is large and vertically oriented. As in other dromaeosaurids, the lateral edge of the quadrate shaft is not straight but instead bears a large triangular process. Unlike other dromaeosaurids, the triangular process in Adasaurus mongoliensis is not centered on the quadrate shaft but is instead apomorphically shifted dorsally (fig. 10A). Unfortunately, the braincase is not visible. It is hoped that this important area of the skull will be CT scanned at some point in the near future. The right ectopterygoid is preserved, but the dorsal surface is compressed against the palate, so presence of a pneumatic recess cannot be ascertained. The lacrimal has an inverted L-shape as in other dromaeosaurids. The descending process of the lacrimal curves anteriorly to a large degree—a feature unique to Adasaurus mongoliensis and Austroraptor cabazai. Kubota and Barsbold (2006) suggest that there is a midline ridge on the dorsal surface of the frontal. Our examination of the holotype was unable to confirm this observation, in part because much of the posterior portion of the frontal is incompletely prepared and is still obscured by a the thin layer of matrix. The completely exposed bone of the frontal lacks a ridge.

As discussed above, the ilium of IGM 100/20 is not pathological, and the often-reproduced illustration of Barsbold (1983a) is inaccurate in a number of ways that are clarified here. The dorsal edge of the ilium is straight and the pubic peduncle is wider and extends farther ventrally than the ischial peduncle (fig. 12). A large supratrochanteric process is absent. The anterior edge of the anterior blade of the ilium has a prominent anteriorly directed, dorsally displaced process similar to that seen in Saurornitholestes langstoni. Coupled with the large ventrally directed flange over the cuppedicus fossa, the anterior edge of the ilium has a “notched” appearance unlike the boxy, or square, margin seen in Velociraptor mongoliensis (IGM 100/25). The pes of IGM 100/21 is complete and well preserved. The ungual of digit II, which is apomorphically enlarged and trenchant in other deinonychosaurs, is small in Adasaurus mongoliensis (fig. 13). The proximal ends of the metatarsals on this pes are partially fused, suggesting that this individual was mature.

Recent phylogenetic analyses typically place Adasaurus mongoliensis in a large polytomy including all other dromaeosaurid taxa except Sinornithosaurus millenii and Microraptor zhaoianus, which are successive outgroups to the unresolved clade (Hwang et al., 2002; Makovicky et al., 2003; Xu and Norell, 2004). Senter et al. (2004) recovers Adasaurus mongoliensis as the sister group to a clade containing Dromaeosaurus, Utahraptor, Achillobator, Deinonychus, and Saurornitholestes. Novas and Pol (2005), Makovicky et al. (2005), and Turner et al. (2007a, 2007b) recover this taxon in an unresolved clade with Achillobator, Dromaeosaurus, and Utahraptor. Kubota and Barsbold (2006, 2007) have suggested a close relationship between Velociraptor mongoliensis and Adasaurus. Reexamination of the holotype material for the present study resulted in 102 changes to the taxon's scoring for existing characters plus numerous additional observations (table 3). This added information is critical for testing whether Adasaurus is in fact a dromaeosaurine, a velociraptorine, or a stem taxon.

TABLE 3

Revised Character Scorings for Adasaurus mongoliensis.

i0003-0090-371-1-1-t03.tif

Graciliraptor lujiatunensis Xu and Wang, 2004

Holotype

IVPP V13474 (fig. 14)

Fig. 14

Select holotype material of Graciliraptor lujiatunensis (IVPP V13474). A, right forelimb; B, left partial maxilla; C, right manus; D, two distal caudal vertebrae. In D, the circle highlights the lamina that extends between the postzygapophyses.

i0003-0090-371-1-1-f14.tif

Distribution

Aptian, lowest member of Yixian Formation, Lujiatun, Beipiao City, western Liaoning, China.

Original Diagnosis

Following Xu and Wang (2004: 113–114), “a laminal structure connecting the postzygapophyses of middle caudals; extremely long and slender middle caudals; ungual of manual digit I much smaller than that of manual digit II; proximal end of metacarpal III strongly expanded; extremely slender tibiotarsus; proximal tibiotarsus shaft rectangular in cross section; astragular medial condyle significantly expanded posteriorly; metatarsal II distally much wider than the other metatarsals; and long slender pedal phalanx III-1.”

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies: extremely long midtail caudals (shared with Microraptor zhaoianus); extremely slender tibiotarsus; medial condyle of tibiotarsus significantly expanded posteriorly*; metatarsal II distally much wider than other metatarsals.

Discussion

Based on a partial skeleton including forelimb and hindlimb material and a partial maxilla, Graciliraptor lujiatunensis is the least well-known Yixian dromaeosaurid (Xu and Wang, 2004). Originally considered the oldest known dromaeosaurid based on earlier stratigraphic work in the Lujiatun beds (Zhou et al., 2003), these beds are now considered contemporaneous with the Jianshangou beds (He et al., 2006) and therefore approximately Aptian in age. Scorings in our matrix are based on examination of the holotype specimen. Graciliraptor has been included in only two phylogenetic analyses (Xu and Wang, 2004; Turner et al., 2007b) and in both of these the taxon was recovered in a clade with Sinornithosaurus and Microraptor.

Of the nine original characters proposed to diagnose Graciliraptor lujiatunensis only four appear to stand up to scrutiny. Moreover, only one character appears to be autapomorphic for the taxon (medial condyle of tibiotarsus significantly expanded posteriorly) but this character is difficult to examine in other microraptorines, as most are preserved in two dimensions. Indeed, future discoveries of Graciliraptor-like specimens and continued work on Microraptor may show the two to be conspecific. Nonetheless, in our opinion, Graciliraptor lujiatunensis can be diagnosed by the unique combination of characters listed above until evidence to the contrary is discovered or published.

A few characters proposed as diagnostic by Xu and Wang (2004) prove to be problematic when considered in the broader context of dromaeosaurid variation. The laminar structure connecting the postzygapophyses of middle caudals noted by Xu and Wang (2004) is widespread among dromaeosaurids (fig. 14D). This laminar structure is a posterior extension of the neural arch that supports the base of the neural spine in successive caudal vertebrae. The structure is present in the dromaeosaurids Rahonavis ostromi (UA 8656), Buitreraptor gonzalezorum (MPCA 245), Adasaurus mongoliensis (IGM 100/20), Velociraptor mongoliensis (IGM 100/986), and Deinonychus antirrhopus (Ostrom, 1969a), as well as in more basal coelurosaurs such as Ornitholestes hermanni (AMNH FARB 619).

The extremely long middle caudals cited by the authors as unique to Graciliraptor are in fact shared with Microraptor (IVPP V13352) and are potentially a synapomorphy for the pair. We are unable to confirm two other observations regarding Graciliraptor. First, it is unclear whether the ungual of manual digit I is much smaller than that of manual digit II. The digit I ungual is the best preserved; the digit II ungual is poorly preserved and was severely crushed, rendering it difficult to gauge how large it is proximally (fig. 14A). Additionally, we could not confirm the observation of a strongly expanded metacarpal III. It looks slender and no more dorsoventrally expanded than metacarpal II.

Both an extremely slender tibiotarsus and a long slender pedal phalanx III-1 was used to diagnose Graciliraptor lujiatunensis. Both features are also present in Microraptor, and it seems reasonable that the slenderness is probably allometric. Lastly, the proximal portion of the tibiotarsus shaft is rectangular in cross section. We find this a weak characteristic. In Buitreraptor gonzalezorum (MPCA 245), Rahonavis ostromi (UA 8656), and Velociraptor mongoliensis (IGM 100/986) the tibia is rectangular in the same spot.

Linheraptor exquisitus Xu et al., 2010

Holotype

IVPP V16923.

Distribution

Campanian, Late Cretaceous, Bayan Mandahu Formation (Wulansuhai Formation), Inner Mongolia, China.

Original Diagnosis

Following Xu et al. (2010), a “dromaeosaurid that can be distinguished from other known dromaeosaurid taxa by the presence of the following autapomorphies: greatly enlarged maxillary fenestra sub-equal in size to external naris; several large foramina on lateral surface of jugal. Differs from other known dromaeosaurids except Tsaagan in the following features: large and anteriorly located maxillary fenestra; lacrimal lacking lateral flange over descending process and with relatively broad medial lamina; sharp angle between anterior and ascending processes of quadratojugal; contact between jugal and squamosal that excludes postorbital from infratemporal fenestra. Differs from Tsaagan in the following features: absence of osseous inner wall partly blocking antorbital fenestra; sharply rimmed ventral margin of antorbital fossa; considerably smaller angle between frontal and jugal processes of postorbital; anteroventrally curved postorbital process of squamosal; considerably shorter quadratojugal process of squamosal; dorsoventrally shorter lateral flange of quadrate; less curved and less posteriorly inclined quadrate shaft; paroccipital process more laterally oriented; angular more extended posteriorly toward glenoid fossa; considerably deeper posterior end of mandible such that glenoid fossa is approximately level with tooth row; pneumatic foramen present on axis vertebra.”

Revised Diagnosis

Not applicable. This taxon is here considered a junior synonym of Tsaagan mangas. (see fig. 15).

Fig. 15

Comparison of Linheraptor exquisitus (IVPP V16923) with Tsaagan mangas (IGM 100/1015). A, skull of Tsaagan mangas in right lateral view; B, skull of Linheraptor exquisitus in right lateral view; C, skull of Tsaagan mangas in left lateral view (reversed for comparative purposes).

i0003-0090-371-1-1-f15.tif

Discussion

Xu et al. (2010) provided eleven putative autapomorphies diagnosing Linheraptor exquisitus. This taxon is strikingly similar to the contemporaneous Tsaagan mangas from the Ukhaa Tolgod locality, Djadokhta Formation of Mongolia. The autapomorphies for Linheraptor were provided to distinguish it from Tsaagan. A review of these features reveals that they fail to differentiate these two taxa.

Xu et al. (2010) indicate that an absence of an internal osseous medial wall within the antorbital fenestra differs from the condition in Tsaagan in which such a medial wall is present posterior to the anterior margin of the internal antorbital fenestra. However, this trait cannot be used to diagnose Linheraptor. The holotype skull (IVPP V16923) was not prepared far enough (or CT scanned) to confirm the presence or absence on the internal osseous medial wall as in Tsaagan and Velociraptor.

Xu et al. (2010) additionally suggest that the ventral margin of the antorbital fossa is more sharply rimmed and demarcated in Linheraptor than in Tsaagan. Examination of the skulls of the IVPP V16923 and IGM 100/1015 clearly shows that little to no difference exists in the degree to which the ventral rim of the antorbital fossa is demarcated. The antorbital fenestra and fossa remain nearly identical between these two specimens.

According to Xu et al. (2010), in Linheraptor the angle between the frontal and jugal processes of the postorbital is smaller (roughly 90°) than the same angle in Tsaagan (roughly 135°). By our calculations, the angle between these two processes is closer to 95° in Linheraptor and 100° in Tsaagan. However, it is not so much that the angle between the frontal and jugal processes are greater in Tsaagan, but rather that the anterior margin of the postorbital is less concave in lateral aspect in Tsaagan relative to Linheraptor. Nevertheless, this is a very subtle difference and well within the range of variation one sees within specimens of Velociraptor (Norell et al., 2006: fig. 6). Therefore, this feature should not be considered diagnostic for Linheraptor exquisitus.

The fourth feature used to distinguish Linheraptor from Tsaagan is the curvature of the postorbital process of the squamosal. In the initial description of Linheraptor it was noted that the postorbital process of the squamosal in Linheraptor curves distinctly ventrally whereas the same process in Tsaagan is straight. We see very little curvature in the postorbital process of the squamosal in Linheraptor. Only the distalmost portion of the process shows any curvature and the “distinct” nature of this curvature may be accentuated by a fracture running through the process in IVPP V16923. The postorbital process of the squamosal in Tsaagan is not completely straight. It too has some curvature on its path along the ventral aspect of the postorbital (Norell et al., 2006: fig. 9). The apparent “straightness” of the process is perhaps accentuated on the left side of the skull due to slight damage at the confluence of the postorbital and quadratojugal processes. Thus, as with the previous putative autapomorphies, we do not consider the squamosal morphology distinct between these two taxa.

Two additional putative autapomorphies for Linheraptor pertain to the size of the quadratojugal process of the squamosal and the dorsoventral height of the lateral flange of the quadrate. Xu et al. (2010) considered the quadratojugal process of the squamosal in Linheraptor to be shorter than that in Tsaagan. We were not able to replicate this observation. When the two skulls are scaled to the same size the quadratojugal process of the squamosal is identical between the two specimens (IVPP V16923 and IGM 100/1015). In the unscaled skulls, the two processes both measure roughly 14 mm in length. With only about 20 mm difference in total skull length between the two animals, we see little difference with respect to this trait especially because some of this length may be ascribable to deformation. Similarly, the lateral flange of the quadrate was said to be dorsoventrally shorter in Linheraptor than in Tsaagan; however, when the skulls are again scaled to the same size there is no difference in the dorsoventral height of the lateral flange of the quadrate (8 mm in height at its anterior edge in both Linheraptor and a scaled Tsaagan).

Xu et al. (2010) suggest that the quadrate shaft is less curved and less posteriorly inclined in Linheraptor relative to Tsaagan. This apparently less curved and less posteriorly inclined quadrate is indeed present when you compare the holotype skull of Tsaagan with that of Linheraptor. However, the difference is not real, but instead it is a preservational artifact of the more severe mediolateral compression in the Tsaagan holotype (see Norell et al., 2006: figs. 3A, F, 14). When one corrects for the compression or views the quadrate of Tsaagan in isolation (e.g., Norell et al., 2006: fig. 10C) the inclination of the quadrate is nearly identical to that of Linheraptor.

One additional feature used to distinguish Linheraptor from Tsaagan is similarly affected by the mediolateral compression of the Tsaagan holotype skull. Xu et al. (2010) suggest that the paroccipital processes are more laterally oriented in Linheraptor than in Tsaagan. The paroccipital processes are in fact laterally oriented in Tsaagan (Norell et al., 2006: figs. 3, 13); however, as noted by Norell et al. (2006: 19), the posterior end of the skull of Tsaagan was compressed mediolaterally during preservation. We conclude that without this compression no difference would exist as a basis for distinction between Tsaagan and Linheraptor.

Two features pertaining to the posterior end of the mandible were used to diagnosis Linheraptor. One was that the angular extends more posteriorly in Linheraptor than in Tsaagan, and the second was that the posterior end of the mandible was overall deeper (dorsoventrally taller) in Linheraptor than in Tsaagan (Xu et al., 2010). In Linheraptor, the angular extends to the posterior margin of the posterior surangular foramen, whereas it extends to the anterior margin of the posterior surangular foramen in Tsaagan. This is roughly a 6 mm difference in the posterior extent of the angular (a similar difference is observed by measuring from the posterior end of the angular to the posterior edge of the glenoid fossa). We do not view this subtle degree of variation as sufficient to be an autapomorphy for a new taxon, especially in skulls that have both suffered some degree of deformation. With regard to the dorsoventral height of the posterior end of the mandible, we were unable to confirm the observation that this portion of the mandible is dorsoventrally taller in Linheraptor. Once again, when the two skulls are scaled to the same size, there is effectively no difference in dorsoventral height when measured from the apex of the coronoid process to the base of the mandible (∼30 mm in Linheraptor and ∼32 mm in Tsaagan when scaled to the skull length of Linheraptor).

The final autapomorphy used to distinguish Linheraptor from Tsaagan is the presence of a pneumatic foramen on the axis of Linheraptor that is not present on the axis of Tsaagan. This appears to be the only clear difference between Linheraptor and Tsaagan. However, vertebral pneumaticity can be variable within dromaeosaurids—notably Velociraptor mongoliensis shows variation in the axis pneumaticity (IGM 100/24 lacks a foramen, but IGM 100/976 has one) and dorsal vertebrae pneumaticity (IGM 100/24 dorsals have foramina, but IGM 100/986 lacks them). We do not think this is sufficient grounds to serve as the sole justification for erecting a new taxon.

Linheraptor exquisitus (known only from IVPP V16923) and Tsaagan mangas (known only from IGM 100/1015) are effectively identical in the areas where the two specimens overlap. As illustrated in the discussion above, the 11 putative autapomorphies for Linheraptor fail to distinguish IVPP V16923 from IGM 100/1015. Moreover, given the presence of three Tsaagan mangas autapomorphies in IVPP V16923 (paroccipital process that is pendulous and not twisted distally; maxillary fenestra that is large and located at the anterior edge of the antorbital fossa; jugal that meets the squamosal to exclude the postorbital from the margin of the infratemporal fenestra), we conclude here that Linheraptor exquisitus is not a valid taxon but is instead a junior synonym of Tsaagan mangas. This conclusion is interesting because it adds additional support to the documented faunal similarity between the Campanian Djadokhta Formation of southern Mongolia and the Campanian Bayan Mandahu Formation of northern Inner Mongolia.

Mahakala omnogovae Turner et al., 2007

Holotype

IGM 100/1033 (figs. 16Fig. 1718).

Fig. 16

Select skull material of Mahakala omnogovae (IGM 100/1033). A, (left to right) right frontal in lateral view, internal view, dorsal view, and left frontal in dorsal view; B, possible right ectopterygoid; C, left maxilla in medial (left) and lateral (right) views; D, partial right pterygoid in dorsal view.

i0003-0090-371-1-1-f16.tif

Fig. 17

Holotype cranium of Mahakala omnogovae (IGM 100/1033). A, right lateral view; B, interpretive line drawing of right lateral view; C, posterior view; D, left ventrolateral view.

i0003-0090-371-1-1-f17.tif

Fig. 18

Select postcranial remains of Mahakala omnogovae (IGM 100/1033). A, left ilium in medial (upper) and lateral (lower) views illustrating a large postacetabular tuber (char. 160.1); B, left femur in posterior (left) and lateral (right) views illustrating the presence of a posterior trochanter (char. 414.0/1); C, left tibia in anterior view; D, left metatarsus in anterior view; E, left pedal phalanges; F, caudal vertebrae 9 through 14 in right lateral view. See appendix 6 for abbreviations.

i0003-0090-371-1-1-f18.tif

Distribution

Campanian, Late Cretaceous, Tögrögiin Member of the Djadokhta Formation, Tögrögiin Shiree, Ömnögov Mongolia.

Original Diagnosis

Following Turner et al. (2007b: 1378), a “small paravian diagnosed by the following combination of characters: a strongly compressed and anteroposteriorly broad ulna tapering posteriorly to a narrow edge*; elongate lateral crest on the posterodistal femur*; anterior caudal vertebrae with subhorizontal, laterally directed prezygapophyses*; a prominent supratrochanteric process; and the absence of a cuppedicus fossa.”

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies: ledgelike depression ventrally located at confluence of metotic strut and posterior tympanic recess on anterior face of paroccipital process*, posteriorly tapering scapula; shortened forelimb (humerus ∼50% femur length); strongly mediolaterally compressed and anteroposteriorly broad ulna tapering posteriorly to narrow edge*; elongate lateral crest on posterodistal end of femur*; anterior caudal vertebrae with subhorizontal, laterally directed prezygapophyses*; prominent supratrochanteric process; and absence of cuppedicus fossa.

Discussion

Turner et al. (2007b) gave a preliminary description of this taxon, which was followed by a longer, more complete and well-illustrated description (Turner et al., 2011). In the accompanying phylogenetic analysis of Turner et al. (2007b), Mahakala was found to be the basalmost dromaeosaurid based on the absence of an accessory tympanic recess dorsal to the crista interfenestralis, the presence of an elongate paroccipital process with parallel dorsal and ventral edges that twist rostrolaterally distally, and the presence of a distinct ginglymus on the distal end of metatarsal II. Mahakala lacks the elongate prezygapophyses and chevrons present in more derived dromaeosaurids and also lacks the subarctometatarsalian pes seen in microraptorine dromaeosaurids. As this taxon was a focus of a contemporary study (Turner et al., 2011) it will not be discussed further here.

Microraptor gui Xu et al., 2003

Holotype

IVPP V13352 (fig. 19).

Fig. 19

Holotype of Microraptor gui (IVPP V13352).

i0003-0090-371-1-1-f19.tif

Distribution

Aptian-Albian, Early Cretaceous, Jiufotang Formation, Xiasanjiazi, Chaoyang County, western Liaoning.

Original Diagnosis

Following Xu et al. (2003: 335), “distinguishable from Microraptor zhaoianus in having prominent biceps tuberosity on radius, much shorter manual digit I, strongly curved pubis, and bowed tibia.”

Revised Diagnosis

Not applicable. This taxon is here considered a junior synonym of Microraptor zhaoianus (see fig. 20 and table 4).

Fig. 20

Select Microraptor specimens illustrating important morphological features that were thought to distinguish M. zhaoianus from M. “gui.” A, Microraptor “gui” holotype IVPP V13352 with inset showing tubercle interpreted to serve as an attachment site for the biceps brachii muscle. B, M. “gui” IVPP V13325 pelvis and hindlimb showing details of the pubis and tibia. C, D, E, M. zhaoianus holotype IVPP V12330. A straight tibia can be seen in C; the box with asterisk is shown in more detail in D. Highlighted square in D represents possible biceps tubercle. Counterslab of D is shown in E. F, uncataloged Microraptor specimen housed at IVPP. G, M. zhaoianus IVPP V13320. Circles denote biceps tubercle location, arrows point to tibiae, and the stars denote the location of the pubis.

i0003-0090-371-1-1-f20.tif

TABLE 4

Distribution of Putative Microraptor Species Autapomorphies among Microraptor Specimens

i0003-0090-371-1-1-t04.tif

Discussion

This taxon was described on the basis of two nearly complete specimens (IVPP V13352, which is the holotype; and IVPP V13320) from the Lower Cretaceous Jehol Group, Jiufotang Formation of the Chaoyang Basin. It is interesting for its preservation of pennaceous, remexlike feathers on its hindlimbs. Fourteen large feathers are preserved on the metatarsus with the proximal ones shorter in length and possessing symmetrical vanes. The distal feathers are longer and possess asymmetrical vanes. Pennaceous feathers are also present on the tibia.

The prominent biceps tubercle present on the radius of IVPP V13352 is unknown in the referred Microraptor gui specimen (IVPP V13320). A large biceps tubercle is also present in an uncataloged IVPP specimen. This character is, however, variably present in Microraptor zhaoianus (IVPP V12330 and CAGS 20-8-001) and its presence is unknown in six of the other known Microraptor specimens (fig. 20). Given the variable presence of this character in Microraptor zhaoianus, its uncertain distribution among other Microraptor specimens, and possible ontogenetic variability of this characteristic we considered it undiagnostic for Microraptor gui, but a variable feature for Microraptor in general.

Manual digit I is not preserved in any of the referred Microraptor zhaoianus specimens (IVPP V12330, CAGS 20-8-001, CAGS 20-7-004), so the proportionally short manual digit I cannot be used to distinguish Microraptor gui from Microraptor zhaoianus. In fact, a proportionally short manual digit I is present in Sinornithosaurus millenii (IVPP V12811, NGMC 91), Microraptor gui (IVPP V13352, IVPP V13320), and two uncataloged IVPP Microraptor specimens to the exclusion of Graciliraptor lujiatunensis and other dromaeosaurids. In Microraptor gui the length of metacarpal I is 43% (IVPP V13352) or 47% (IVPP V13320) that of metacarpal II. In two uncataloged IVPP specimens of Microraptor metacarpal I is 44% and 48% respectively, whereas in, Sinornithosaurus millenii metacarpal I is 49% (NGMC 91) or 42% (IVPP V12811) that of metacarpal II.

As written, “a strongly curved pubis” is ambiguous and therefore problematic. This character refers to a strongly posteriorly curved pubis. Many Microraptor specimens are compressed dorsoventrally (BPM 1 3-13, CAGS 20-8-001, CAGS 20-7-004, IVPP V12330, IVPP V13320, IVPP V12727, IVPP V13476, IVPP V200211, and two uncataloged IVPP specimens) as is one specimen of Sinornithosaurus (NGMC 91). In specimens that are mediolaterally compressed, the pubis of Microraptor (IVPP V13352, uncat. 2) and Sinornithosaurus (IVPP V12811) are posteriorly curved, so that the posterior surface is concave. Thus, like a proportionally short manual digit I, a strongly posteriorly curved pubis is likely a Microraptor + Sinornithosaurus synapomorphy, although a lesser degree of posterior curvature is present in many dromaeosaurids (e.g., Unenlagia, Velociraptor, Deinonychus) and in some basal troodontids (e.g., Sinovenator, Sinusonasus, Anchiornis). The presence of some degree of posterior curvature of the pubis may prove to be more broadly characteristic of basal deinonychosaurs.

A bowed tibia is present in only one specimen of Microraptor gui (IVPP V13320); the holotype specimen (IVPP V13352) lacks a bowed tibia (fig. 20). The tibiae of Microraptor zhaoianus also display this variability—IVPP V12330 has a bowed tibia while CAGS 20-8-001 and CAGS 20-7-004 lack a bowed tibia. Five additional Microraptor specimens display an unbowed tibia (IVPP V13352, IVPP V12727, IVPP uncataloged 1, IVPP uncataloged 2, and IVPP uncataloged 3). Given the predominance of unbowed tibiae among Microraptor specimens and that in both cases where bowing is observed, the specimens are poorly preserved and are preserved in a splayed posture, we view this bowing as preservational and not diagnostic of a particular Microraptor taxon. Indeed, many of the specimens from the “Jehol localities” are severely plastically deformed (for instance, see the Sapeornis specimen described by Yuan, 2008) and subtle differences in curvature and orientation are often difficult to interpret.

Of the originally proposed characters diagnosing Microraptor gui none remain to distinguish it from Microraptor zhaoianus. Therefore, we consider Microraptor gui a junior synonym of Microraptor zhaoianus. All specimens of Microraptor zhaoianus and Microraptorgui” (when the relevant anatomy is preserved) possess a basal constriction of the teeth (except IVPP V13320), midcaudals that are three to four times the length of the dorsal vertebrae, an accessory crest distal to the lesser trochanter, and a strongly posteriorly curved pubis.

Microraptor zhaoianus Xu et al., 2000

Holotype

IVPP V12330 (fig. 21).

Fig. 21

Holotype skull of Microraptor zhaoianus (IVPP V12330). A, skull; B, interpretive line drawing adapted from Xu et al. (2000).

i0003-0090-371-1-1-f21.tif

Distribution

Aptian-Albian, Early Cretaceous, Yixian and Jiufotang Formations, Xiasanjiazi, Chaoyang County, western Liaoning.

Original Diagnosis

Following Xu et al. (2000: 705), “distinguishable from all other dromaeosaurids in anterior serrations absent on all teeth; posterior teeth have a basal constriction; middle caudals are about three to four times as long as the anterior dorsals; accessory crest of femur at the base of the lesser trochanter; tail with less than 26 vertebrae; and it has a strongly recurved and slender pedal ungual with prominent flexor tubercle.” This diagnosis was based on IVPP V12330.

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies: anterior serrations absent on all teeth; posterior teeth with basal constriction; middle caudal vertebrae about three to four times as long as anterior dorsals*; accessory crest on femur at base of lesser trochanter; tail with less than 26 vertebrae; extremely long and bowed metatarsal V*; and strongly recurved and slender pedal ungual with prominent flexor tubercle.

Discussion

Microraptor zhaoianus is one of the smallest nonavian dinosaurs known. The holotype specimen (IVPP V 12330) has a femur length of only 53 mm and an estimated body length of 470 mm (although in the initial publication a typo suggested that the animal was only 47 mm long). Subsequently, larger individuals of Microraptor have been found, some with femoral lengths nearly twice that of the holotype (IVPP V13320) and a thorough description was given by Hwang et al. (2002). As discussed above, we regard all specimens of Microraptor as belonging to a single species Microraptor zhaoianus. The characters originally used to diagnosis M. zhaoianus are present in specimens of Microraptorgui,” and as outlined above the characters used to distinguish M.gui” from M. zhaoianus are not valid or are more widespread.

Shanag ashile Turner et al., 2007

Holotype

IGM 100/1119 (fig. 22).

Fig. 22

Holotype of Shanag ashile (IGM 100/1119).

i0003-0090-371-1-1-f22.tif

Distribution

Berriasian–Barremian; Öösh Formation, Öösh locality, Ovorkhangai Aimag, Mongolia.

Diagnosis

Following Turner et al. (2007a: 4), a “small dromaeosaurid diagnosed by the following combination of characters: triangular, anteriorly tapering maxilla; lateral lamina of nasal process of maxilla reduced to small triangular exposure; absence of a promaxillary fenestra*; presence of interalveolar pneumatic cavities*; incipient dentary groove on posterolateral surface of dentary.”

Discussion

This taxon is known from a single fragmentary specimen with only cranial material preserved. The material consists of a right maxilla, dentary and splenial. Postcranial material from a small arctometatarsalian theropod recently described from Öösh by (Prieto Marquez et al., 2011) may be referable to this taxon, but there is no overlap between the two specimens aside from a small fragment of maxilla. Character scoring in our analysis is based entirely on the holotype material from Öösh and originally identified by Turner et al. (2007a).

Sinornithosaurus haoiana Liu et al., 2004

Holotype

D 2140.

Distribution

Early Cretaceous, Yixian Formation, Toutai, western Liaoning, China.

Original Diagnosis

Following Liu et al. (2004: 783), “distinguished from S. millenii in that: (1) the main body of the premaxilla is higher, its length being slightly longer than its height; (2) the anterior margin of the premaxilla is vertical; (3) the maxillary process of the premaxilla is very long; (4) the maxilla is separated from the external naris; (5) maxillary fenestra is circular and relatively small; (6) the ascending process of the quadratojugal is remarkably longer than the jugal process; (7) the ratio of the dentary length/height is distinctly small; and (8) the pubic peduncle of ilium is longitudinally narrower than acetabulum.”

Revised Diagnosis

Not applicable. This taxon is here considered a junior synonym of Sinornithosaurus millenii.

Discussion

This taxon was based on a single specimen (D 2140) from the Yixian Formation near Toutai, China. This specimen is broadly similar to other specimens of Sinornithosaurus. It possesses a rough surface of pits and ridges on the anterolateral surface of the antorbital fossa, a posteriorly bifurcated dentary, a large promaxillary fenestra with a thickened posterior rim, and many other Sinornithosaurus millenii autapomorphies. The eight autapomorphies provided by Lui et al. (2004) do not distinguish this specimen from any other S. millenii specimens.

For example, the main body of the premaxilla is longer than high in both S.haoiana” and S. millenii (see IVPP V12811; Xu and Wu, 2001: 1741). The premaxillae in D 2140 are disarticulated and displaced. It does appear true that the anterior margin of the premaxilla is more vertical in D 2140 than in other Sinornithosaurus specimens, but this is variable across specimens (see IVPP V12811 and NGMC 91) and therefore may not serve as a distinguishing feature for a new species.

Contrary to Liu et al. (2004), whereas the maxillary process of the premaxilla is very long in S.haoiana,” it is also long in all other specimens of S. millenii, and therefore the maxilla is separated from the border of the external naris (NGMC 91 and IVPP uncataloged; see fig. 20C). Liu et al. (2004) correctly note that the maxillary fenestra in D 2140 is circular and relatively small, but this cannot be used to distinguish this specimen from other Sinornithosaurus specimens. The maxillary fenestra is heavily damaged in the holotype skull of Sinornithosaurus millenii, and it is not possible to clarify this morphology in either NGMC 91 or the uncataloged IVPP specimen (fig. 23).

Fig. 23

Skulls of Sinornithosaurus millenii. A, holotype skull (IVPP V12811); B, referred specimen (NGMC 91); C, referred specimen (IVPP uncataloged). 1, likely promaxillary fenestra; 2, structure originally identified as maxillary fenestra; 3, long and posteriorly directed lateral process of parietal; 4, area corresponding to where a diastema should be, based on Xu and Wu (2001).

i0003-0090-371-1-1-f23.tif

The last three putative synapomorphies listed by Liu et al. (2004) likewise are not sufficient to distinguish S.haoiana” from S. millenii. The ascending process of the quadratojugal is approximately equal in length to the jugal process of the quadratojugal in S. millenii (∼11.5 mm × ∼10.5 mm, respectively). The apparently short ascending process of S. millenii that was depicted in the line drawing of Xu and Wu (2001: fig. 4E) does not match the much longer process in the specimen itself. Therefore, the slightly longer ascending process of the quadratojugal noted in S.haoiana” is insufficient to recognize a new species. The small length∶height ratio in the dentary is present in other specimens of Sinornithosaurus and in fact is broadly present among paravian taxa. Lastly, the pubic peduncle of the ilium in the holotype of Sinornithosaurus is narrower than the acetabulum just as in S.haoiana” and for that matter the other microraptorine Microraptor zhaoianus (Hwang et al., 2002).

Thus, the reexamination of the putative synapomorphies of Sinornithosaurushaoiana” reveals that they are either present in Sinornithosaurus millenii or variable among the number of Sinornithosaurus specimens. Coupled with the presence of discrete Sinornithosaurus millenii autapomorphies in D 2140, we conclude that S.haoiana” is a junior synonym of S. millenii.

Sinornithosaurus millenii Xu et al., 1999

Holotype

IVPP V12811.

Distribution

Early Cretaceous, Yixian Formation, Sihetun, western Liaoning, China.

Original Diagnosis

Following Xu et al. (1999: 262) and Xu and Wu (2001: 1740), a small dromaeosaurid differing from other dromaeosaurids in the presence of ornamentlike pits and ridges on the anterolateral surface of the antorbital fossa; a deep excavation on the posteroventral margin of the premaxilla; a diastema between the premaxillary and maxillary teeth*; a semicircular maxillary fenestra with a straight ventral margin; a large promaxillary fenestra; the posterolateral process of the parietal long and sharply posteriorly directed; the columnlike margin of the pterygoid process of the quadrate; a large excavation on the posterolateral surface of the parasphenoid process; the bifurcated posterior margin of the dentary*; unserrated premaxillary teeth; a distinctive groove posterior to the anterior carina on the lingual surface of the premaxillary tooth crowns; supracoracoid fenestra of coracoid; manual phalanx III-1 more than twice the length of phalanx III-2; pronounced tubercle near the midshaft of the pubis; posterodorsal process of the ischium; sub-arctometatarsalian metatarsal III.

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies: presence of rough surface of pits and ridges on anterolateral surface of antorbital fossa; deep excavation on posteroventral margin of premaxilla; large promaxillary fenestra with thickened posterior rim; posterolateral process of parietal long and sharply posteriorly directed; columnlike margin of pterygoid process of quadrate; large excavation on posterolateral surface of parasphenoid process (likely an anterior continuation of the basipterygoid recess); bifurcated posterior margin of dentary*; premaxillary teeth unserrated; distinctive groove posterior to anterior carina on lingual surface of premaxillary tooth crowns; supracoracoid fenestra of coracoid present; manual phalanx III-1 more than twice the length of phalanx III-2; pronounced lateral tubercle near midshaft of pubis; posterodorsal process of ischium present; partially arctometatarsalian metatarsal III (figs. 23, 24).

Fig. 24

Select microraptorine specimens illustrating important morphological features. A, forelimb of Sinornithosaurus (NGMC 91); B, uncataloged IVPP Sinornithosaurus; C, Microraptor (IVPP V12727); D, pelvis of Sinornithosaurus (IVPP V12811).

i0003-0090-371-1-1-f24.tif

Discussion

Sinornithosaurus millenii was the first dromaeosaurid described from China and the first dromaeosaurid recognized as preserving filamentous integumentary structures. It is also one of the only Jehol theropods that were collected by professional paleontologists. In the holotype specimen (IVPP V12811) the filaments have been displaced slightly from the corresponding part of the body. Xu et al. (2001) reexamined these filamentous appendages and demonstrated that there were two distinct types of compound structures composed of multiple filaments that are unique to paravian feathers—filaments joined in a basal tuft and filaments joined at their bases in series along a central shaft. The plesiomorphic feather types further conformed to the predictions of independent developmental models of feather origins.

Xu and Wu (2001) gave a detailed treatment of the cranial anatomy of Sinornithosaurus millenii; however, a similarly detailed treatment of this taxon's postcranial anatomy has not been conducted. A few inconsistencies exist in the reconstruction of the skull. The quadratojugal was reconstructed as not contacting the squamosal. It was specified in the text, however, that it was unclear whether contact between these elements existed (Xu and Wu, 2001: 1745). The mandible was reconstructed as possessing a very pronounced arced profile posterior to the external mandibular fenestra. Additionally, the angular was reconstructed in an orientation nearly 45° tilted relative to the anteroposterior plane of the dentary. Such a conformation of the mandibular bones would be very odd for a dromaeosaurid. Examination of the specimen shows that the elements of the mandible are clearly disassociated and there is nothing particular about the exposed morphology of the surangular or angular that would force a distinct angle to be present in the mandible. Given this fact, plus comparisons to the mandibles of Microraptor zhaoianus (IVPP uncataloged) and NGMC 91, it is our view that the mandible of Sinornithosaurus millenii would have been dorsoventrally straight across its length.

In 2001, Ji et al. described a very well-preserved dromaeosaurid with exquisitely preserved feather integument that showed a distribution of feather types across the specimen (NGMC 91). Ji et al. (2001) noted its similarities to Sinornithosaurus and that some of the morphological differences were consistent with allometric changes given the smaller relative size of NGMC 91 compared to IVPP V12811. Ji et al. (2001), citing its juvenile nature, did not erect a new taxon for NGMC 91. NGMC 91 shares a number of apomorphies with Microraptor and Sinornithosaurus. These include a coracoid fenestra, a metacarpal I plus phalanx I-1 that is shorter than metacarpal II, a radius that is less than half the width of the ulna, and a manual phalanx III-1 that is more than two times the length of manual phalanx III-2 (fig. 24). NGMC 91 cannot be referred to Microraptor zhaoianus, because it lacks elongate middle caudals that are three to four times the length of the dorsal vertebrae. It does possess a posteriorly bifurcated dentary, which is an apomorphy of Sinornithosaurus (fig. 23). For this reason, we regard NGMC 91 as a subadult specimen of Sinornithosaurus millenii and the scoring used for this taxon in our analysis is based both on the holotype and NGMC 91.

Two other aspects of Sinornithosaurus millenii anatomy are worth mentioning, the accessory antorbital fenestra and the premaxilla-maxilla diastema. In the detailed description of the holotype skeleton (IVPP V12811), Xu and Wu (2001) described the maxillary fenestra as positioned anteriorly in the antorbital fossa and possessing a straight ventral margin. The structure identified as the promaxillary fenestra was characterized as well developed and larger than the maxillary fenestra. Complicating matters is that the structure labeled as the maxillary fenestra in the line interpretation of the holotype skull is a fractured portion of the underlying right maxilla—not the structure present in the line drawing that corresponds to the maxillary fenestra in the authors' reconstruction of the skull. Examination of the holotype skull reveals that the structure that can most reliably be identified as a maxillary fenestra is in a heavily damaged area of the left maxilla (fig. 23). Therefore, any characterization of its morphology should be viewed critically. Consequently, we removed the original described maxillary fenestra morphology from the diagnosis of the taxon. Referred specimens of Sinornithosaurus provide little help. NGMC 91 does not have well-preserved external surfaces of the maxillae as the skull is split along the midline. The exposed right maxilla has what appears to be a dorsally displaced opening just anterior to antorbital fenestra (therefore, consistent with a maxillary fenestra), but the bone is too damaged to make a definitive identification. The presence of a promaxillary fenestra in this specimen cannot be determined. A third specimen, an uncataloged IVPP fossil, is referable to Sinornithosaurus millenii (fig. 20F). This specimen shows what we interpret to be the anteriorly placed, expanded promaxillary fenestra, although in this specimen the dorsal border of the fenestra is damaged and displaced partially from the main body of the maxilla. No maxillary fenestra is apparent on this specimen, but a crushed zone immediately anterior to the antorbital fenestra may be the site of the opening.

Reexamination of IVPP V12811, NGMC 91, and the uncataloged IVPP Sinornithosaurus specimen clarifies the nature of the premaxilla-maxilla contact in Sinornithosaurus millenii. Xu and Wu (2001) observed an excavation on the posterior portion of the premaxilla. The authors suggest that this excavation may be a diastema between the premaxillary and maxillary tooth rows, and they included this morphology in the diagnosis. We have chosen to remove this feature. The excavation noted by Xu and Wu (2001) is, in fact, the transition from the premaxillary body to the posterior process of the premaxilla. When placed in life articulation, the premaxillary body would have been flush with the anterior margin of the maxilla and the long posterior process of the premaxilla would have separated the body of the maxilla from the external narial opening (see NGMC 91 and uncataloged IVPP specimen) as in other dromaeosaurids. Therefore, the premaxillary tooth row would have been continuous with the maxillary tooth row, excluding the possibility of a diastema.

Sinornithosaurus has recently been the subject of some very strange speculations on a potentially venomous feeding habit (Gong et al., 2010). We find the conclusion of this paper highly suspect as have others (Gianechini et al., 2011). It is worth noting here that the long anterior maxillary teeth noted by the authors as present in Sinornithosaurus are clearly a preservational artifact. The anterior maxillary teeth are not apomorphically long; rather, most of the known Sinornithosaurus specimens have the maxillary teeth partially released from their respective alveoli (something that is common in theropod fossils). This exposes a large portion of the tooth's laterally concave root and gives the impression of long, fanglike teeth with venom-conducting grooves. Instead of venom-conducting furrows these teeth simply have the charactersic figure-eight cross-sectional teeth seen in the roots of dromaeosaurid and many other theropod teeth.

Tianyuraptor ostromi Zheng et al., 2010

Holotype

STMI-3 (fig. 25).

Fig. 25

Holotype of Tianyuraptor ostromi (from Zheng et al., 2010).

i0003-0090-371-1-1-f25.tif

Distribution

Early Cretaceous, Yixian Formation, Dawangzhangzi, Lingyuan, western Liaoning, China.

Diagnosis

Following Zheng et al. (2010), “a medium-sized dromaeosaurid that differs from other dromaeosaurids in the following derived features: length of the middle caudal vertebrae more than twice that of the dorsal vertebrae, a small and extremely slender furcula, and an elongate hindlimb about three times as long as the dorsal series.”

Discussion

Only a preliminary description of this taxon has been published and as a result it is poorly diagnosed. We have not had the opportunity to examine the specimen firsthand, so our observations are based on the preliminary description. Regarding the characters used to diagnose this new taxon, the elongate caudal vertebrae noted by the authors are more widely distributed, as they are present in Microraptor. The extremely slender furcula is interesting, but in our opinion its position along the right coracoid and right sternal plate renders its identification ambiguous. Comparison of the hindlimb length versus total dorsal series length is a bit problematic as an autapomorphy because it will be difficult to apply to new discoveries if the specimens are incomplete.

Although not mentioned in the diagnosis, the short forelimbs of Tianyuraptor are uncommon among dromaeosaurids (only Austroraptor and Mahakala have similarly reduced forelimbs). This feature, combined with other aspects of its morphology, suggest microraptorine affinities and lead us to consider this taxon valid but in need of a more thorough treatment of its anatomy. Prior to the present study, the phylogenetic position of Tianyuraptor remained unresolved, with some character data suggesting microraptorine affinities and other data placing Tianyuraptor closer to more derived Laurasian dromaeosaurids.

Tsaagan mangas Norell et al., 2006

Holotype

IGM 100/1015 (figs. 26, 27).

Fig. 26

Holotype skull of Tsaagan mangas (IGM 100/1015). A, right lateral view; B, left lateral view; C, dorsal view; D, anterior (left) and posterior (right) views. 1, large maxillary fenestra located at the anterior edge of the antorbital fossa (char. 28.0); 2, ascending process of jugal meets the squamosal; 3, oval shaped foramen magnum; 4, paroccipital process pendulous but not twisted as in other dromaeosaurids.

i0003-0090-371-1-1-f26.tif

Fig. 27

Interpretive line drawing of the skull of Tsaagan mangas.

i0003-0090-371-1-1-f27.tif

Distribution

Campanian, Djadokhta Formation, Xanadu sublocality, Ukhaa Tolgod, Ömnögov, Mongolia.

Original Diagnosis

Following Norell et al. (2006: 2), “paroccipital process pendulous and not twisted distally, basipterygoid process elongate and anteroventrally directed, maxillary fenestra large and located at the anterior edge of the antorbital fossa, jugal meets the squamosal to exclude the postorbital from the margin of the infratemporal fenestra.”

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters and autapomorphies: paroccipital process pendulous and not twisted distally; basipterygoid process elongate and anteroventrally directed; maxillary fenestra large and located at anterior edge of antorbital fossa*; jugal meets squamosal to exclude postorbital from margin of infratemporal fenestra; oval-shaped foramen magnum; low coracoid tuber (biceps tubercle); weak subglenoid shelf*; dorsoventrally oriented path of supracoracoid nerve through coracoid*.

Discussion

Tsaagan mangas was only the second dromaeosaurid taxon reported from the Djadokhta Formation since Velociraptor mongoliensis was described in 1924 (Osborn, 1924b). It is known from a well-preserved skull, cervical series, and partial scapulocoracoid. Tsaagan mangas has been included in iterations of the TWiG matrix (Norell et al., 2001, and onward) under its specimen number IGM 100/1015. In more recent versions Tsaagan is typically found closely related to Velociraptor mongoliensis and Deinonychus antirrhopus (Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b). It has been described in detail by Norell et al. (2006) and requires no more comment here.

Velociraptor mongoliensis Osborn, 1924

Holotype

AMNH FARB 6515.

Distribution

Campanian, Late Cretaceous, Djadokhta Formation, Shabarakh Usu, Mongolia.

Original Diagnosis

Following Osborn (1924b: 1–2), “skull and jaws of diminutive megalosaurian type. Cranium abbreviated; orbits greatly enlarged; face elongated; four fenestrae in the side of the cranium, one fenestra in the jaw. Teeth recurved, serrate on one or both borders, alternating in replacement; 3? + in premaxillaries, 9? + in maxillaries, 14 in dentaries. Ungual phalanges very large, laterally compressed, strongly recurved, super-raptorial in type.”

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters and autapomorphies (modified in part from Barsbold and Osmólska, 1999: 192): supratemporal fossa (and fenestra) subcircular, bound by laterally convex supratemporal arcade; frontal long, almost four times longer than wide across orbital portion, and almost four times as long as parietal; anterior border of internal antorbital fenestra broadly rounded; maxillary fenestra not located in a caudally open depression; premaxilla with long maxillary process reaching well beyond caudal margin of external nares; dentary very shallow, its depth constituting 1/8 to 1/7 of its length, ventral margin convex (dentary relatively deeper and with straight ventral margin in other dromaeosaurids); first and second premaxillary teeth larger than third and fourth; lateral wall of braincase possesses a deep prootic recess*; V-shaped furcula with reduced and asymmetrically developed hypocledium*; flangelike m. ambiens tubercle located proximally on anterior face of pubis*; well-developed anterior tuberosity proximally located on ischium* (termed the obturator tuberosity by Hutchinson (2001a); rounded longitudinal ischial ridge (shared with Deinonychus antirrhopus).

Discussion

Published shortly after the initial brief description of Dromaeosaurus albertensis (Matthew and Brown, 1922), Velociraptor mongoliensis is the second described dromaeosaurid, although at the time Osborn did not recognize its close relationship with Dromaeosaurus. It remains one of the best-studied species of the group, and is known from at least nine skeletons of varying levels of completeness (figs. 28, 29). It is iconic and one of the most familiar dinosaurs among nonscientists.

Fig. 28

Skull of “fighting dinosaur” specimen of Velociraptor mongoliensis (IGM 100/25) in A, right lateral view; B, left lateral view; C, dorsal view; D, posterior view. Important features of Velociraptor include: 1, rounded maxillary fenestra not recessed in a depression (char. 28.1); 2, maxillary fenestra separated from the anterior border of the antorbital fossa; 3, a very long maxillary process of premaxilla; 4, anterior two teeth longer than posterior two teeth; 5, accessory depression in supratemporal fossa (char. 466.1); 6, circular supratemporal fenestra; 7, long nasal process of frontal.

i0003-0090-371-1-1-f28.tif

Fig. 29

Lateral views of the skulls of Velociraptor mongoliensis: (top) holotype skull (AMNH FR 6516); (middle) IGM 100/25 (the “fighting dinosaur” skull); (bottom) IGM 100/982.

i0003-0090-371-1-1-f29.tif

Barsbold and Osmólska (1999) and Norell et al. (2004) have given a detailed treatment of the cranial anatomy, whereas Norell and Makovicky (1997, 1999) have provided extensive description of postcranial anatomy. Clearly the characters given by Osborn (1924b) to diagnose this taxon are plesiomorphies and of little value now. The revised diagnosis provided above is adapted from the cranial diagnosis provided by Barsbold and Osmólska (1999) and characters noted by Norell and Makovicky (1997, 1999). A number of authors (Sues, 1977; Barsbold and Osmólska, 1999; Senter et al., 2004; Senter, 2007) have regarded Velociraptor mongoliensis as having depressed nasals behind the external naris or as having an “upturned” snout. As Norell et al. (2006: 7) point out, this morphology is the result of the nasals being wider anteriorly than in the midsection, so that when mediolaterally crushed, as in the holotype skull (AMNH FARB 6516) or in the “fighting dinosaur” skull (IGM 100/25), the snout has an unusual upturned appearance (fig. 29).

Fig. 30

Interpretive line drawing of the skull of Velociraptor mongoliensis.

i0003-0090-371-1-1-f30.tif

Velociraptor osmolskae Godefroit et al., 2008

Holotype

IMM 99NM–BYM–3/3.

Distribution

Campanian, Late Cretaceous, Bayan Mandahu Formation, Inner Mongolia, China.

Original Diagnosis

Following Godefroit et al. (2008: 433), “Long rostral plate of maxilla, with elongation index (L/H ratio) 1.38. Promaxillary fenestra subequal in size to the maxillary fenestra and teardrop shaped; long axis of the promaxillary fenestra perpendicular to the dorsal border of the maxilla; long axis of maxillary fenestra parallel to this border. Ten maxillary teeth with short unserrated carina on the apical end of the mesial edge and with incipient serrations on the distal carina.”

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters and autapomorphies (modified from Godefroit et al., 2008): promaxillary fenestra teardrop shaped and subequal in size to maxillary fenestra*; long axis of promaxillary fenestra perpendicular to dorsal border of maxilla; long axis of maxillary fenestra parallel to dorsal border of the maxilla.

Discussion

Velociraptor osmolskae is the second dromaeosaurid reported from the Djadokhta Formation–equivalent Bayan Mandahu Formation. Jerzykiewicz et al. (1993) reported the presence of V. mongoliensis from Bayan Mandahu, although as noted by Godefroit et al. (2008) that material has never been adequately prepared. Given this, it is possible that only a single Velociraptor species is present in the Bayan Mandahu Formation. Although, V. osmolskae is known only from paired maxillae and a lacrimal it does appear to be distinct from V. mongoliensis. The two species share a long facial lamina of the maxilla anterior to the antorbital fossa and a very shallow caudally open fossa surrounding the maxillary fenestra. V. osmolskae differs from V. mongoliensis in the shape and size of the maxillary and promaxillary fenestrae (as evinced by the revised diagnosis above). Although there is considerable variation in the snout morphology of V. mongoliensis (see Norell et al., 2006), the promaxillary fenestra is always very small in these specimens. In contrast, this fenestra is very large in V. osmolskae and approximates the size of the maxillary fenestra. We currently view V. osmolskae as valid but note the possibility that future discoveries may render it synonymous with V. mongoliensis as several of these pneumatic features vary considerably where large samples are known.

EUROPEAN DROMAEOSAURIDS

Balaur bondoc Csiki et al., 2010

Holotype

EME PV.313 (fig. 31).

Fig. 31

Select material of Balaur bondoc exhibiting diagnostic features for the taxon. A, posterior dorsal vertebrae showing the dromaeosaurid trait of a stalked parapophysis and extensive vertebral pneumaticity; B, left lower leg exhibiting the derived states of a fused tibiotarsus, a fused proximal metatarsals, and a digit I modified for hyperextension.

i0003-0090-371-1-1-f31.tif

Distribution

Maastrichtian, Late Cretaceous, Sebes Formation, Alba County, Romania.

Original Diagnosis

Following Csiki et al. (2010), a “dromaeosaurid theropod with the following autapomorphies (asterisk denotes autapomorphies unique among all theropods): hypertrophied coracoid tubercle*; sinuous ridge on lateral surface of distal humerus extends for 1/3 of the length of the bone*; prominent ridge on medial surface of distal half of humerus*; anterior surface of ulna flattened and bisected by longitudinal ridge*; fused carpometacarpus; reduced, splintlike metacarpal III*; mc III contacting mc II distally, buttressed by overhanging ridge on mc II*; distal articular surface not extending onto plantar surfaces of metacarpals I and II; manual ungual II with Y-shaped lateral and medial grooves*; phalanges of manual digit III reduced and digit nonfunctional; extremely retroverted pubes and ischia whose long axes are nearly horizontal*; pubic peduncle [of the ilium] laterally everted such that broad cuppedicus fossa faces laterally and dorsally*; pubis reoriented so that lateral surface faces ventrally and pubic tubercle located directly below acetabulum*; ischial obturator tuberosity expressed as enlarged, thin flange that contacts or nearly contacts pubis ventrally*; tarsometatarsus substantially wider (1.5×) than distal tibiotarsus*; fused metatarsus (mt II–V); robust ridges on plantar surfaces of metatarsals II–IV*; metatarsals II and III not ginglymoid; articular region of mts II–III narrower than entire distal end*; first digit of pes functional with enlarged phalanges but vestigial metatarsal I*; and short, hooklike mt V.”

Discussion

This is a bizarre dromaeosaurid from the ancient European archipelago of the Late Cretaceous. The animal is perhaps most distinctive for its double sickle claw on the foot due to hypertrophy of the ungual of the first digit in addition to the typical hypertrophy of the ungual of the second digit. Csiki et al. (2010) provided an initial description of this taxon and a more detailed description of this taxon is underway (Brusatte et al., in review).

Additional details of the morphology of Balaur will not be discussed here, however, the description of Brusatte et al. (in review) will likely result in a modified diagnosis for Balaur. Some of the features included in the original diagnosis as autapomorphies appear to be more widely distributed among theropods (e.g., the reduced metacarpal III is similar to that of caenagnathids, and Y-shaped claw grooves are common to many coelurosaurs). Moreover, some of the autapomorphies identified from the shoulder girdle and pelvis many need revision in light of likely preservational distortion, initially undetected in the holotype material.

In the initial, preliminary, publication, Balaur bondoc was recovered as the sister taxon of the Campanian Velociraptor mongoliensis. This analysis was based on the dataset of Turner et al. (2007b). Future work on Balaur will prove important because this taxon marks the best-known European dromaeosaurid to date and has important biogeographic implications. This paper will be well illustrated and will provide adequate coverage of this important specimen.

Pyroraptor olympius Allain and Taquet, 2000

Holotype

MNHN BO001.

Distribution

Campanian-Maastrichtian, Late Cretaceous, La Boucharde, France.

Original Diagnosis

Following Allain and Taquet (2000) this taxon is diagnosed by “the presence of a deep depression on the lateral face of the ulna; a ventrally concave, distally grooved and asymmetrical metatarsal II; tooth serrations present posteriorly but restricted on the anterior carina; and an ulna that is subequal in length to metatarsal II.”

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters: ventrally concave metatarsal II subequal in length to ulna; tooth serrations present in posterior teeth but restricted on anterior carina.

Discussion

Prior to the discovery of Balaur bondoc, this relatively poorly known taxon was the best-represented European dromaeosaurid (figs. 32Fig. 3334). Pyroraptor olympius is known from Upper Campanian to Lower Maastrichtian sandstones of La Boucharde, France. The described Pyroraptor olympius material contains the remains of at least two individuals. The paratype of Pyroraptor olympius consists, in part, of a right phalanx II-2 (MNHN BO 002). Material referred to Pyroraptor olympius by Allain and Taquet (2000) included five “pedal phalangeal elements.” It was never specified which of the associated catalog numbers (MNHN BO 006–010) pertain to which element, but one of these is an additional right phalanx II-2 that is approximately 20%–25% larger than the paratype phalanx. The two phalanges are identical in all respects save that the larger phalanx has a more mediolaterally robust flexor heel that is not as medially displaced nor as ventrally extensive as in the smaller MNHN BO 002. Also referred to Pyroraptor olympius was a dorsal vertebra (MNHN BO 017). This vertebra is a bit atypical for a dromaeosaurid, although it is incomplete and poorly preserved. Most of the vertebral centrum is either distorted or unpreserved. The neural spine is not as squared off in lateral view as other dromaeosaurids such as Velociraptor and it cannot be determined whether the spine expanded into a spine table dorsally as in other dromaeosaurids (Norell and Makovicky, 1999). Additionally, the prezygapophyses slant dorsomedially more strongly than in other dromaeosaurids and the parapophyses are not preserved in MNHN BO 017 so that it can be determined whether they are “stalked” or pedicellate as in other dromaeosaurids.

Fig. 32

Select material of Pyroraptor olympius. A, manual phalanx (MNHN BO 011); B, tooth; C, ungual phalanx of pedal digit II; D, holotype specimen, ungual phalanx of pedal digit II (MNHN BO 001); E, dorsal vertebra (MNHN BO 017); F, phalanx II-2.

i0003-0090-371-1-1-f32.tif

Fig. 33

Forelimb elements of Pyroraptor olympius, ulna (MNHN BO 004) and radius (MNHN BO 013). Arrow indicates the preservational artifact initial interpreted as a unique depression.

i0003-0090-371-1-1-f33.tif

Fig. 34

Right metatarsal elements of Pyroraptor olympius. A, right metatarsal II (MNHN BO 003) in medial view; B, right metatarsal II (MNHN BO 003) in posterior view; C, right metatarsal III in anterior view. Arrow indicates the concave posterior margin of metatarsal II characteristic of Pyroraptor.

i0003-0090-371-1-1-f34.tif

Additional undescribed Pyroraptor remains include the distal portion of a right metatarsal III (MNHN uncataloged) and a nearly complete (left?) metatarsal I (this element appears to have been referred to Pyroraptor as a partial distal end of metacarpal I, MNHN BO 012).

The deep depression on the lateral surface of the ulna noted by Allain and Taquet (2000) as a diagnostic feature is a preservational artifact (fig. 33). The deep depression sits within a larger, shallower depression. In this depression concentric compression fractures are apparent both near the deepest portion of the depression and along the extremities. Although a distinct depression may have indeed been present, given the apparent preservational distortion, this character should not be used to diagnose this taxon.

A distally grooved and asymmetric metatarsal II is characteristic of dromaeosaurids in general (Ostrom, 1969a) and not a unique character of Pyroraptor olympius. Furthermore, tooth serrations present on the posterior carina, but absent on the anterior carina, is widespread among dromaeosaurids. The exceptions are Dromaeosaurus albertensis and Achillobator giganticus where serrations are present on both carinae and Buitreraptor gonzalezorum where serrations are absent on both carinae. Therefore, this character is diagnostic for a much more inclusive clade (probably Dromaeosauridae) and is not unique to Pyroraptor olympius.

It is clear that Pyroraptor olympius is a dromaeosaurid, however, at present it is hard to adequately diagnose this taxon based on clear apomorphies. Many of the characteristics proposed by Allain and Taquet (2000) prove to be widespread among dromaeosaurids. Two characteristics remain as possible apomorphies or what may serve as a unique combination of characters, namely metatarsal II ventrally concave and subequal to the length of the ulna. These features may prove sufficient to refer any future material to this taxon at which point a more detailed diagnosis may be possible.

Variraptor mechinorum LeLoeuff and Buffetaut, 1998

Holotype

MDE-D168.

Distribution

Late Campanian to Early Maastrichtian, Late Cretaceous, Grès à Reptiles Formation, La Bastide Neuve.

Original Diagnosis

Following LeLoeuff and Buffetaut (1998: 106), “the cervico-dorsal vertebrae have prominent epipophy[ses] and [well] developed[ed] hypapophysis; the cervico-dorsals bear two pleurocoels; the cervico-dorsals to the last dorsal bear a hyposphene-hypantrum articulation; the centra shorten from the anterior to the posterior dorsals; the sacrum consists of five coossified sacral vertebrae; the sacrocaudal vertebra has a trapezoidal centrum; the transverse processes of the sacrocaudal are aliform; the humerus has a well-developed deltopectoral crest and internal tubercle and also bears a strongly developed medial tubercle.”

Revised Diagnosis

Not applicable. This taxon is here considered a nomen dubium.

Discussion

LeLoeuff and Buffetaut (1998) based Variraptor mechinorum on isolated and extremely poorly preserved remains from two separate localities from southern France. The holotype consists of an incomplete and badly crushed posterior dorsal vertebra and sacrum. These elements preserve no characters diagnostic of Dromaeosauridae, or other less inclusive clades of coelurosaurs. The unassociated referred material includes a right humerus, a cervicodorsal vertebra, a posterior dorsal vertebra, and a femur. This material was used by the authors to augment the description and diagnosis of Variraptor.

Like the holotype material, the referred specimens lack specific apomorphies diagnostic of Dromaeosauridae. The cervicodorsal vertebra has a well-developed hypapophysis and is thus referable only to Maniraptora. Allain and Taquet (2000) noted the nondiagnostic nature of the Variraptor mechinorum holotype and concluded that the taxon is a nomen dubium. We agree with Allain and Taquet (2000) and follow their interpretation.

NORTH AMERICAN DROMAEOSAURIDS

Atrociraptor marshalli Currie and Varricchio, 2004

Holotype

TMP 95.166.1 (fig. 35).

Fig. 35

The rostral area of a variety of dromaeosaurids. A, cf. Bambiraptor feinbergorum (MOR 553S-7.30.91.274); B, Bambiraptor feinbergorum (AMNH FR 30556); C, Atrociraptor marshalli (TMP 95.166.1), reversed image of right maxilla; D, Velociraptor mongoliensis (IGM 100/25); E, Saurornitholestes langstoni (TMP 94.12.844), reversed image of right maxilla; F, Deinonychus antirrhopus (YPM 5232); G, Achillobator giganticus (MNUFR 15). Reproduced from Currie and Varrichio (2004: 120).

i0003-0090-371-1-1-f35.tif

Distribution

Late Campanian or Early Maastrichtian, Horseshoe Canyon Formation, Drumheller, Alberta.

Original Diagnosis

Following Currie and Varricchio (2004: 115), a “small velociraptorine, dromaeosaurid theropod that differs from Saurornitholestes and Velociraptor in having a shorter, deeper face. Subnarial body of premaxilla is taller than its anteroposterior length as in Deinonychus and possibly Dromaeosaurus. Internarial and maxillary processes of premaxilla subparallel and oriented more dorsally than posteriorly. Larger maxillary fenestra than in any other velociraptorine. Maxillary fenestra is directly above the promaxillary fenestra, rather than well behind it as in all other dromaeosaurids. Maxillary teeth more strongly inclined toward the throat than in all other dromaeosaurids except Bambiraptor and Deinonychus. Maxillary dentition is essentially isodont.”

Revised Diagnosis

Atrociraptor marshalli is a small dromaeosaurid that cannot currently be diagnosed with autapomorphies, but it is sufficiently characterized by a (currently) unique combination of characters: proportionally short and dorsoventrally high maxilla; body of premaxilla longer than tall (shared with Deinonychus); nasal and maxillary processes of premaxilla strongly slanted dorsally (shared with Deinonychus); maxillary fenestra placed close to anterior margin of antorbital fossa (shared with Achillobator, Tsaagan, and Dromaeosaurus) and located dorsal to promaxillary fenestra; maxillary teeth strongly inclined posteriorly (shared with Bambiraptor and Deinonychus).

Discussion

This fragmentary taxon was recovered from the upper Campanian or lower Maastrichtian Horseshoe Canyon Formation of Alberta, Canada, in 1995 and given a brief description by Currie and Varricchio (2004). Atrociraptor consists of partial premaxillae, the right maxilla, right dentary, fragmentary left dentary, teeth, and several bone fragments. Currie and Varricchio (2004) referred to this taxon as a “small velociraptorine” with a short, deep face. The teeth are strongly inclined caudally, a trait it shares with Bambiraptor feinbergorum and Deinonychus antirrhopus.

To date Atrociraptor marshalli has been included in only three phylogenetic analysis—the one conducted by Currie and Varricchio (2004) based on 42 characters, the analysis of Senter (2007), and most recently in an analysis by Longrich and Currie (2009). Currie and Varricchio (2004) recovered a single fully resolved most parsimonious cladogram with Atrociraptor as the sister taxon of Deinonychus, with Dromaeosaurus the basalmost dromaeosaurid. Senter (2007) found Atrociraptor as the sister taxon of a clade composed of Achillobator, Dromaeosaurus, and Utahraptor. Longrich and Currie (2009) recovered Atrociraptor as part of a clade that also include Saurornitholestes and Bambiraptor, which they dubbed Saurornitholestinae. As these results are highly disparate, no consensus exists regarding the affinities of Atrociraptor to other dromaeosaurids.

The position of the maxillary fenestra in Atrociraptor marshalli is near the rostral boundary of the antorbital fossa. It is difficult to tell because the dorsal margin of the maxilla right above the maxillary fenestra is broken, but the location appears to be similar to the extreme anterior placement of the maxillary fenestra in Tsaagan mangas (IGM 100/1015).

Bambiraptor feinbergorum (Burnham et al., 2000)

Holotype

AMNH FARB 30556.

Distribution

Mid to late Campanian, Late Cretaceous, Two Medicine Formation, Montana.

Original Diagnosis

Following Burnham et al (2004), “jugal with row of foramina along ventral margin; scapula with large, medially directed acromion; distinct, short scapulocoracoid suture; coracoid with neck or peduncle forming part of glenoid; coracoid foramen absent; 13 dentary teeth, nine maxillary teeth; ratio of humerus plus ulna to femur large; pubis with distal shaft and boot rotated posterodorsally; ischium with small proximal dorsal process; femur strongly recurved laterally and posteriorly.

Revised Diagnosis

Provisional, awaiting future descriptive work on both Bambiraptor and Saurornitholestes. The only difference between Bambiraptor and Saurornitholestes is the proportional length of the frontal, which is longer in Bambiraptor than in Saurornitholestes.

Discussion

Bambiraptor feinbergorum is quite small (less than a meter long) and generally regarded as a juvenile to subadult (Burnham, 2004; Currie and Varricchio, 2004; Norell and Makovicky, 2004). Burnham et al. (2000) based this taxon on a relatively complete and well-preserved skeleton from the middle to late Campanian Two Medicine Formation of Montana and gave it a preliminary description. A longer description was later provided by Burnham (2004) with attempts at functional interpretation regarding some of the taxon's anatomy.

Unfortunately, this later description is not very detailed and the author misidentified portions of the morphology present in the holotype. For instance, the correct sides for the nasals and quadratojugals were misidentified. Additionally, two of the supposedly diagnostic characters for the taxon—coracoid foramen absent and coracoid with a constricted neck proximally—were the results of misinterpretation of the preserved element. Both the left and right coracoids are damaged proximally giving the appearance that the blade of the coracoid is constricted proximally. When the crushing and damage of the element are accounted for it is similar to other basal paravian coracoids. Also, the coracoid foramen is only apparently absent because of damage to the coracoid. The “notched” proximal surface is in fact part of the medial wall of the n. supracoracoideus foramen.

In a number of respects Bambiraptor presents a complex history. When first reported, it was referred to Velociraptor (Burnham et al., 1997) with Feduccia (1999) figuring and briefly discussing aspects of its morphology. Later, Burnham et al. (2000) erected a new taxon, Bambiraptor feinbergi, for the material. However, Bambiraptor feinbergi is very similar to the nearly contemporaneous Saurornitholestes langstoni (Sues, 1978) and the two taxa differ only slightly in regard to the amount of frontal participation in the orbit. As noted by Norell and Makovicky (2004) this trait could potentially vary ontogenetically.

Moreover, as originally named by Burnham et al. (2000) the specific epithet for this taxon was “feinbergi.” However, as the etymology of this name was in honor of two individuals, the correct spelling should be “feinbergorum.” The taxonomic list of dromaeosaurids given in The Dinosauria (Norell and Makovicky, 2004) follows this and refers to the specific epithet as “feinbergorum.

Lastly, confusion exists over the proper collection numbers for the holotype and referred material. When first described Bambiraptor feinbergorum was exhibited at the Florida Institute of Paleontology, Graves Museum of Archaeology and Natural History. As such it received a FIP number, with the holotype receiving the initial accession number (FIP 001) and the referred material receiving the successive numbers (FIP 002–036). The American Museum of Natural History was later gifted with the material referred to Bambiraptor. In Burnham's (2004) more detailed description he references the holotype of Bambiraptor feinbergorum as AMNH 001 (as does Makovicky et al., 2005) and the referred material as AMNH 002-036. However, no such numbers exist for Bambiraptor, with the holotype instead reposited as AMNH FARB 30556. It should also be noted that Norell et al. (2006) and Turner et al. (2007a) erroneously referred to the holotype as “AMNH FARB 30554.”

Currently, Bambiraptor has been included in only three phylogenetic analyses—Currie and Varricchio (2004), Senter et al. (2004), and Longrich and Currie (2009). Currie and Varricchio (2004) included only six dromaeosaurid taxa and a supraspecific Troodontidae as the ingroup and recovered a single most parsimonious tree. In this case, Bambiraptor was recovered as the sister taxon to an Atrociraptor + Deinonychus clade supported by posteriorly directed maxillary teeth (40-1), and an anterior ramus of the maxilla that is shorter anteroposteriorly than dorsoventrally (20-1). Senter et al. (2004) and Senter (2007) recovered Bambiraptor as the sister taxon to a clade composed of Microraptor + Sinornithosaurus. This was supported by four unambiguous synapomorphies—a proximally constricted coracoid, manual phalanx III-1 greater than or equal to twice the length of phalanx III-2, manual phalanx I-1 bowed, and metatarsal V greater than or equal to 1/2 the length of metatarsal IV. The characters supporting this grouping are problematic. As discussed above, the constricted coracoid is a preservational artifact. Bambiraptor does have a dorsoventrally “bowed” or arched manual phalanx I-1. However, the supposedly “bowed” manual phalanx I-1 in Microraptor zhaoianus is ambiguous at best and in fact appears to be a preservational artifact owing to the two-dimensional nature of the specimens. Sinornithosaurus millenii (NGMC 91) may have bowed I-1 phalanges, but this too is complicated by two-dimensional preservation.

There is a possibility that Bambiraptor is a juvenile specimen of Saurornitholestes (Burnham et al., 2000; Norell and Makovicky, 2004). As noted by Norell and Makovicky (2004) and discussed below, Saurornitholestes langstoni currently lacks an adequate diagnosis. The original diagnosis proposed by Sues (1978) is not useful as it is based almost entirely on plesiomorphies. A precise estimate of the ontogenetic age of Bambiraptor has proven difficult because of remodeling of the fibula (personal obs.). A clear resolution of whether Bambiraptor feinbergorum is synonymous with Saurornitholestes necessitates detailed treatment of material known for each of these taxa. A confounding factor with Bambiraptor is that it may at least partially be based on a chimera, given that there are three identically sized tibiae included in the type specimen. Nevertheless, we are currently of a mind to provisionally regard both Saurornitholestes and Bambiraptor as valid until future work can resolve this issue.

Deinonychus antirrhopus Ostrom, 1969

Holotype

YPM 2505 (fig. 36).

Fig. 36

Select skull remains of Deinonychus antirrhopus. A, right maxilla (YPM 5232) in lateral view; B, left jugal (YPM 5210) in medial view; C, right lacrimal in lateral view; D, left astragalocalcaneum (YPM 5226) in anterior view. Important features of Deinonychus antirrhopus include: 1, a maxillary fenestra located dorsal to the level of the promaxillary fenestra; 2, lobate anteriormost process of jugal beneath antorbital fenestra; 3, a very prominent lacrimal boss; 4, a large and wide calcaneum.

i0003-0090-371-1-1-f36.tif

Distribution

Late Aptian or Early Albian, Cloverly Formation, Montana.

Original Diagnosis

Following Ostrom (1969a: 12), “a small, bipedal theropod with a moderately large head, moderately long and well-developed hind limbs, fore limbs elongate, manus long and slender in construction. Pes of medium length with four digits, the fifth represented by a vestigial metatarsal. Digital formula 2-3-4-5-0. Digits III and IV subequal in length, II specialized and bearing a very large, trenchant and strongly recurved ungual, I reduced and directed backward. Pes functionally didactyl (III and IV). Distal end of metatarsal II deeply grooved; metatarsal II not greatly compressed proximally. Articular facets of II developed to permit unusual extension but very limited flexion between first and second phalanges. Manus with three very long digits (formula 2-3-4), digits IV and V lost. Metacarpal I short and irregular in shape. Metacarpal III long, slender and divergent from II. Carpus consists of radiale and ulnare only. Radiale with well-defined asymmetrical ginglymus proximally for articulation with radius. Humerus and radius-ulna not reduced. Skull with large, subcircular to oval orbits and three antorbital fenestrae. Supraorbital rugosities on postorbital and lachrymal. Preorbital bar slender and in weak contact with a thin, platelike jugal. Quadratojugal very small, T-shaped, and apparently not in contact with squamosal. Nasals long, narrow, and unfused. Inferior premaxillary process forms lower margin of external naris. Pterygoid very long and slender, ectopterygoid complex and pocketed ventrally. Palatine expanded, with subsidiary palatine fenestra medially. Fifteen maxillary teeth, four asymmetrical, subincisiform premaxillary teeth, 16 subisodont dentary teeth. All teeth with anterior and posterior serrations; denticles of posterior serrations nearly twice as large as denticles of anterior serrations on all teeth. Twenty-two or 23 presacral vertebrae, 3 or 4 sacrals and approximately 40 caudals. Cervical vertebrae of moderate length, massive, platycoelous, and sharply angled. Dorsals short, and platycoelous. All caudals except the first 8 or 9 bear extremely long (up to 10 segments), rodlike, prezygapophyseal processes. Chevrons also elongated into long, paired, double bony rods extending forward beneath the preceding 8 or 9 segments. Ischium with triangular obturator process. Pubis (if correctly identified) short and greatly expanded into a subcircular, scoop-shaped element, with a distinct obturator foramen.”

Revised Diagnosis

A large dromaeosaurid diagnosed by the following combination of characters and autapomorphies: 15 maxillary teeth; four asymmetrical, subincisiform premaxillary teeth; 16 nearly isodont dentary teeth; all teeth with anterior and posterior serrations; denticles of posterior serrations nearly twice as large as denticles of anterior serrations on all teeth; lobate anteriormost process of jugal beneath antorbital fenestra*; lacrimal boss prominent; prefrontal greatly reduced*; maxillary fenestra located dorsal to level of promaxillary fenestra; large, wide calcaneum.

Discussion

This taxon is the best-known North American dromaeosaurids and is represented by multiple individuals from several localities. The original diagnosis for this taxon, however, is comprised of plesiomorphic characteristics. Ostrom (1969a) based this taxon on a number of specimens preserving elements from across the entire body, although at the time a braincase was lacking. These characters were assayed from multiple individuals, as the type specimen was accumulated in a multi-individual bonebed. This material was recovered from the Lower Cretaceous Cloverly Formation of Montana. It is currently known from at least eight articulated and disarticulated skeletons and skulls, which includes the partial skeleton and braincase from the Antlers Formation of Oklahoma (Brinkman et al., 1998) (fig. 37). The Antlers Formation material was referred to Deinonychus antirrhopus based on postcranial similarity and the presence of posterior serrations on all teeth that are twice as large as the anterior serrations (identified here as one of the autapomorphies of this species). The age of the Antlers Formation remains poorly constrained relative to the Cloverly Formation. It is possible that future additional material or analysis of the Antlers taxon could reveal it to be taxonomically distinct from Deinonychus antirrhopus.

Fig. 37

Braincase of Deinonychus antirrhopus (OMNH 50268). A, dorsal view; B, posterior view; C, ventral view; D, anterior view.

i0003-0090-371-1-1-f37.tif

The osteology of Deinonychus was given extensive treatment by Ostrom (1969a, 1969b, 1974, 1976a). Maxwell and Witmer (1996) and Witmer and Maxwell (1996) provided a brief description of additional cranial remains. This, plus the additional material discussed above, was examined for scoring Deinonychus in the present study.

Currie and Varricchio (2004) recovered Deinonychus antirrhopus as derived within dromaeosaurids, as the sister taxon of Atrociraptor marshalli. This was supported by one unambiguous synapomorphy—a premaxilla with a subnarial depth that is higher than long (30-1). In that analysis, Deinonychus and Atrociraptor also shared with Bambiraptor maxillary teeth that are strongly inclined posteroventrally (40-1). Senter et al. (2004) and Currie and Varricchio (2004) have only six dromaeosaurid taxa in common in their analyses. Senter et al. (2004) recover Bambiraptor well outside the least inclusive clade containing Deinonychus, and Dromaeosaurus was recovered closer to Deinonychus than in the analysis of Currie and Varricchio (2004). Senter et al. (2004) recover Deinonychus as the sister taxon of an Achillobator + (Utahraptor + Dromaeosaurus) clade. This relationship is supported by a single unambiguous synapomorphy; “dentary straight” (25-0). However, the robustness of this putative synapomorphy is highly suspect. Of the four taxa, only Deinonychus is scored as possessing character state 25-0—it is coded as 25-1 in Achillobator and as unknown (?) in Dromaeosaurus and Utahraptor. However, the known specimen of Achillobator lacks a dentary, so this character state cannot be considered anything other than uncertain (?). This leaves Deinonychus as the only taxon in the group to exhibit the putative synapomorphy. Therefore, it is equally parsimonious to interpret a straight dentary as autapomorphic for Deinonychus, with this conclusion the only one supported by the current data.

In the phylogenetic analyses of Xu et al. (2002a), Hwang et al. (2002), and Xu and Norell (2004), Deinonychus was recovered in a clade with Dromaeosaurus and Achillobator when the largely incompletely scored Adasaurus was removed from the analysis. This clade was unambiguously supported by the presence of D-shaped premaxillary teeth (unknown in Achillobator) and a loss of opisthopuby (unknown in Dromaeosaurus) (Norell and Makovicky, 2004). The scoring of the latter trait was based on the reconstruction offered by Ostrom (1974), but further examination of MCZ 4371 indicates the pubis was more reverted than in Ostrom's reconstruction.

Dromaeosaurus albertensis Matthew and Brown, 1922

Holotype

AMNH FARB 5356 (figs. 38Fig. 39Fig. 4041).

Fig. 38

Rostrum of Dromaeosaurus albertensis (AMNH FARB 5356). A, left premaxilla, maxilla, and jugal in lateral view; B, right maxilla, jugal, and quadratojugal in lateral view; C, right mandible in lateral view. Important features of Dromaeosaurus albertensis include: 1, a deep and thickened premaxilla; 2, a stout quadratojugal; 3, anteroposterior short lateral lamina of the maxilla anterior to the antorbital fossa; 4, presence of either an enlarged promaxillary fenestra or an extremely anteroventrally placed maxillary fenestra with no promaxillary fenestra depending on interpretation of identity of opening; 5, presence of a distinct surangular foramen (char. 74.1)—a deinonychosaurian synapomorphy; 6, a laterally exposed splenial (char. 75.1)—a deinonychosaurian synapomorphy.

i0003-0090-371-1-1-f38.tif

Fig. 39

Select skull remains of Dromaeosaurus albertensis (AMNH FARB 5356). A, braincase in posterior and right lateral views; B, left quadrate in medial and posterior views; C, frontal in dorsal view; D, left ectopterygoid in dorsal and ventral views. Important features of Dromaeosaurus albertensis include: 1, paroccipital process elongate and slender with parallel dorsal and ventral edges (char. 56.0), which distally curves ventrally becoming pendant (char. 57.1); 2, a very weakly expressed dorsal tympanic recess; 3, prominent preotic pendent; 4, expression of the anterior tympanic recess or basipterygoid recess absent on the basisphenoid or basipterygoid processes; 5, tip of frontal flat; 6, deep notches anteriorly on frontal for articulation with lacrimal; 7, postorbital process of frontal more sharply demarcated from the dorsomedial orbital margin.

i0003-0090-371-1-1-f39.tif

Fig. 40

Preserved left second pedal elements and metatarsal of Dromaeosaurus albertensis (AMNH FARB 5356). A, phalanges II-1 and II-2 in medial view; B, phalanges II-1 and II-2 in lateral view; C, phalanges II-1 and II-2 in dorsal view; D, phalanges II-1 and II-2 in ventral view; E, metatarsal II in anterior view. Important features of Dromaeosaurus albertensis include: 1, a modified phalanx II-1; 2, modified phalanx II-2 for hyperextension of the ungual (char. 204.1); 3, ginglymoid articulation on metatarsal II (char. 201.1)—a deinonychosaur synapomorphy.

i0003-0090-371-1-1-f40.tif

Fig. 41

Interpretive line drawing of the skull of Dromaeosaurus albertensis.

i0003-0090-371-1-1-f41.tif

Distribution

Campanian, Oldman Formation, Alberta, Canada.

Original Diagnosis

Following Colbert and Russell (1969: 40), “skull moderate length and height. Rugosity present on dorsolateral rim of lacrimal. Orbit circular, larger than first antorbital fenestra. Supratemporal arcade long. Quadratomandibular articulation slightly depressed. Pterygoid wing of palatine narrow. Dental formula: four premaxillary, nine maxillary, 11 dentary teeth. Anterior carina of maxillary and dentary teeth displaced medially near base of crown. Sixteen denticles per 5 mm on anterior and posterior carinae. Pes similar to that of Deinonychus, but with relatively shorter phalanges.”

Revised Diagnosis

A medium-sized dromaeosaurid diagnosed by the following combination of characters and autapomorphies (modified in part from Currie, 1995: 577): nine maxillary teeth; anterior carina of maxillary or dentary tooth close to midline of tooth near tip, twists toward lingual surface*; premaxilla deeper and thicker than other dromaeosaurids; quadratojugal stout; tip of frontal flatter and margin of supratemporal fossa less pronounced; postorbital process of frontal sharply demarcated from the dorsomedial orbital margin; posteromedial process of palatine slender; anterior and posterior tooth denticles subequal in size; anteroposterior short lateral lamina of maxilla anterior to antorbital fossa; nasals with V-shaped suture posteriorly between frontals; deep notches anteriorly on frontal for articulation with lacrimal; presence of either an enlarged promaxillary fenestra or an extremely anteroventrally placed maxillary fenestra with no promaxillary fenestra depending on interpretation of identity of opening; dorsal tympanic recess very weakly expressed; moderately developed preotic pendant; expression of anterior tympanic recess and/or basipterygoid recess absent on the basisphenoid or basipterygoid processes*.

Discussion

A preliminary description of this dromaeosaurid was presented by Matthew and Brown (1922) based on partially prepared material recovered by Barnum Brown from the Upper Cretaceous Oldman Formation of Alberta Canada. The type (AMNH FARB 5356) is based on a partial skull with lower jaws, hyoids, associated pedal elements, and a left metacarpal I. Colbert and Russell (1969) gave Dromaeosaurus albertensis a more thorough and complete description while Currie (1995), after repreparation of the holotype and CT analysis, was able to add new information on cranial and braincase morphology and improve upon the incorrect reconstruction of the skull by Colbert and Russell (1969).

Apart from the holotype, few other specimens of Dromaeosaurus albertensis exist. These include an isolated frontal (NMC 12349; Sues, 1978), a partial dentary (Currie, 1987), and numerous isolated teeth (Currie et al., 1990), all of which are difficult to definitively refer to the type. Dromaeosaurus appears to be distinct from other dromaeosaurids based on the general lack of pneumaticity in the braincase (only the caudal tympanic recess is present) and maxillary/dentary teeth carinae that curve lingually. The anterior and posterior semicircular canals in Dromaeosaurus albertensis (AMNH FARB 5356) are oriented largely in the vertical plane whereas those of Velociraptor mongoliensis (Norell et al., 2004; IGM 100/982) and Tsaagan mangas (Norell et al., 2006) are rotated more posteriorly. Scorings in this analysis are based entirely on the holotype material (AMNH FARB 5356). Furthermore, we have interpreted the preserved fenestra in the maxilla as a promaxillary fenestra. Changing the interpretation of this feature does not effect the placement of Dromaeosaurus within Dromaeosauridae.

Currie and Varricchio (2004) recovered Dromaeosaurus as the basalmost dromaeosaurid in their analysis, while Senter et al. (2004) recovered Dromaeosaurus as the most derived. Senter et al. (2004) groups Dromaeosaurus with Utahraptor as sister taxa based on one unambiguous synapomorphy—mesial and distal keels of posterior teeth with an equal number of denticles per 5 mm (20-0). In the analyses of Xu et al. (2002a), Hwang et al. (2002), and Xu and Norell (2004), Dromaeosaurus is recovered in a clade with Deinonychus and Achillobator when Adasaurus is excluded from the analysis. This clade is unambiguously supported by the presence of D-shaped premaxillary teeth (unknown in Achillobator) and a loss of opisthopuby (unknown in Dromaeosaurus) (Norell and Makovicky, 2004). In more current versions of the TWiG dataset (Novas and Pol, 2005; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b) and variants thereof (Longrich and Currie, 2009), Dromaeosaurus albertensis groups in an unresolved clade containing Achillobator giganticus, Utahraptor ostrommaysorum, and Adasaurus mongoliensis.

Hesperonychus elizabethae Longrich and Currie, 2009

Holotype

UALVP 48778.

Distribution

Campanian, Dinosaur Park Formation, Dinosaur Provincial Park, Canada.

Diagnosis

Following Longrich and Currie (2009: 5003), “pubic peduncle of ilium with medial surface deeply excavated; posterior wing of ilium with medial shelf split to form anterior posterior processes*; lateral tubercles of pubis wing-like and curving anteriorly*; pubic apron shifted onto posterior surface of pubis; pubic symphysis teardrop-shaped in lateral view; ischiadic process of pubis reduced to a narrow lamina.”

Discussion

This small dromaeosaurid is known from a single incomplete pelvic girdle and isolated referred pedal elements. Nevertheless, the morphology of the pelvis is convincingly microraptorine in aspect, an observation that is borne out by the phylogenetic analysis of Longrich and Currie (2009). Hesperonychus, like other microraptorines, has a pubis that curves posteriorly bending sharply beginning midshaft, a spatulate pubic symphysis, and a large lateral process roughly midway down the shaft of the pubis. The discovery of a Late Cretaceous North American microraptorine is significant for a number of reasons. Hesperonychus extends the temporal range of Microraptorinae by almost 45 million years and suggests a remarkable level of morphological conservatism within this dromaeosaurid subclade. Additionally, the presence of this taxon in North America is perhaps unexpected, but results in a significant geographic range extension and substantiates a more complex biogeographic interaction between Asia and western North America. With the great similarity between the Late Cretaceous faunas of western North America and Asia, and the dense sampling record in the Late Cretaceous it is perhaps more surprising that a Late Cretaceous microraptorine has not been found in Asia.

Saurornitholestes langstoni Sues, 1978

Holotype

TMP 74.10.5.

Distribution

Campanian (Judithian), Judith River Formation, Dinosaur Provincial Park, south-central Alberta, Canada.

Original Diagnosis

Following Sues (1978: 383), “a very small, lightly built theropod. Frontal triangular, not basined between the median suture and the orbital rim. Posterior part of the frontal well rounded and slightly inflated, without frontoparietal crest. Lateral walls of the anterior part of the endocranial cavity flaring laterally. Ectopterygoid complex, pocketed ventrally. Teeth with well-developed denticles (24–26 per 5 mm) on posterior carinae and tiny denticles (c. 35 per 5 mm) on anterior carinae.”

Revised Diagnosis

Awaiting detailed description of unpublished material.

Discussion

Saurornitholestes langstoni has been reported from the Oldman Formation, Alberta (Sues, 1978), the Judith River Formation, Montana (Russell, 1969), Dinosaur Park Formation, Alberta (Currie, 2005), Kirtland Formation, New Mexico (Sullivan and Lucas, 2000), the Aguja Formation, Texas (Sankey, 2001), and the Prince Creek Formation, Alaska (Fiorillo and Gangloff, 2000). Sullivan (2006) has subsequently referred the Kirtland Formation material to a new species of Saurornitholestes (see below).

As noted by Norell and Makovicky (2004), Saurornitholestes langstoni has not had definite diagnostic characters proposed to support it. The original diagnosis proposed by Sues (1978), is not useful as it is based almost entirely on plesiomorphy. The anterior denticle morphology and spacing (5 to 6 denticles/mm), however, seems to be effective in referring isolated teeth to Saurornitholestes (Sankey, 2001; Sankey et al., 2002). Currie and Varricchio (2004) recently described a maxilla for Saurornitholestes langstoni (TMP 94.12.844). Additional postcranial material awaits further description (TMP 67.20.36, TMP 88.121.39, MOR 660). Our view from the totality of the published and unpublished material is that Saurornitholestes langstoni is a valid taxon. Scorings in this analysis are based on the holotype, published descriptions, and examination of the unpublished TMP and MOR osteological specimens. Recent work (Zanno et al., in press) has drawn into question the unambiguous referral of teeth to Saurornitholestes and Dromaeosaurus.

Currie (1995) considered Saurornitholestes as a close Velociraptor relative. Makovicky et al. (2003) and Novas and Pol (2005) found the phylogenetic position of Saurornitholestes langstoni to be very labile among dromaeosaurids more derived than Sinornithosaurus and Microraptor. Norell et al. (2006) and Turner et al. (2007a) found Saurornitholestes in an unresolved position but still more derived than microraptorines. Makovicky et al. (2005) recovered Saurornitholestes langstoni in a clade with Achillobator giganticus, Dromaeosaurus albertensis, Utahraptor ostrommaysorum, and Adasaurus mongoliensis. Senter (2007) recovered a similarly composed clade including Atrociraptor marshalli. Turner et al. (2007b) found a phylogenetic position for Saurornitholestes langstoni most similar to that envisioned by Currie (1995)—in a clade with Tsaagan mangas, Velociraptor mongoliensis, and Deinonychus antirrhopus. In contrast, Longrich and Currie (2009) recovered Saurornitholestes in a clade with the North American taxa Atrociraptor and Bambiraptor.

Saurornitholestes robustus Sullivan, 2006

Holotype

SMP VP-1955.

Distribution

Campanian, Late Cretaceous, De-na-zin Member of the Kirtland Formation, San Juan Basin, New Mexico.

Original Diagnosis

Following Sullivan (2006: 253), “a species of Saurornitholestes distinguished from Saurornitholestes langstoni by its larger and more robust frontal (twice as thick).”

Revised Diagnosis

Not applicable. This taxon is here considered a nomen dubium.

Discussion

The frontal described by Sullivan (2006) is extremely weathered and damaged. It lacks synapomorphies of Saurornitholestes and even fails to preserve synapomorphies of Dromaeosauridae (e.g., the postorbital process is damaged; therefore, it is impossible to tell whether the frontal has a sharply demarcated postorbital process and the supposedly sigmoidal ridge on the postorbital process is too weathered to confidently homologize with the structure present in dromaeosaurids). Simply put, this is a damaged and weathered theropod frontal. We consider this taxon to be a nomen dubium.

Utahraptor ostrommaysorum (Kirkland et al., 1993)

Holotype

CEU 184v.400 (CEUM 1430).

Distribution

Barremian, Early Cretaceous, Cedar Mountain Formation, Utah.

Original Diagnosis

Following Kirkland et al. (1993), “claws on hand more specialized as cutting blades than in other dromaeosaurs. Lachrymal has distinctly parallel mesial and outer sides, giving it an elongate subrectangular appearance in top view. Premaxilla has base of nasal opening parallel to premaxillary tooth row.”

Revised Diagnosis

A large dromaeosaurid diagnosed by the following combination of characters and autapomorphies: elongate nasal process of premaxilla; quadratojugal L-shaped, without posterior process*; dorsal vertebrae lack pleurocoels; well-developed notch present between lesser trochanter and greater trochanter; distal end of metatarsal III smooth, not ginglymoid.

Discussion

The holotype and hypodigm are based on a single disarticulated, but seemingly associated skeleton from one site (Gaston Quarry) and disarticulated material of multiple individuals from a second site (Dalton Wells) (figs. 42, 43) and even more new material has been collected (J. Choiniere, personal commun.). Additional material pertaining to at least nine individuals is currently under study by Brooks Britt at BYU (Britt et al., 2001) (fig. 43). This material was recovered from Dalton Wells and Yellow Cat Quarries of the Cedar Mountain Formation. In addition to this, material referred to Nedcolbertia and two indeterminate coelurosaurs are present in this quarry (Eberth et al., 2006). Based on cervical morphology, one of these taxa may be an ornithomimid (personal obs.: A.H.T.).

Fig. 42

Select holotype and hypodigm material of Utahraptor ostrommaysorum. A, right premaxilla (CEUM 1430); B, possible right quadratojugal (CEUM 3538); C, possible palatine (CEUM 4023); D, left premaxilla (CEUM 1370). Note the lack of a distinct posterior process on the quadratojugal.

i0003-0090-371-1-1-f42.tif

Fig. 43

Referred material of Utahraptor ostrommaysorum. A, left premaxilla (BYU 7510 14585) in lateral (top) and medial (bottom) views; B, right femur (BYU 7510 14281) in lateral view. In B, the arrow indicates the notch between the greater and lesser trochanter, which is characteristic of Utahraptor ostrommaysorum.

i0003-0090-371-1-1-f43.tif

Utahraptor is the largest dromaeosaurid known with the largest specimen recovered having a femur length of 565 mm. This individual, however, is only slightly larger than the Mongolian dromaeosaurid Achillobator giganticus (femur length  =  550 mm). After its initial description, little attention has been paid to the osteology of Utahraptor. This may be due to the disarticulated nature of the referred material and the difficulty of referring the isolated elements to a specific taxon given that other theropods are known from the quarry.

Within the hypodigm of Utahraptor ostrommaysorum, there is at least one additional small individual (manual phalanx 184v.783/ CEUM 3657), which likely does not pertain to Utahraptor. This phalanx cannot be referred to Utahraptor nor any other coelurosaur based on apomorphies. Britt et al. (2001) noted that the ungual originally identified as an autapomorphically enlarged manual ungual is in fact the hypertrophied digit II pedal ungual. These authors also noted that the referred surangular may be an unidentifiable bone fragment and that the lacrimal is a Gastonia postorbital. We have confirmed the latter two points firsthand. Indeed, the “surangular” is impossible to confidently identify. While it may pertain to a surangular it is also possible that it is a portion of the splenial. As noted by Britt et al. (2001) the quadratojugal (CEU 184v.667/CEUM 3528) lacks the triradiate shape common to most maniraptorans, given the absence of a distinct quadrate process. The femur of Utahraptor ostrommaysorum (e.g., BYU VP15417) bears a well-developed lesser trochanter that, unlike other dromaeosaurids, is separated from the greater trochanter by a distinct notch. As in other dromaeosaurids the parapophyses are distinctly projected on pedicles on the dorsal vertebrae. Unlike other dromaeosaurids, the dorsal vertebrae of Utahraptor ostrommaysorum lack pleurocoels.

Britt et al. (2001) proposed that the pubis of Utahraptor ostrommaysorum would have been retroverted. They based this inference on the orientation of the pubic peduncle of the ilium. This could not be confirmed based on examination of BYU VP 14389 and we regard the orientation of the pubis as ambiguous for this taxon.

The ischium (BYU VP 10978) referred to Utahraptor ostrommaysorum by Britt et al. (2001) is unusual for a dromaeosaurid. The obturator process is very proximally placed compared to most maniraptorans. The ischium lacks the longitudinal ridge that typically divides the dromaeosaurid ischium into an anterior and posterior part (Norell and Makovicky, 1999; Makovicky et al., 2005: char. 168). The shaft of the ischium is also rodlike versus the flat, platelike ischium seen in paravians. In these aspects this ischium is plesiomorphic. Given that multiple coelurosaur taxa are present in at the Dalton Wells site, it seems more likely that this ischium belongs to some other basal coelurosaur (e.g., an ornithomimid) than to Utahraptor ostrommaysorum, which would necessitate the presence of an extremely plesiomorphic ischium. The referral of this ischium to the Ornithomimidae is supported by the presence of a distinct semicircular scar on the posterior part of the proximal end of the ischium (present in ornithomimids and tyrannosaurids). Further indirect support for this referral is the presence in the quarry of ornithomimidlike cervical vertebrae (elongate and strongly opisthocoelous) and a postorbital tentatively referred to Ornithomimidae.

Nearly all early iterations of the TWiG matrix found a largely unresolved Dromaeosauridae (Norell et al., 2001; Xu et al., 2002a; Makovicky et al., 2003; Hwang et al., 2002, 2004b; Kirkland et al., 2005; Xu and Norell, 2004). However, more recent versions typically find Utahraptor as a derived dromaeosaurids closely related to Dromaeosaurus albertensis, Achillobator giganticus, and Adasaurus mongoliensis (Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b). Senter (2007) found Utahraptor related to a similar set of taxa but specifically found a resolved relationship between Dromaeosaurus and Utahraptor.

The specimens used in the present analysis all derive from the holotype, hypodigm, and referred material from Dalton Wells and Yellow Cat Quarries (Kirkland et al., 1993; Britt et al., 2001). Given the multitaxon nature of these quarries, specimens that could conceivably be referred to other theropod clades were avoided and codings from these were not included in the matrix.

SOUTH AMERICAN DROMAEOSAURIDS

Austroraptor cabazai Novas et al., 2009

Holotype

MML 195.

Distribution

Campanian-Maastrichtian, Late Cretaceous, Allen Formation, Río Negro Province, Patagonia, Argentina (fig. 44; table 5).

Fig. 44

Geographic distribution of Gondwanan dromaeosaurids illustrated on a paleogeographic globe of the mid-Cretaceous (adapted from Smith et al., 1994).

i0003-0090-371-1-1-f44.tif

TABLE 5

Temporal and Geographical Distributions of Gondwanan Dromaeosaurids

i0003-0090-371-1-1-t05.tif

Diagnosis

A large dromaeosaurid diagnosed by the following combination of characters and autapomorphies (following Novas et al., 2009: 1102–1103): “lacrimal highly pneumatized, with descending process strongly curved rostrally*, and caudal process flaring out horizontally above orbit* (differing from Laurasian dromaeosaurids, but unknown for other unenlagiines); postorbital lacking dorsomedial process for articulation with the frontal*, and with squamosal process extremely reduced (differing from Laurasian dromaeosaurids, but unknown for other unenlagiines); maxillary and dentary teeth small, conical, devoid of serrations and fluted (as in Buitreraptor); humerus short, representing slightly less than 50 per cent of femur length (a smaller ratio than in other dromaeosaurids and paravians); pedal phalanx II-2 transversely narrow, contrasting with the extremely robust phalanx IV-2 (differing from other dromaeosaurids, including unenlagiines, but resembling the condition of advanced troodontids).”

Discussion

This taxon is based on disarticulated skeletal remains, which are proposed to represent a single, albeit incompletely preserved, individual (fig. 45). Estimated at nearly 5 m long, Austroraptor is one of the largest dromaeosaurid described to date with only Achillobator and Utahraptor approaching it in size. The phylogenetic analysis conducted by Novas et al. (2009) recovered Austroraptor as a derived member of the South American dromaeosaurid clade Unenlagiinae. As support for the placement of Austroraptor in Dromaeosauridae, these authors cited the extension of the supratemporal fossa over most of the frontal process of the postorbital, teeth with unconstricted crown-root transition, epipophyses of anterior cervical vertebrae placed distally on the postzygapophyses, and the posterior margin of the cervical centra level with the posterior margin of the neural arch. In the postcranium, Austroraptor shares with other unenlagiines dorsal vertebrae with shortened transverse processes, transversely expanded distal ends of the dorsal neural spines, which form a spine table, and a proximally pinched metatarsal III (Novas et al., 2009). Additionally, Austroraptor has ventrolateral ridges along the cervical centra as in Buitreraptor and Unenlagia paynemili.

Fig. 45

Select material of Austroraptor cabazai. A, left maxilla in lateral view; B, right humerus in anterodorsal view; C, right postorbital in lateral view; D, left pedal phalanx II-2 in dorsal view; E, left pedal phalanx II-2 in lateral view; F, left pedal phalanx IV-2 in dorsal view.

i0003-0090-371-1-1-f45.tif

Novas et al. (2009) noted that Austroraptor cabazai differs from other dromaeosaurids and derived paravians in several respects. The humerus is less than half the length of the femur, thus this taxon exhibits relatively short forelimbs. This condition is uncommon in dromaeosaurids (exceptions: Mahakala omnogovae and Tianyuraptor ostromi) and in particular contrast to the very long forelimbs in the other unenlagiines Rahonavis ostromi and Buitreraptor gonzalezorum. Additionally, the deltopectoral crest is plesiomorphic relative to other dromaeosaurids, given that it projects anteriorly with a flat lateral face as opposed to the laterally excavated and laterally oriented deltopectoral crest seen in other unenlagiines. The lacrimal has a large excavation containing two foramina in the posterodorsal corner, a condition not known in other dromaeosaurids. Novas et al. (2009) also noted that enamel on the tooth surface of Austroraptor is fluted in a manner similar to that found in spinosaurid theropods. Not noted by the authors is that the postorbital of Austroraptor is bizarre and seemingly very plesiomorphic. All maniraptorans possess a postorbital with a frontal process that curves anterodorsally forming a dorsally concave temporal bar. The postorbital of Austroraptor lacks this anterodorsal upturning and in fact has greatly reduced frontal and squamosal processes. The very short squamosal process suggests that either a very long postorbital process on the squamosal was present (a trait that would be dissimilar to most dromaeosaurids) or that there was overall shortening of the temporal region.

Perhaps the most bizarre feature of Austroraptor is the strangely disproportionate pedal phalanges (fig. 45D, E, F). Phalanx IV-2 is over twice the width of phalanx II-2. Phalanx II-2 is clearly deinonychosaurian in its morphology; however, phalanx IV-2 is nearly three times the expected width based on similarly sized dromaeosaurids. Contrary to the suggestion of Novas et al. (2009) that a similar condition is present in derived troodontids, taxa such as Zanabazar mongoliensis and Sinornithoides show very little difference in the width between phalanx II-2 and IV-2. Given the loose association of the holotype material, the marked size discrepancy in phalangeal elements and plesiomorphic aspect of some of the cranial elements (e.g., postorbital) it is possible that the holotype could be a chimera. However, given that there is not at present any phylogenetic uncertainty (at least within the context of current datasets including Austroraptor), as could be expected if the specimen was a chimera, new discoveries will be needed to fully reassess the association of the Austroraptor holotype elements.

Buitreraptor gonzalezorum Makovicky et al., 2005

Holotype

MPCA 245 (figs. 46, 47).

Fig. 46

Select material of Buitreraptor gonzalezorum (MPCA 245). A, skull in left lateral view; B, right humerus in anterodorsal view; C, proximal view of left ulna; D, dorsal view of distal end of humerus. Important features include: 1, proximal median process on the posterior edge of the ischium (char. 165.1); 2, obturator process of ischium located at the distally (char. 169.3) and triangular obturator process with a short base and long anteriorly directed process (char. 234.1); 3, presence of a small bicipital scar (char. 384.1); 4, proximal surface of ulna divided into two distinct fossae (char. 144.1); 5, distal margin of the humerus developed into well-projected flexor process (char. 374.1).

i0003-0090-371-1-1-f46.tif

Fig. 47

Interpretive line drawing of the skull of Buitreraptor gonzalezorum.

i0003-0090-371-1-1-f47.tif

Distribution

Cenomanian-Turonian, Late Cretaceous, Candeleros Formation, Río Negro Province, Patagonia, Argentina.

Original Diagnosis

Following Makovicky et al. (2005: 1008) Buitreraptor “differs from other theropods in the following unique combination of traits: skull long, exceeding femoral length by 25%; teeth small, unserrated, without root-crown constriction; quadrate with large lateral flange and pneumatic foramen; posterior cervical centra with ventrolateral ridge; furcula pneumatic; brevis shelf expanded and lobate, projects laterally from caudal end of ilium.”

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies (modified from Makovicky et al., 2005: 1008): skull long, exceeding femoral length by 25%; teeth small, unserrated, without root-crown constriction; quadrate with large lateral flange and pneumatic foramen*; posterior cervical centra with ventrolateral ridge*; furcula pneumatic*; brevis shelf expanded and lobate, projects laterally from caudal end of ilium; differs from other dromaeosaurids by possessing a large maxillary fenestra*; a continuous transition from frontal margin to postorbital process (shared with Troodontidae); dentary bears a deep subalveolar groove (also shared with Troodontidae); flexor process present on humerus.

Discussion

This taxon is based on a nearly complete articulated adult skeleton as well as a partially articulated right hindlimb, sacrum, and pelvis. Found in the Cenomanian-Turonian Candeleros Formation of northwestern Patagonia, Buitreraptor gonzalezorum is the oldest dromaeosaurid known from Gondwana. This taxon proved to be a very important discovery because it provided evidence that united previously described Gondwanan dromaeosaurids in a single monophyletic Gondwanan lineage. Makovicky et al. (2005) named this Gondwanan dromaeosaurid lineage Unenlagiinae. This improved phylogenetic context clarified some of the previously problematic morphological data from the incomplete taxa Unenlagia and Neuquenraptor and led to the suggestion that these two taxa may be synonyms.

The phylogenetic analysis of Makovicky et al. (2005) recovered the purported avialan Rahonavis ostromi (from the Late Cretaceous of Madagascar) as a dromaeosaurid included in the Unenlagiinae clade with Buitreraptor and Unenlagia. Rahonavis was previously thought to be a primitive avialan based largely on the long forelimb proportions of the animal, the presence of ulnar papillae (attachment bumps for the follicular ligament of feathers), and features of the ilium. Both the ulnar papillae and ilial characters are known to have wider distributions among paravians (Makovicky et al., 2005; Turner et al., 2007b, 2007c). Furthermore Buitreraptor and many basal dromaeosaurids, described subsequent to the discovery of Rahonavis, have long forelimb proportions similar to that of Rahonavis.

Neuquenraptor argentinus Novas and Pol, 2005

Holotype

MCF PVPH 77 (figs. 48, 49).

Fig. 48

Select hindlimb material of Neuquenraptor argentinus (MCF PVPH 77). A, left femur in lateral view; B, distal end of left tibiotarsus in anterior view; C, distal end of left tibiotarsus in posterior view; D, distal end of left tibiotarsus in lateral view; E, distal end of left tibiotarsus in medial view.

i0003-0090-371-1-1-f48.tif

Fig. 49

Select pedal elements of Neuquenraptor argentinus (MCF PVPH 77). A, left metatarsus in anterior view; B, left metatarsus in medial view; C, left metatarsus in posterior view; D, left metatarsus in lateral view; E, pedal digit II in medial view; F, pedal phalanx II-2 in proximal view; G, pedal phalanx II-2 in ventral view.

i0003-0090-371-1-1-f49.tif

Distribution

Coniacian, Late Cretaceous, Portezuelo Formation, Sierra del Portezuelo, Neuquén Province, Argentina.

Diagnosis

Following Novas and Pol (2005: 858), “a probable dromaeosaurid with the following combination of characters: metatarsal II with lateral expansion over the caudal surface of metatarsal III (autapomorphic); metatarsal III proximally pinched; extensor sulcus on proximal half of metatarsus; distal end of metatarsal III is incipiently ginglymoid (to a lesser degree than other dromaeosaurids); pedal digit II with phalanges 1 and 2 subequal in length, and bearing a trenchant ungual phalanx.”

Discussion

This taxon provided important information regarding the presence of dromaeosaurids in South America because of the preservation of unambiguous deinonychosaurian synapomorphies on the pes. The holotype material is fragmentary and incomplete, rendering its exact phylogenetic position within Dromaeosauridae ambiguous.

Neuquenraptor argentinus was distinguished from Unenlagia based on differences in femoral proportions; however, both the proximal and distal ends of the femur of Neuquenraptor are incomplete. This renders potential statements of proportional differences tenuous. Makovicky et al. (2005) concluded that both femora are nearly identical and are similar in size. Firsthand examination of the femora of both specimens confirms these observations.

Available material referable to Unenlagia (Calvo et al., 2004; and see discussions below) is almost identical to Neuquenraptor in both size and proportion. Although the cooccurrence and size similarities between the overlapping skeletal elements of Neuquenraptor and Unenlagia appear to be compelling evidence for possible synonymy as noted by Makovicky et al. (2005), they cautioned for the need for more material to test this hypothesis more rigorously. Additional undescribed material of Neuquenraptor fails to provide a sufficient amount of new information (personal obs.: A.H.T.). Whereas we think it is likely these two taxa will prove to be synonyms, we feel it prudent to reserve formalizing this until additional overlapping material is found. Makovicky et al. (2005) tentatively treated Neuquenraptor as a junior synonym of Unenlagia for a secondary phylogenetic analysis. Norell et al. (2006) and Turner et al. (2007a, 2007b) have followed this in subsequent treatments of dromaeosaurid relationship. We follow these authors and previous analyses in continuing to treat these taxa as synonyms for the phylogenetic analysis. Furthermore, phylogenetic sensitivity to Neuquenraptor/Unenlagia is explored in greater detail in the Discussion.

Unenlagia comahuensis Novas and Puerta, 1997

Holotype

MCF PVPH 78 (figs. 50Fig. 5152).

Fig. 50

Select elements of Unenlagia comahuensis (MCF PVPH 78). A, right scapula in lateral view; B, left humerus in anterior view; C, close-up of proximal end of humerus in dorsal view. Arrow indicates the laterally oriented glenoid fossa (char. 138.1) characteristic of paravians.

i0003-0090-371-1-1-f50.tif

Fig. 51

Pelvis of Unenlagia comahuensis (MCF PVPH 78). A, right ilium, pubis, and ischium in lateral view; B, medial view of postacetabular blade of right ilium. Important features include: 1, proximal median process on the posterior edge of the ischium (char. 165.1); 2, ridge bounding the cuppedicus fossa confluent with the acetabular rim (char. 163.1).

i0003-0090-371-1-1-f51.tif

Fig. 52

Select hindlimb elements of Unenlagia comahuensis (MCF PVPH 78). A, left tibia in anterior view; B, left femur in anterior view; C, close-up of proximal end of left femur in lateral view; D, close-up of proximal end of left femur in posterior view. Arrow indicates the posterior trochanter of the femur.

i0003-0090-371-1-1-f52.tif

Distribution

Turonian-Coniacian, Late Cretaceous, Río Neuquén Formation (now Río Neuquén Subgroup and Portezuelo Formation), Sierra del Portezuelo, Neuquén Province, Argentina.

Original Diagnosis

Following Novas and Puerta (1997: 390), “possesses tall neural spines in posterior dorsals and anterior sacral vertebrae, being nearly twice the height of the centrum; deep lateral pits in the base of the neural spines of these vertebrae; twisted scapular shaft; inflected dorsal margin of postacetabular iliac blade.”

Revised Diagnosis

A large dromaeosaurid diagnosed by the following combination of characters and autapomorphies (modified from Novas and Puerta, 1997: 390, incorporating observations from Norell and Makovicky, 1999, and Makovicky et al., 2005): tall neural spines present in posterior dorsals and anterior sacral vertebrae, being nearly twice the height of centrum; deep lateral pits in base of neural spines of posterior dorsal and anterior sacral vertebrae*; twisted scapular shaft; inflected dorsal margin of postacetabular iliac blade; lobate brevis shelf projecting from end of ilium and beyond end of postacetabular lamina; proximodorsal process large, hooked and separated from iliac peduncle of ischium by notch; obturator process with short base and long process extending anteriorly.

Discussion

This taxon is represented by a fragmentary and poorly preserved postcranial skeleton from the Río Neuquén Formation, which has produced other nonavian theropods like the basal alvarezsaurid Patagonykus puertai. In the parsimony analysis of Novas and Puerta (1997) Unenlagia comahuensis was found as the sister taxon to Avialae.

Unenlagia comahuensis possesses a number of avialanlike features, which lead Novas and Puerta (1997) to regard this taxon as an important transitional nonavian theropod. Norell and Makovicky (1999), however, showed that many of the so-called avialanlike features in Unenlagia are also present in dromaeosaurids. These features include the presence of a pubic apron, the posteriorly deflected proximomedial corner of each pubic apron, an expanded cuppedicus fossa on the pubic peduncle (fig. 51), and a laterally oriented glenoid fossa (fig. 50). Furthermore, Unenlagia comahuensis preserves a number of dromaeosaurid synapomorphies not present in basal birds; these include stalked parapophyses and a mediolateral expansion of the tip of the neural spine in the posterior dorsal vertebrae (Norell and Makovicky, 1999).

Contrary to earlier analyses that found a sister-group status between Avialae and Unenlagia (Novas and Puerta, 1997; Forster et al., 1998), iterations of the TWiG dataset (Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Xu and Norell, 2004; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b) have consistently found Unenlagia nested among other dromaeosaurids. The Unenlagiinae dromaeosaurid clade recovered by Makovicky et al. (2005) including Unenlagia, Rahonavis, and Buitreraptor emerged after the discovery of the much more complete South American Buitreraptor gonzalezorum and the associated character analysis of that study. Calvo et al. (2004) recently described an additional species of Unenlagia. For the purposes of our analysis, these two species have been fused into a single Unenlagia genus-level terminal taxon.

Unenlagia paynemili Calvo ET AL., 2004

Holotype

MUCPv-349.

Distribution

Turonian–Early Coniacian, Late Cretaceous, Portezuelo Formation, Futalognko site, Neuquén Province, Argentina.

Diagnosis

Following Calvo et al. (2004: 550), “distinguishable from Unenlagia comahuensis by possessing more gracile bones, the angle between the anterior rim of the deltopectoral crest and humerus shaft about 116° (opposed to 140° of Unenlagia comahuensis) (see fig. 50), a small anterior process on the distal end of pubis, and the distal end of the postacetabular blade of ilium broader and rounded, and a shallower brevis fossa.”

Discussion

This taxon is based on a left humerus and left pubis (MUCPv-349) (figs. 53, 54) and referred material including a dorsal vertebra (MUCPv-416), the posterior end of a right ilium (MUCPv-409) (fig. 54), a manual ungual of digit I (MUCPv-343), and pedal phalanx II-1 (MUCPv-415) and phalanx II-2 (MUCPv-1066) (fig. 54).

Fig. 53

Select pelvic material of Unenlagia paynemili. A, right ilium (MUCPv 409) in medial view; B, right ilium (MUCPv 409) in lateral view; C, right pubis (MUCPv 349) in lateral view.

i0003-0090-371-1-1-f53.tif

Fig. 54

Select postcranial material of Unenlagia paynemili. A, left humerus (MUCPv 349) in anterior view; B, manual ungual (II?) (MUCPv 343); C, pedal phalanx II-1 (MUCPv 415) and phalanx II-2 (MUCPv 1066) in articulation in medial view; D, pedal phalanx II-2 (MUCPv 1066) in proximal view; E, pedal phalanx II-2 (MUCPv 1066) in ventral view.

i0003-0090-371-1-1-f54.tif

The new pedal material referred to Unenlagia by Calvo et al. (2004) includes pedal phalanges that are almost identical to phalanx II-1 and phalanx II-2 of Neuquenraptor in both size and proportions. The phalanx II-2 was not reported in the initial publication and the manual ungual was misidentified as a pedal ungual pertaining to digit I. For the purposes of our analysis the two species of Unenlagia have been fused into a single Unenlagia taxon.

AFRICAN DROMAEOSAURIDS

Rahonavis ostromi Forster ET AL., 1998

Holotype

UA 8656 (figs. 55, 56).

Fig. 55

Select postcranial material of Rahonavis ostromi (UA 8656). A, left scapula in lateral view; B, left ilium in lateral view; C, proximal part of right femur in lateral view; D, right tibia in proximal view. Important paravian features include: 1, acromion of scapula laterally everted (char. 133.1) and projecting anteriorly past the articular surface of the coracoid (char. 354.0); 2, lobate anterior edge of ilium (char. 156.1); 3, tuber along the dorsal edge of the ilium (char. 160.1).

i0003-0090-371-1-1-f55.tif

Fig. 56

Select pedal elements of Rahonavis ostromi (UA 8656). A, left metatarsus in anterior view; B, left phalanx II-2 in ventral and lateral views; C, ungual phalanx of digit two in lateral view.

i0003-0090-371-1-1-f56.tif

Distribution

Maastrichtian, Late Cretaceous, Maevarano Formation, Mahajanga Basin, Madagascar.

Original Diagnosis

Following Forster et al. (1998: 1919) “distinguished from all other avians by retention of a robust, hyperextensible, pedal digit II; from all other avians except Patagonykus by hyposphene-hypantra articulations on dorsal vertebrae; from Archaeopteryx by six fused sacral vertebrae and a greatly reduced fibula lacking contact with the calcaneum; from nonavian theropods, Archaeopteryx, and alvarezsaurids by its relatively elongate ulna with ulnar papillae and mobile scapulocoracoid articulation; from all other avians except Archaeopteryx and alvarezsaurids by retention of a long tail lacking a pygostyle; and from nonavian theropods by neural canals at least 40% of the height of the dorsal vertebral centra, proximal tibia of equal width and length, lack of a medial fossa on the fibula, and a reversed pedal digit I.”

Revised Diagnosis

A small dromaeosaurid diagnosed by the following combination of characters and autapomorphies: deeply concave glenoid fossa on scapula; elongate acromion process (length greater than length of glenoid)*; long muscle scar above glenoid on lateral face of scapula*; proximally kinked scapular shaft—this is marked by a low tubercle along the dorsal margin of the shaft*; elongate ulna with ulnar papillae; ulnar distal condyle subtriangular in distal view and twisted more than 54° with respect to the proximal end; a neural canal at least 40% of the height of the dorsal vertebral centra; six sacral vertebrae; undivided trochanteric crest, that is distally shifted on the femoral shaft*; prominent and proximally placed fingerlike lateral ridge on femur*; mediolaterally broad cnemial crest with short lateral process*; proximal end of tibia of equal width and length; a greatly reduced fibula lacking contact with the calcaneum; lack of a medial fossa on the fibula.

Discussion

Upon description, Rahonavis ostromi (originally “Rahona,” but this was amended due to preoccupation by a moth genus) was considered a transitional basal avialan (Forster et al., 1998; Chiappe, 2002). The reason is that, in addition to the new taxon's purported “avian-like” features, it retained the dromaeosaurids feature of an enlarged and hyperextensible claw on digit II, like that seen in dromaeosaurids.

Many of the characters used by Forster et al. (1998) and Chiappe (2002) to support the placement of Rahonavis within Avialae are now known to have wider distributions. Of the traits proposed by Forster et al. (1998) all are known to have fairly wide occurrences within maniraptorans with the possible exception of the ulnar distal condyle being subtriangular in distal view and twisted more than 54° with respect to the proximal end. The midshaft diameter of the fibula, reduced to 1/5 or less that of the tibia, is also present in dromaeosaurids like Velociraptor mongoliensis (IGM 100/986) and many Chinese dromaeosaurids (as well as several troodontids), and the lack of a deep fossa on the medial side of the proximal end of the fibula is common to most derived maniraptorans ranging from oviraptorids (Citipati osmolskae IGM 100/978) to dromaeosaurids (Velociraptor mongoliensis IGM 100/986) to alvarezsaurids (Patagonykus puertai MCF PVPH 37). Four additional characters—preacetabular process of ilium twice as long as postacetabular process, postacetabular process shallow (less than 50% of the depth at the acetabulum) and drawn back into a pointed process, pubic foot only projects caudally, and loss of a femoral neck also show a wide distribution. Although originally identified as possessing a reversed hallux, the morphology of metatarsal I in Rahonavis ostromi doesn't differ from other basal paravians with well-represented pedal elements (e.g., Velociraptor mongoliensis IGM 100/985, Archaeopteryx lithographica [Mayr et al., 2007], Mei long IVPP 12733). These taxa do not have fully reversed halluces (also see Middleton, 2002).

Makovicky et al. (2005) recovered Rahonavis as a dromaeosaurid related to the other Gondwanan maniraptorans Unenlagia and Buitreraptor. This analysis incorporated two of the traits Chiappe (2002) proposed uniting Rahonavis with birds more derived than Archaeopteryx—a fibula that does not reach tarsals and metatarsal II with a tubercle on extensor surface.

Chiappe (2002) proposed that the presence of a well-developed muscle scar for the m. brachialis anticus below the lateral cotyla of the ulna united Rahonavis with birds more derived than Archaeopteryx. Makovicky et al. (2005) noted that a small depression is present in this region in Buitreraptor (MPCA 245) and Bambiraptor (AMNH FARB 30556) yet did not include the character in the final analysis. However, the authors did conduct an exploratory analysis with Rahonavis and Confuciusornis scored as having a developed scar and all other taxa that preserve the ulna scored as lacking it as per Chiappe (2002). That analysis recovered the same set of most parsimonious trees placing Rahonavis as a dromaeosaurid.

A ratio of less than 70% between radial and ulnar shaft diameters was also considered diagnostic of Rahonavis and pygostylian birds (Chiappe, 2002). Similar ratios are present in Buitreraptor (67%; MPCA 245) and other basal deinonychosaurs, such as Microraptor zhaoianus (55%–62%; IVPP V13320) and Graciliraptor lujiatunensis (50%; IVPP V13474), suggesting a wider paravian distribution of this trait. Although discussed but not included in the analysis by Makovicky et al. (2005), we have incorporated a character to test the distribution of this morphology.

Rahonavis was reported to have a mobile scapulocoracoid joint, a particularly interesting character because this is generally regarded as a fairly derived avialan morphology occurring somewhere around Ornithothoraces. Archaeopteryx lithographica and Confuciusornis sanctus have fused nonmobile scapula-coracoid articulations, whereas Jeholornis and Jixiangornis have incipient ball-and-socket articulation between the scapula and coracoid, but in all three cases the scapula and coracoid are well sutured (albeit not fused) and therefore were probably immobile. If Rahonavis does indeed have a mobile scapulocoracoid joint, this distribution of characters in basal avialans strongly suggests separate origins.

Adding to this conclusion is the fact that the supposed mobile scapulocoracoid joint of Rahonavis is not morphologically similar to that in ornithothoracines. In those taxa with a true mobile joint, the scapula and coracoid are unfused and the scapula bears a well-developed ball-shaped tubercle that fits into a pit-shaped cotyla on the coracoid. Apparently no such morphology is present in Rahonavis, although one cannot comment on the coracoid morphology directly for that taxon because none are known. However, the coracoid facet on the scapula is plesiomorphically flat to very weakly concave in Rahonavis and therefore seems unlikely to have enabled mobility of the joint, or at least not in a way that is homologous to that in derived avialans.

The Wadi Milk Dromaeosaurid

This putative dromaeosaurid is known from the Albian-Cenomanian Wadi Milk Formation of northern Sudan (Rauhut and Werner, 1995). The remains of this potential taxon are isolated teeth and a few postcranial elements including a pedal phalanx II-2 and digit II ungual as well as two unguals of uncertain location in the pes. Phalanx II-2 has a deeply grooved distal articular surface and has a modest flexor heel posteriorly as is present in deinonychosaurs. In general appearance it resembles Pyroraptor olympius. The flexor process is relatively short compared to most dromaeosaurids and the anterior articular surface is not as highly modified as that seen in either Velociraptor mongoliensis or Deinonychus antirrhopus. But both the anterior and distal articular surfaces are enlarged and the shaft connecting them is constricted as seen in dromaeosaurids, but not in troodontids. The ungual of the second digit is also enlarged and trenchant as seen in other deinonychosaurs. Based on this information, we agree with Rauhut and Werner's referral of this material to Dromaeosauridae.

Based on tooth morphology, these authors further referred these remains to Velociraptorinae. We disagree with this. The denticles on the teeth are present on both the anterior and posterior carinae and the posterior denticles are much larger than the anterior ones. Although at the time of that publication (Rauhut and Werner, 1995) this morphology may have been unique to Velociraptorinae, this is no longer the case. Denticle size asymmetry is pervasive among dromaeosaurids. Therefore, given this material is insufficient to diagnosis a new taxon, we consider the Wadi Milk material referable to Dromaeosauridae incertae sedis. Nonetheless, this material is important because of its impact on our understanding the geographic distribution of dromaeosaurids, particularly in the light of the recent discoveries of other Gondwanan dromaeosaurids (Novas and Pol, 2005; Makovicky et al., 2005).

ANTARCTIC DROMAEOSAURIDS

The Naze Dromaeosaurid

Case et al. (2007) recently described a potential dromaeosaurid based on two poorly preserved teeth and a partial left pes from the Early Maastrichtian, Camp Lamb Member of the Snow Hill Island Formation. Fragments of metatarsals II, III, and IV were recovered along with phalanx III-1, part of a phalanx from digit IV and part of a distal phalanx from digit III, the posterior half of phalanx II-2, and an distally incomplete digit II ungual. Although, Case et al. (2007) referred this material to Dromaeosauridae, we have reservations. We agree that the digit II ungual is large and trenchant, but this only allows referral of the material to Deinonychosauria. However, large trenchant claws are also known in therizinosaurs and the stem avialans Patagonykus and Vorona. The distal articular surface of metatarsal III does appear to be incipiently ginglymoid (a possible dromaeosaurid synapomorphy), but the lack of a ginglymoid articulation on metatarsal II is inconsistent with dromaeosaurid morphology because even the most basal dromaeosaurids have ginglymoid articulations on the distal end of metatarsal II (Turner et al., 2007b). Additionally, it is unclear whether phalanx II-2 has an elongate flexor heel as would be expected for a dromaeosaurid of this size. It is our opinion that this taxon cannot be confidently referred to Dromaeosauridae and should be considered Deinonychosauria incertae sedis.

PART 2: PARAVIAN PHYLOGENY IN THE CONTEXT OF COELUROSAUR EVOLUTION TAXON SAMPLING

This section is intended to provide a brief outline of the taxonomic sampling scheme employed in this study as well as provide a rationale for such an approach. Further information on the included taxa is listed in appendix 1.

A total of 190 specimens of coelurosaurian theropods were examined firsthand at the collections of 19 different institutions in Argentina, Canada, People's Republic of China, France, Mongolia, the United Kingdom, and the United States. The studied specimens represent a large fraction of known coelurosaur diversity.

The principle analysis in our study includes 109 species-level taxa and two generic-level taxa (Unenlagia and Crax). The generic-level taxa were scored at this level because in both cases multiple species of each respective genus were used. For Unenlagia, this was done to reduce the amount of missing data for it as a terminal taxon, given that Unenlagia comahuensis and Unenlagia paynemili are both based on largely incomplete species and Neuquenraptor argentinus is considered as possibly synonymous with Unenlagia (e.g., Makovicky et al., 2005; Turner et al., 2007a, 2007b). The goal of this analysis was to provide the most comprehensive treatment of paravian phylogeny including the largest sample of valid dromaeosaurid, troodontid, and basal avialan taxa available.

Serving at the core of this taxon-sampling regime are 58 coelurosaurian taxa from the Theropod Working Group (TWiG) matrix (Norell et al., 2001). This TWiG matrix served as the base for the datasets of Makovicky et al. (2003), Hwang et al. (2004b), Makovicky et al. (2005), Norell et al. (2006), Turner et al. (2007a), and Turner et al. (2007b), which should be viewed as precursors to and the foundation of the dataset analyzed here. Twenty-two of the avialan taxa in the dataset are derived from the taxon-sampling regime of Clarke et al. (2006).

The coelurosaur outgroup choice was based on previous analyses of the TWiG matrix and confirmed by analyses of basal groups of theropod dinosaurs (Carrano et al., 2002; Rauhut, 2003; Smith et al., 2007). The taxa chosen were the well-known Allosaurus fragilis and Sinraptor dongi. Since both taxa are equally closely related to Coelurosauria, the better-known and more extensively studied Allosaurus fragilis was used to root the trees in this analysis.

Taxon sampling among the basal clades of Coelurosauria closely follows that of the various TWiG iterations (Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Hwang et al., 2004b; Makovicky et al., 2005; Norell et al., 2006). The largely exhaustive taxon sampling within Ornithomimosauria, Compsognathidae, and Alvarezsauroidea is maintained. Therizinosauria were not heavily sampled and are scored from a few of the best-represented and described taxa—Segnosaurus galbinensis, Alxasaurus elesitaiensis, and Erlikosaurus andrewsi. Oviraptorosauria remains represented basally by Incisivosaurus gauthieri and Caudipteryx zoui. More derived members of the clade include Avimimus portentosus, Microvenator celer, Chirostenotes pergracilis, Ingenia yanshani, Citipati osmolskae, Rinchenia mongoliensis, Oviraptor philoceratops, and Conchoraptor gracilis.

Continuing with the additions of Turner et al. (2007a, 2007b) to the basalmost coelurosaurs present in the matrix, putative basal tyrannosauroids in this dataset include Dilong paradoxus and Eotyrannus lengi. Derived tyrannosaurids are represented by Tyrannosaurus rex, Tarbosaurus bataar, Albertosaurus sarcophagus, Gorgosaurus libratus, and Daspletosaurus torsus. The two basal tyrannosauroids were studied firsthand for this project while the latter five taxa were based largely on previously published accounts although supplemented at times with reference to available material especially that at the AMNH.

Two other basal coelurosaur taxa of uncertain affinities were added. These are Coelurus fragilis (Marsh, 1879) and Proceratosaurus bradleyi (von Huene, 1926). Ideas on the phylogenetic position of Coelurus fragilis are varied (an issue compounded by its incompleteness), and historically this taxon has been difficult to place within the context of other seemingly closely related forms such as Ornitholestes hermanni and Proceratosaurus bradleyi. The cladistic analyses that have included Coelurus fragilis recover it in a number of different positions basally in Coelurosauria. Rauhut (2003) and Smith et al. (2007) found Coelurus as more closely related to compsognathids than to other coelurosaurs, whereas Senter (2007) found Coelurus plus Tanycolagreus topwilsoni (Carpenter et al., 2005) as the sister group to Tyrannosauroidea. Turner et al. (2007b) found Coelurus as more derived than the tyrannosaurids in their analysis (except for Dilong paradoxus which was not depicted as being a tyrannosauroid) but basal to all other coelurosaurs. Makovicky and Sues (1998), Zanno et al. (2009), and Zanno (2010a) recovered Coelurus as a basal maniraptoran.

Only a few authors have explicitly considered the phylogenetic position of Proceratosaurus bradleyi. Holtz (1998) found Proceratosaurus to be positioned at the most basal node in the coelurosaur tree. In this analysis, the majority of the terminal taxa examined were supraspecific and therefore provided a weaker test of the position of Proceratosaurus than an species-level approach like that taken here. More recently Li et al. (2009), Rauhut et al. (2010), and Brusatte et al. (2010) found this taxon to be a basal tyrannosauroid.

Previous approaches to the phylogenetic relationships of Paraves have had various levels of taxon sampling among the three major constituent clades. The two sampling regimes typically fall into two categories: (1) analyses interested primarily in coelurosaurian relationships and the placement of birds within coelurosaurs (e.g., TWiG iterations); and (2) analyses interested in the interrelationships of birds (Avialae) with only a few nonavialan outgroups (i.e., Clarke, 2004; Clarke et al., 2006; Zhou et al., 2008). Although sufficient for the purposes of those analyses, that level of sampling is insufficient to test for the multiple origins of flight (Mayr et al., 2005), optimization of the Microraptor-like four-wing morphology (Xu et al., 2003; Longrich, 2006), or the placement of avialanlike taxa like Rahonavis ostromi, Anchiornis huxleyi, and new basal taxa with seemingly troodontid and avialan similarities.

Our taxon sampling regime among basal paravians is exhaustive, comprising 52 nonavian paravian taxa (i.e., paravians that do not belong to crown-group Aves). Six crown-group avian taxa were included to help character optimization in this part of the tree by providing accurate codings for characters that affect optimizations near the base of Avialae. This portion of the dataset therefore comprises more than half of the total coelurosaurs sampled for this project. The complete sampling of Dromaeosauridae was discussed at length in Part 1. A brief account of the troodontid and avialan taxa included in the analysis is given below.

Troodontidae

There are currently 15 recognized troodontid species. All but two of these taxa are known from Cretaceous deposits of Asia. Troodon formosus, the eponymous taxon for the group, was the first troodontid described (Leidy, 1856) and is known from the Campanian deposits of the Judith River Formation and its stratigraphic equivalents in western North America (Makovicky and Norell, 2004) as well as Maastrichtian units like the Prince Creek Formation of Alaska (Fiorillo, 2008). Troodon remains one of the few troodontids represented by multiple individual specimens, although a thorough treatment of these multiple specimens has not been published. Nonetheless, most of the character scorings for this taxon are based on published descriptions (e.g., Currie and Zhao, 1993) and existing scorings derived from the initial TWiG work (in Norell et al., 2001; Makovicky et al., 2003, etc.). It is worth noting that ongoing work may indicate that all the material considered here as Troodon formosus may pertain to more than one species of Troodon (e.g., Zanno et al., 2011, in press). Because of the highly nested phylogenetic position of Troodon formosus, splitting the Troodon terminal into two separate species should have little effect on Troodontidae interrelationships.

Only one specimen of the basal troodontid Sinovenator changii (Xu et al., 2002a) has been published, but at least three other specimens exist for this taxon (IVPP uncataloged, and PKU VP 1058). Two specimens exist for Byronosaurus jaffei (IGM 100/983 and IGM 100/984; Makovicky et al., 2003). All of these specimens were examined firsthand for this project. All other troodontids species are known from single specimens. Saurornithoides mongoliensis (Osborn, 1924b), Zanabazar junior (Barsbold, 1974; see Norell et al., 2009), Sinornithoides mongoliensis (Russell and Dong, 1993), and Mei long (Xu and Norell, 2004) were examined firsthand. The recently described Xixiasaurus henanensis was scored based on it published description (Lu et al., 2010). Barsbold et al. (1987) discussed, but did not name, a somewhat plesiomorphic troodontid (IGM 100/44) from the late Albian to early Cenomanian Baruungoyot Svita, Mongolia. This taxon occupies an important position within troodontid phylogeny and is included here based on character scoring from previous iterations of the TWiG matrix.

Five taxa left out of the current project are Borogovia gracilicrus, Urbacodon itemirensis, Tochisaurus nemegtensis, Geminiraptor suarezarum, and Sinusonasus magnodens. Sinusonasus closely resembles Sinovenator changii, but we were not able to examine the holotype firsthand, so did not include it because the illustrations from the description were not sufficient to perform even a conservative scoring of the taxon. The other four taxa, aside from their autapomorphies, preserve character states that are common to all troodontids more derived than Sinovenator changii, were unavailable to be examined firsthand, and therefore were not included in the dataset.

The phylogenetic analysis of Turner et al. (2007b) recovered Jinfengopteryx elegans (Ji et al., 2005), originally described as an avialan, as a basal troodontid. There are three undescribed troodontids that play an important role in understanding troodontid and paravian phylogeny. One is a Late Jurassic taxon from the Morrison Formation of Wyoming. This taxon is under study by employees of the Wyoming Dinosaur Center and appears to occupy a position near Sinornithoides based on the phylogenetic analysis of Hartman et al. (2005). This taxon was not examined and is not included here. Two undescribed troodontids, IGM 100/1126 and IGM 100/1323, were collected from the Late Cretaceous of Mongolia at Ukhaa Tolgod (Hwang et al., 2004a). These taxa are well represented by complete skulls and postcranial material. IGM 100/1323 lacks forelimb material, but IGM 100/1126 preserves most of both hands. The two taxa these specimens represent share numerous characters with troodontids and most closely resemble basal members of the clade such as Mei and Sinovenator. Nevertheless, a number of putatively avianlike traits have been noted for these taxa (Hwang et al., 2004a). The two specimens were scored for the first time here and included in the phylogenetic analysis. Both of these specimens are currently being described in detail, including through the use of CT data and it is certain that more complete character codings for these taxa will emerge.

Avialae

A total of 23 basal avialans and six avians were included in the study. This represents the largest number of basal avialans included in an analysis that considered all of Paraves. This is 27 more avialan taxa than previous versions of the TWiG dataset, 23 more than the analysis of Turner et al. (2007b), and 22 more than the recent analysis by Senter (2007). Generally, these additions follow the taxon sampling regime of Clarke et al. (2006), but with the inclusion of the basal and enigmatic pygostylians Hongshanornis longicresta (Zhou and Zhang, 2005) and Liaoningornis longidigitris (Hou, 1996), the basal enantiornithine Pengornis houi (Zhou et al., 2008), and the important transitional, long-tailed, basal birds Jeholornis prima (Zhou and Zhang, 2002a) and Jixiangornis orientalis (Ji et al., 2002b). This taxonomic sampling brings the dataset into close approximation with two other recent analyses of Avialae (You et al., 2006, and Zhou and Zhang, 2006a). We were unable to examine Gansus yumenensis and Ambiortus dementjevi firsthand so they were not included and therefore our dataset does not match You et al. (2006) completely. Likewise, we were not able to examine Archaeorhynchus spathula, so that taxon was not added; our dataset thus does not encompass the entire dataset of Zhou and Zhang (2006a), which otherwise is nearly that of Clarke et al. (2006).

Previous approaches to paravian and avialan phylogenetic relationships were comparatively restricted and did not completely sample the diversity of long-tailed basal avialans. The sampling of basal long-tailed birds here is exhaustive save for Dalianraptor cuhe (Gao and Liu, 2005), which has been only briefly described and poorly figured. Archaeopteryx lithographica (Meyer, 1861) was included based on the 10 described and available specimens and firsthand examination of the London (BMNH 37001), Eichstätt (JM 2257), Munich (S6), and Thermopolis (WDC-CSG-100) specimens.

Two birds Jeholornis prima (Zhou and Zhang, 2002a) and Shenzhouraptor sinensis (Ji et al., 2002a) occur in the Early Cretaceous Jiufotang Formation. Described just a few months apart, these two taxa exhibit near identical morphology and have been considered synonymous (Zhou and Zhang, 2006b) and potentially synonymous (Chiappe and Dyke, 2006). We agree with the interpretation of these authors and consider Shenzhouraptor sinensis a junior synonym of Jeholornis prima. Jeholornis prima was scored based on firsthand examination of IVPP V13274, IVPP V13353 and the published descriptions of Zhou and Zhang (2002a, 2003a) and Ji et al. (2002a).

Jixiangornis orientalis (Ji et al., 2002b), from the Yixian Formation, has been included in only one phylogenetic analysis, which found it to be more derived than Jeholornis. This taxon was added to the current dataset based on the published description of the holotype plus images of an undescribed CAGS specimen. Contrary to Zhou and Zhang (2006b) Jixiangornis is not synonymous with Jeholornis. The two taxa are distinguished by a number of differences. For example, Jeholornis has six sacral vertebrae and Jixiangornis has seven. Jixiangornis has uncinate processes whereas no uncinate processes are present in Jeholornis. The sternal plates of Jeholornis are unfused, but Jixiangornis has a single sternal element. Jeholornis has a hooked ventral edge to the anterior blade of the ilium whereas Jixiangornis does not. Jeholornis has a deep ventrally concave cuppedicus fossa, yet Jixiangornis lacks a cuppedicus fossa. In Jeholornis, the astragalus and calcaneum are unfused to each other and the tibia, whereas there are various degrees of fusion in Jixiangornis. Jeholornis has gastralia whereas Jixiangornis lacks them. These differences are many and significant and definitively support the validity of Jixiangornis.

Here, basal short tailed birds are represented by Confuciusornis sanctus (Hou et al., 1995a, 1995b; Chiappe et al., 1999) and Sapeornis chaoyangensis (Zhou and Zhang, 2002b), which were scored firsthand (five and two specimens respectively). More derived pygostylians such as Vorona berivotrensis (Forster et al., 1996), Neuquenornis volans (Chiappe and Calvo, 1994), Gobipteryx minuta (Elzanowski, 1976; see also Chiappe et al., 2001), Patagopteryx deferrariisi (Alvarenga and Bonaparte, 1992), and Hongshanornis longicresta (Zhou and Zhang, 2005) were scored firsthand. Whereas the remaining basal pygostylians Liaoningornis longidigitris, Pengornis houi, Cathayornis yandica, and Concornis lacustris were scored from published descriptions.

Of the 15 ornithurine taxa included in the analysis, nine were scored based on firsthand examination (Apsaravis ukhaana Norell and Clarke, 2001; see also Clarke and Norell, 2002; Yanornis martini Zhou and Zhang, 2001; Yixianornis grabaui Zhou and Zhang, 2001; Crypturellus undullatus Temminck, 1815; Lithornis, Gallus gallus Linnaeus, 1758; Crax Linnaeus, 1758; Anas platyrhynchus Linnaeus, 1758; Chauna torquata Oken, 1816). Scoring for the remaining taxa (Songlingornis linghensis Hou, 1997; Hesperornis regalis Marsh, 1880; Baptornis advenus Marsh, 1877a; Ichthyornis dispar Marsh, 1872; Iaceornis marshi Clarke, 2004) was based on published descriptions.

Paravian problematica

Three putative paravian taxa of problematic taxonomic affinity are considered in our study. Epidendrosaurus ningchengensis, Epidexipteryx hui, and Pedopenna daohugouensis are known from the Middle to Late Jurassic Daohugou Formation in Inner Mongolia China. Epidendrosaurus has typically been recovered as a basal avialan either just outside (Senter, 2007; Zhang et al., 2008) or just inside (Choiniere et al., 2010) the Archaeopteryx node, or in an unresolved position basally in Avialae (Xu and Zhang, 2005) (fig. 4). Epidexipteryx was described in a short publication (Zhang et al., 2008) and has been recovered as the sister taxon to Epidendrosaurus forming a clade called Scansoriopterygidae (fig. 4B). The phylogenetic position of Pedopenna was tested by Xu and Zhang (2005) and found to lie in a polytomy with an unresolved avialan clade and deinonychosaurs. The holotype of Epidendrosaurus ningchengensis is an extremely juvenile individual (femur length less than 16 mm long).

A number of similarities exist between Epidendrosaurus and Epidexipteryx; however, the juvenile nature of Epidendrosaurus material makes it difficult to determine whether the differences between these taxa are ontogenetic or taxonomic. Moreover, inclusion of Epidendrosaurus in a phylogenetic analysis is problematic because juveniles do not necessarily preserve all the adult morphology needed to accurately place a taxon phylogenetically (Balanoff et al., 2008; Bever and Norell, 2009). Of the three problematic paravian taxa discussed here, we have included the largely complete and nearly full-grown Epidexipteryx into the primary phylogenetic analysis. We have chosen not to include Epidendrosaurus in the primary analysis but instead explore its potential phylogenetic position in a separate exploratory analysis. Similarly, Pedopenna (known from very incomplete remains—a left pes and distal portion of the tibia and fibula) were also included in the exploratory analysis discussed below.

CHARACTER SAMPLING

Previous approaches to paravian systematics have typically been split between nonavialan taxa or strictly avialans, and the character sampling has been drawn along this line as well. Therefore, a significant effort was made in this study to combine these two datasets (primarily the TWiG dataset for nonavialan taxa and Clarke et al., 2006, for avialan taxa). Additional new characters pertinent to basal paravian relationships were added to increase the number of characters relevant to this portion of the tree. This was aided by the study of new Mongolian dromaeosaurids (Norell et al., 2006; Turner et al., 2007a, 2007b), examination of additionally specimens of Microraptor zhaoianus, and two undescribed Mongolian troodontids (IGM 100/1323 and IGM 100/1126).

To this end, a compilation of published character sets was made from previous studies (Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Xu and Norell, 2004; Makovicky et al., 2005; Novas and Pol, 2005; Norell et al., 2006; Turner et al., 2007a, 2007b; Currie and Varricchio, 2004; Currie et al., 2003; Rauhut, 2003; Smith et al., 2007; Nesbitt et al., 2009). These characters were then scrutinized for redundancy, violation of character independence, weak homology statements, arbitrary character-state delimitation (particularly within continuously varying traits), or inconsistency with personal observations. A total of 560 characters were obtained combining new information and published characters. After detailed scrutiny of this character set, a final set of 474 characters remained (appendix 2). Many of the previously used characters were redefined and new character states were incorporated accounting for the additional morphological variation exhibited over the wide range of forms exhibited in the most basal of coelurosaurs (e.g., Coelurus fragilis) to the most derived members (e.g., Gallus gallus). The character list used here is composed by a fairly even sample of the different anatomical regions (table 6).

TABLE 6

Character Breakdown by Anatomical Region

i0003-0090-371-1-1-t06.tif

CLADISTIC ANALYSIS

Tree Search Strategy

The phylogenetic analysis of paravian theropods within the larger context of Coelurosauria was analyzed with equally weighted parsimony using TNT v. 1.0 (Goloboff et al., 2008a, 2008b). A heuristic tree search strategy was conducted performing 1000 replicates of Wagner trees (using random addition sequences, RAS) followed by TBR branch swapping (holding 10 trees per replicate). The best trees obtained at the end of the replicates were subjected to a final round of TBR branch swapping. Zero-length branches were collapsed if they lacked support under any of the most parsimonious reconstructions (i.e., rule 1 of Coddington and Scharff, 1994). This tree search strategy aims to obtain all the most parsimonious resolutions.

Most Parsimonious Topologies

This search strategy resulted in 1190 most parsimonious trees of 2024 steps (CI  =  0.300, RI  =  0.740), found in 134 out of the 1000 replications of RAS+TBR. Additional TBR branch swapping of these 1190 trees found 90,970 additional optimal topologies resulting in a total of 92,160 most parsimonious topologies. Due to the desire to sample densely the available paravian taxa, a number of incomplete taxa were included in the analysis. This results in three large zones of tree instability within derived oviraptorosaurs, basal dromaeosaurids, and basal pygostylian avialans. Combined, these zones of instability provide for the extremely large pool of most parsimonious reconstructions.

Tree Summary

Although a very large number of trees were recovered during the tree search, the strict consensus is highly resolved (fig. 57). The largest areas of poor agreement between trees is among basal dromaeosaurids, ornithurine avialans, and to a lesser degree derived oviraptorosaurs and basal coelurosaurs. This lack of resolution in the strict consensus is due to uncertainty in the position of only a few incomplete taxa, most importantly for paravians is the uncertainty of Pyroraptor olympius and Limenavis patagonicus. When the placement of these taxa are excluded during the strict consensus calculation the resolution markedly improves while still accurately representing the shared topology among all 92,160 fundamental trees (fig. 58). The benefit of this approach as opposed to pruning of these fragmentary taxa from the analysis is that it retains the character optimizations implied by these taxa as well as testing for their relationships among the more completely known coelurosaur taxa.

Fig. 57

Strict consensus topology of 92,160 most parsimonious reconstructions of coelurosaurian relationships found in the phylogenetic analysis of 474 characters and 111 taxa. Tree length equals 2024 steps; CI  =  0.300; RI  =  0.740.

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Fig. 58

Reduced strict consensus topology of the same 92,160 most parsimonious reconstructions. Pyroraptor olympius and Limenavis patagonicus have been excluded from the consensus.

i0003-0090-371-1-1-f58.tif

The broad phylogenetic relationships depicted in the consensus tree are congruent with the fundamental topology of previous TWiG analyses (Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Xu and Norell, 2004; Novas and Pol, 2005; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b). This is the case even though total character sampling and taxon sampling was nearly doubled. The monophyly of the major clades of Coelurosauria were recovered including Dromaeosauridae, Troodontidae, Avialae, Oviraptorosauria, Therizinosauria, Alvarezsauroidea, Compsognathidae, Ornithomimosauria, and Tyrannosauroidea. Relationships of note are the sister-group status of Dromaeosauridae and Troodontidae (constituting the Deinonychosauria), which together with Avialae constitute Paraves.

Important conclusions include: Pyroraptor olympius never falls within the derived Laurasian dromaeosaurid clade; Shanag ashile is more derived than unenlagiines but outside of microraptorines; Unenlagiinae is monophyletic with Rahonavis the most basal member; interrelationships of Microraptorinae are unresolved and Tianyuraptor nests within the clade; a monophyletic Laurasian clade comprised of a monophyletic Velociraptorinae and Dromaeosaurinae is present; Rahonavis ostromi is a dromaeosaurid and placing it into Avialae is markedly unparsimonious; Bambiraptor feinbergorum nests with velociraptorines and is not closely related to Chinese microraptorines; a new clade of basal Troodontidae is discovered consisting of Jinfengopteryx elegans and two undescribed Mongolian troodontids IGM 100/1323 and IGM 100/1126; Xiaotingia zhengi and Anchiornis huxleyi are sister taxa at the base of Troodontidae; Sapeornis chaoyangensis is more basal within Avialae than both Jeholornis prima and Jixiangornis orientalis, in spite of the presence of a pygostyle, and therefore is the only known nonpygostylian avialan with a pygostyle.

Areas of Uncertainty

Four regions of coelurosaur phylogeny exhibited disagreement among the fundamental trees and resulted in topological uncertainty in our strict consensus. These areas are basal coelurosaurs, derived oviraptorosaurs, basal dromaeosaurids, and ornithurine avialans. The character matrix employed in this study was not constructed with resolution of all these parts of the tree (except basal dromaeosaurids) in mind. Furthermore, phylogenetic resolution of basal coelurosaurs, derived oviraptorosaurs, and therizinosaurs are the subject of ongoing research with datasets constructed specifically to elucidate these relationships (Zanno, 2010a; Balanoff, 2011; Choiniere et al., 2010). Therefore, only two of these areas of uncertainty will be discussed further: basal dromaeosaurids and ornithurine avialans.

Basal Dromaeosauridae

In the strict consensus, the relationships among dromaeosaurids more basal than the Laurasian Velociraptorinae + Dromaeosaurinae clade are completely unresolved. This is due to the highly variable position of Pyroraptor olympius. Pyroraptor olympius is known from a few pedal phalanges, a metatarsal II, a radius and ulna, and a vertebra (Allain and Taquet, 2000). Examination of the fundamental trees shows that Pyroraptor olympius can occupy at least 23 alternate positions (fig. 59). Only one of these positions, however, is supported by unambiguous character data. Pyroraptor olympius shares with all dromaeosaurids the development of a ginglymoid distal end on metatarsal II (char. 201.1). This is a quintessential dromaeosaurid synapomorphy first noted by Ostrom (1969a) in Deinonychus antirrhopus and appears to relate to a reduction in medial and lateral excursion of the hypertrophied digit II in dromaeosaurids. The lability of this incomplete taxon is currently due to the absence of data and not character conflict. This is evinced by the fact that deletion of Pyroraptor from the analysis does not result in shorter trees.

Fig. 59

Cladogram summarizing the 23 alternate positions that Pyroraptor olympius can take among dromaeosaurids.

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Ornithurine Avialans

In the strict consensus, all non-neognath ornithurines form a polytomy. When the alternate positions of Limenavis patagonicus are ignored, the resolution of non-neognath ornithurines greatly improves. Limenavis can occupy eight locations among ornithurines (fig. 60); however, only one of these is supported by an unambiguous synapomorphy. Limenavis groups with Ornithurae (Hesperornis + Aves) based on a slightly raised bicipital scar on the ulna (char. 421.1).

Fig. 60

Cladograms illustrating the eight alternate positions that Limenavis patagonicus can take in the most parsimonious set of trees. Topology highlighted in the box is the only one with unambiguous support of the position of Limenavis patagonicus.

i0003-0090-371-1-1-f60.tif

Tree Description

The main point of interest for this analysis is paravian relationships, so that is the portion of the phylogeny that will be described in the most detail. Nonetheless, the characters supporting the major coelurosaurian clades along the spine of the tree leading to Paraves will be discussed here. We will begin at the base of the tree moving crownward. A complete list of synapomorphies can be found in appendix 4.

As discussed above, the consensus topology depicting the interrelationships of Coelurosauria is well resolved when two poorly known/highly incomplete taxa are ignored (Pyroraptor olympius and Limenavis patagonicus). This reduced strict consensus (fig. 58) forms the basis for the discussion to follow. For each clade discussed below, if formally named, the definition is given prior to the discussion of synapomorphies for this clade. Unless specifically noted, these definitions follow the author(s) that named the clade. Additionally, character numbers are cited for each of the diagnostic characters. The numbering of these characters corresponds to the list of used characters included in appendix 2. In this description, character numbers are followed by a period then the synapomorphic character state number.

Coelurosauria Huene, 1914

Definition

A stem-based monophyletic group containing Passer domesticus (Linnaeus, 1758) and all theropods closer to it than to Allosaurus fragilis Marsh, 1877b (sensu Holtz, 1994).

Coelurosauria is an extremely well-supported group with eight unambiguous synapomorphies and very strong nodal support (see below) (figs. 61, 66). All coelurosaurs are united by the presence of a number of derived features of the skull, vertebral column, and hindlimb. In the skull, they are characterized by extensive palatal shelves on the maxilla that form a long secondary palate (char. 25.1). This feature becomes particularly well developed in some paravian taxa like troodontids (e.g., Byronosaurus jaffei Makovicky et al., 2003). This is a completely nonhomoplastic character that has remained a consistent synapomorphy for Coelurosauria across multiple datasets and a number of iterations of the TWiG dataset (Sereno, 1999; Norell et al., 2001; Makovicky et al., 2003; Hwang et al., 2004b; Turner et al., 2007a).

Fig. 61

Reduced strict consensus cladogram of basal coelurosaur relationships. Maniraptoran taxa have been collapsed into a single terminal.

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The quadratojugals of coelurosaurs are typically a reversed L shape, as they lack a horizontal process posterior to the ascending process (char. 35.0), although reversals to the plesiomorphic condition are spread across various clades. Ornitholestes hermanni, Erlikosaurus andrewsi, Caudipteryx zoui show reversals to the Y-shaped quadratojugal and a reversal supports Dromaeosauridae monophyly, although the trait is not known in the basal Mahakala omnogovae. A reduced prefrontal ossification is optimized as ancestral for coelurosaurs (char. 40.1). Originally considered by Gauthier (1986) as a synapomorphy for Maniraptora, this optimization is based on the presence of a reduced prefrontal in the basally placed Tyrannosauroidea and Dilong paradoxus, which exhibit reduced prefrontal ossifications. The character witnesses notable reversals in ornithomimosaurs, Sinosauropteryx prima, Shuvuuia deserti, Sinornithosaurus millenii, Deinonychus antirrhopus, and possibly Erlikosaurus. Also near the posterior margin of the snout, the frontals narrow anteriorly as a wedge between the nasals (char. 41.0), this character reverses to the transverse frontal/nasal suture at the clade including Alvarezsauroidea + Therizinosauria + Oviraptorosauria + Paraves.

Also ancestral for coelurosaurs are the so-called D-shaped premaxillary teeth (char. 91.1). This is better defined as asymmetrical tooth crowns, because of their rounded, labial-sided, and flat lingual surface. This optimization is common to previous TWiG matrices (Norell et al., 2001, Hwang et al., 2004b; Makovicky et al., 2005; Turner et al., 2007a) and is due to occurrence of this morphology among basal members of Coelurosauria, notably Tyrannosauroidea, Proceratosaurus bradleyi, Dilong paradoxus, Pelecanimimus polydon, and Ornitholestes hermanni. Coelurosaurs including and more derived than the node marked by the common ancestor of Alvarezsauroidea + Therizinosauria + Oviraptorosauria + Paraves exhibit a reversal to the plesiomorphic subcircular morphology, with some modifications to spatulate teeth in some therizinosaurs and extremely modified unusual teeth in basal oviraptorosaurs.

In the postcranial skeleton the cervical and anterior trunk vertebrae are amphiplatyan (char. 101.0) compared to the opisthocoelous vertebrae seen in the proximate outgroups. Noted by Gauthier (1986), this feature has remained a consistent synapomorphy for Coelurosauria (Holtz, 1998; Norell et al., 2001; Hwang et al., 2004b; Makovicky et al., 2005; Turner et al., 2007a). Derived alvarezsaurids, however, exhibit a reversal to the plesiomorphic opisthocoelous condition, whereas in ornithurines and a few basal euornithines exhibit further transformation to a heterocoelous condition.

In non-coelurosaurian tetanurans, the ascending process of the astragalus is confluent with the condylar portion. In coelurosaurs, the condylar portion and the ascending process are separated by a transverse groove (in more basal coelurosaurs like tyrannosauroids and ornithomimosaurs) or a well-defined fossa (as in maniraptorans) (char. 197.1). In the pelvis, coelurosaurs share a triangular obturator process on the ischium, the caudal portion of which is confluent with the shaft of the ischium (char. 233.1). This feature was noted as a synapomorphy for coelurosaurs by Sereno (1999: char. 43) and later incorporated into the TWiG matrix by Makovicky et al. (2005: char. 233). Some specimens of Sinosauropteryx and some derived therizinosaurs, however, exhibit a reversal to the quadrangular obturator process of more basal tetanurans (Currie and Chen, 2001).

Unnamed Clade (Compsognathidae + Maniraptoriformes)

This group is united by four unambiguous synapomorphies. Ancestrally adult members of this group possess a lacrimal that lacks a supraorbital crest (char. 37.0), although a reversal to some degree of lacrimal crest is present in Shuvuuia deserti, all troodontids more derived than Anchiornis huxleyi, and some derived ornithurines. Likewise members of this group lack an enlarged foramen or foramina at the angle of the lacrimal above the antorbital fenestra (char. 38.0). A reversal to the presence of a foramen is seen in the dromaeosaurid Austroraptor cabazai and the oviraptorosaurs Rinchenia and Citipati. The antitrochanter on the ilium, located posterior to the acetabulum, is prominent in members of this clade (char. 162.1). Among compsognathids this feature is scorable only in Juravenator starki, but is then widespread among maniraptoriforms. Reversals to a more weakly developed antitrochanter are seen in Archaeornithomimus, Haplocheirus, Segnosaurus, Anchiornis, Sapeornis, and Cathayornis. Lastly, the supratemporal fossa has only a limited extension onto the dorsal surfaces of the frontal and postorbital (char. 245.0). This is reversed in most dromaeosaurids, Citipati, and IGM 100/1126.

Maniraptoriformes Holtz, 1995

Definition

A node-based monophyletic group containing Passer domesticus (Linnaeus, 1758), Ornithomimus edmontonicus Sternberg, 1933, and all their descendents.

A total of seven unambiguous synapomorphies united this clade, but overall nodal support remains relatively low (see below). Four cranial characters are shared among the coelurosaurs in this group, including the absence of a jugal recess in the posteroventral corner of the antorbital fossa (char. 33.1), the absence of a coronoid ossification (char. 76.2), the absence of distinct interdental plates (char. 90.0)—reversed in some therizinosaurs and Archaeopteryx lithographica, and the quadrate cotyle of the squamosal is open laterally exposing the quadrate head (char. 216.1). Postcranially, maniraptoriforms have cervical epipophyses placed proximal to the postzygapophyseal facets (char. 95.1) and broad and short cervical ribs (char. 124.1). In lateral view, the coracoid has a shallow ventral blade with an elongate posteroventral process (char. 136.2), which becomes further modified in more derived maniraptorans and avialans.

Maniraptora Gauthier, 1986

Definition

A stem-based monophyletic group containing Passer domesticus (Linnaeus, 1758) and all coelurosaurs closer to it than to Ornithomimus edmontonicus Sternberg, 1933.

In Gauthier's formulation (Gauthier, 1986), as followed above, this group contained all coelurosaurs more derived than ornithomimosaurs. He considered Compsognathus longipes, Coelurus fragilis, and Ornitholestes hermanni as likely members of this group. Holtz (1998), however, failed to recover either Ornitholestes or Coelurus as maniraptorans. Sereno (1997, 1999) likewise did not find Ornitholestes as a maniraptoran, but did not consider Coelurus in his analysis. Makovicky and Sues (1998) found both Ornitholestes hermanni and Coelurus fragilis as maniraptorans.

All TWiG matrices (e.g., Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Hwang et al., 2004b; Xu and Norell, 2004; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007c) have found Ornitholestes as a maniraptoran except that by Turner et al. (2007b), where its position was unresolved. All TWiG matrices that included compsognathids (Hwang et al., 2004b; Xu and Norell, 2004; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007c) recover compsognathids as maniraptorans again except that by Turner et al. (2007b) in which they were basal to the Ornithomimosauria-Maniraptora split. The only TWiG matrices to include Coelurus fragilis previous to this study were those by Turner et al. (2007b) and Makovicky et al. (2009), both of which found it outside Maniraptora and Maniraptoriformes. Senter (2007), which is based largely on the TWiG backbone with many characters from Holtz (1998), finds Ornitholestes as a maniraptoran, but Coelurus and compsognathids as non-maniraptorans. Zanno et al. (2009) and Zanno (2010a) use the TWiG matrix as a backbone and both analyses found Coelurus and Ornitholestes as maniraptorans and compsognathids outside Maniraptora. Conversely, the recent analysis of Choiniere et al. (2010), also using TWiG as a backbone, recovered Ornitholestes and compsognathids as maniraptorans, but Coelurus very basal among coelurosaurs.

The present analysis finds Ornitholestes as the basalmost maniraptoran (fig. 62), but Coelurus located more basally among coelurosaurs (although not as basal as the analysis of Choiniere et al., 2010). Four synapomorphies unite Ornitholestes with all other maniraptorans. These include a quadratojugal with a horizontal process posterior to the ascending process forming an inverted T or Y shape (char. 35.1)—reversed in Archaeopteryx, Mei, Sinornithoides, Utahraptor, Citipati, Rinchenia, Conchoraptor, and Incisivosaurus—and the lateral border of the quadrate shaft possesses a broad, triangular process along the lateral edge of the shaft that contacts the squamosal and the quadratojugal above an enlarged quadrate foramen (char. 53.1)—reversed in most troodontids, Incisivosaurus, and avialans. Additionally, a short and deep paroccipital process with a convex distal end (char. 56.1) and anterior trunk vertebrae with large hypapophyses (char. 102.1) also characterize maniraptorans.

Fig. 62

Reduced strict consensus cladogram illustrating the relationships among nonparavian maniraptorans. Paravian taxa have been collapsed into a single terminal.

i0003-0090-371-1-1-f62.tif

Fig. 63

Strict consensus cladogram illustrating the troodontid relationships. Dromaeosaurid taxa have been collapsed into a single terminal.

i0003-0090-371-1-1-f63.tif

Unnamed Clade (Alvarezsauroidea + Therizinosauria + Oviraptorosauria + Paraves)

Perle et al. (1994), Chiappe et al. (1996, 1998), Novas (1997), and Holtz (1998) found alvarezsaurids close to Avialae (Metornithes). Subsequent analyses including Sereno (1999) and all versions of the TWiG matrix (Norell et a., 2001; Novas and Pol, 2002; Hwang et al., 2002; Makovicky et al., 2003; Hwang et al., 2004b; Makovicky et al., 2005; Norell et al., 2006; Turner et al., 2007a, 2007b) place alvarezsaurids in a more basal position, with Sereno (1999) finding an ornithomimosaur + alvarezsaurid clade, although this topology has not been supported in any subsequent phylogenetic analysis. Senter (2007) also recovers this group but with slightly different interrelationships among the constituent clades, as do Zanno et al. (2009), Zanno (2010a), and Choiniere et al. (2010).

In our study, this derived maniraptoran clade is supported by 12 unambiguous synapomorphies and considerable nodal support (see below). In this clade the frontal process of the postorbital in lateral view curves anterodorsally with the dorsal border of temporal bar concave (char. 4.1). The maxillary process of the premaxilla is reduced, so that the maxilla participates broadly in external naris (char. 20.1), although this is reversed in oviraptorosaurs and ornithurines to the plesiomorphic condition, whereas in derived Laurasian dromaeosaurids it is even further transformed. Fused parietals (char. 46.1), maxillary and dentary teeth without serrations anteriorly (char. 83.1), and premaxillary tooth crowns suboval to subcircular in cross section (char. 91.0) also optimizes at this node.

Two characters supporting the group have been found in previous versions of TWiG. The teeth are constricted between root and crown (char. 88.0), and prezygapophyses are reduced on the distal caudal vertebrae (char. 120.2). This last character is a reversal of the plesiomorphic elongate state in basal avialans more derived than Archaeopteryx lithographica and are further transformed into the hyperelongate prezygapophyses characteristic of a subset of dromaeosaurid taxa more derived than Mahakala omnogovae.

A number of pelvic transformations also support this clade. The supraacetabular crest on the ilium is reduced at this point on the coelurosaur tree and does not form a hood over the acetabulum (char. 157.1). The brevis fossa becomes shelflike (char. 161.0) and well developed along the full length of postacetabular blade with the lateral overhang extending along full length of the fossa, completely covering the medial edge when viewed laterally (char. 217.1). The distal ends of the ischia approach one another, but do not form a symphysis (char. 174.1).

Lastly, an unkinked pterygoid with the braincase articulation inline with the main axis of the bone (char. 339.1) optimizes at this node. The character is unknown for many coelurosaurs and is highly homoplastic among nonavialan coelurosaurs where it can be observed. A reversal to a kinked morphology is present in nearly all paravians excluding crown-group Aves.

Unnamed Clade (Therizinosauria + Oviraptorosauria + Paraves)

This clade was not present in Holtz (1998), Sereno (1999), or in the more recent analyses by Senter (2007), Zanno et al. (2009), and Hu et al. (2009), with the latter three finding therizinosaurs to occupy a more basal position. In the case of Holtz (1998) this group was not recovered because alvarezsaurids were found nested up the tree with avialans and in Sereno (1997) and Sereno (1999) therizinosaurs were found closely related to Ornithomimosaurs. A Therizinosauria + Oviraptorosauria + Paraves clade was recovered by Makovicky and Sues (1998), Norell et al. (2001), Makovicky et al. (2005), Norell et al. (2006), Turner et al. (2007a, 2007b), but remained unresolved in Hwang et al. (2004b) because of a lack of consensus in the position of alvarezsaurids. Kirkland et al. (2005) also recovered a large polytomy at the base of Maniraptora, whereas Zanno et al. (2009) recovered oviraptorosaurs as the sister taxon to Paraves and alvarezsaurids occupying the next position just outside this group. The phylogenetic analysis of Hu et al. (2009), which is largely based on the dataset of Senter (2007), also finds this topology. The strict consensus of Choiniere et al. (2010) depicts a polytomy between therizinosaurs, oviraptorosaurs, alvarezsauroids, and paravians. Two alternate most parsimonious topologies are present—one that mimics the relationships recovered in the present study with alvarezsauroids basal to therizinosaurs + oviraptorosaurs + paravians and a second topology with therizinosaurs + oviraptorosaurs sister to an alvarezsauroid + paravian clade.

Two cranial synapomorphies support this grouping, one is the absence of a basisphenoid recess (char. 9.2), although this is reversed in oviraptorosaurs, basal therizinosaurs, and derived Laurasian dromaeosaurids, and the other is a downturned symphyseal end of the dentary (char. 66.1). The downturned dentary optimizes at this node because of the condition present in oviraptorosaurs, therizinosaurs, and Epidexipteryx. Maniraptorans more derived than the Epidexipteryx node show a reversal to the plesiomorphic straight dentary morphology.

Therizinosaurs, oviraptorosaurs, and paravians all possess a coracoid that in lateral view is subquadrangular with an extensive ventral blade (char. 136.1). Likewise members of this clade possess a proximodorsal “lip” on some manual unguals (char. 153.1). The supraacetabular crest on the ilium is further reduced from the ancestral maniraptoran condition and is altogether absent (char. 157.2). The ischial shaft in these taxa is wide, flat, and platelike (char. 166.1).

Unnamed Clade (Oviraptorosauria + Paraves)

Seven synapomorphies unite Oviraptorosauria with Paraves. Four of these synapomorphies pertain to changes in cranial morphology. The quadratojugal lacks a horizontal process posterior to the ascending process forming an L-shaped quadratojugal (char. 35.0). Unfortunately, the feature is not known for many paravians and shows a reversal to a T-shaped morphology in most dromaeosaurids. The prefrontal is absent in members of this clade (char. 40.2) (except some specimens of Deinonychus and in Sinornithosaurus) and the dorsal surface of the parietals is dorsally convex with a very low sagittal crest along the midline (char. 45.1). The coronoid ossification is expressed as a thin splint of bone in members of this clade (and completely lost in Archaeopteryx lithographica) (char. 76.1) and the squamosal overhangs the quadrate cotyle and covers the head of the quadrate in lateral view (char. 216.0).

In the axial skeleton, members of this clade exhibit short, wide, and slightly inclined transverse processes of the anterior dorsal vertebrae (char. 107.1). This feature is shared convergently with the basal alvarezsauroid Haplocheirus sollers and is reversed in derived Laurasian dromaeosaurids and Microraptor zhaoianus to the plesiomorphic long and thin condition.

The presence of a semilunate carpal (a single distal carpal present capping all or portions of metacarpals I and II) diagnoses this clade (char. 146.1). The presence of a semilunate carpal in theropods more basal than this node is problematic. The semilunate carpal identified by Hwang et al. (2004b) in Huaxiagnathus orientalis is reinterpreted here to be a large distal carpal I and the ulnare of those same authors is reinterpreted to be a much smaller distal carpal II. This is a similar configuration to that present in Allosaurus fragilis (Chure, 2001). Coelurus fragilis has a large “semilunate” carpal that is very similar to that seen in members of the Oviraptorosauria + Paraves clade. However, the distal articulation surface is angled (like that in carpal I of Allosaurus) and not flat like those one sees in the semilunate carpal of derived paravians like Velociraptor. It is preserved in isolation from the other manual elements, so it is ambiguous whether this represents a true semilunate carpal homologous with that present in this clade or it is like more basal tetanurans that have an enlarged distal carpal I. In the present analysis, this feature was scored as unknown (?) in Coelurus fragilis, but this may prove to be homologous and therefore a possible feature drawing Coelurus up-tree into a topology similar to that recovered by Makovicky and Sues (1998), Zanno et al. (2009), and Zanno (2010a). The carpal of Coelurus is very similar to the distal carpal I in Falcarius (Zanno, 2010b), which in some individuals fuses to distal carpal II to form a true semilunate carpal. This suggests that the correct interpretation of the Coelurus carpal in question is that it is a distal carpal I. Whether this carpal fused to distal carpal II is unknown. The inclusion of Falcarius in the present dataset could result in this character optimizing at a different position on the tree or it may be that some version of a partially fused “semilunate” carpal complex is characteristic of a more inclusive coelurosaur clade.

Paraves Sereno, 1997

Definition

A stem-based monophyletic group containing Passer domesticus (Linnaeus, 1758) and all coelurosaurs closer to it than to Oviraptor philoceratops 262263Osborn, 1924 (sensu Holtz and Osmólska, 2004).

Beginning with Ostrom's (1969a) work documenting the morphological similarities between the dromaeosaurid Deinonychus antirrhopus and Archaeopteryx lithographica specifically and birds generally, taxa that today constitute Paraves have been grouped in close phylogenetic association. Gauthier (1986) recovered a Dromaeosauridae + Troodontidae + Avialae clade as did many subsequent analyses (e.g., Sereno, 1997, 1999; Makovicky and Sues, 1998; Xu et al., 1999, 2000; Hwang et al., 2002; Makovicky et al., 2003; Hwang et al., 2004b; Xu and Norell, 2004; Makovicky et al., 2005; Novas and Pol, 2005; Norell et al., 2006; Turner et al., 2007a, 2007b, 2007c). Studies that have failed to recover this clade (e.g., Holtz, 1998, 2001; Senter et al., 2004) do so because the troodontids sampled in their datasets group with ornithomimids. This clade was not found by Senter in his more recent analysis (Senter, 2007) and has been convincingly demonstrated to be the result of attraction between convergent branches because of insufficient taxon sampling at the base of Troodontidae and Ornithomimosauria (Makovicky et al., 2003).

In the present analysis, Epidexipteryx hui is recovered as the basalmost paravian outside the split between the traditional paravian clades Avialae and Deinonychosauria. This result is interesting because it departs from previous phylogenetic work suggesting Epidexipteryx is a basal avialan. The basal position of Epidexipteryx results in low nodal support for paravian monophyly (see below) and a novel set of synapomorphies supporting the clade. These features include large dentary and maxillary teeth (char. 84.1), the posterolateral surface of the coracoid ventral to the glenoid fossa is expanded to form a triangular subglenoid fossa bounded laterally by an enlarged coracoid tuber (char. 134.1), a humerus that is longer than the scapula (char. 139.1), a calcaneum and astragalus that are fused to each other but not to the tibia (char. 198.1), and the position of the frontoparietal suture is at the level of the postorbital process of the frontal (char. 464.1).

Paraves, exclusive of Epidexipteryx hui, is much better supported with high GC values, three unambiguous synapomorphies and six ambiguous synapomorphies. These six ambiguous synapomorphies (chars. 137.1, 144.1, 156.1, 160.1, 414.0/1, and 474.1) are unknown in Epidexipteryx and may ultimately optimize along the Paraves stem. They are discussed here because they have important evolutionary implications and entail the more traditional suite of paravian characteristics.

Paraves, exclusive of Epidexipteryx hui, is marked by a suite of modifications to the shoulder girdle typically associated with the origin of the “avian” flight stroke (Ostrom, 1976b; Jenkins, 1993). The acromion margin of the scapula has a laterally everted anterior edge (char. 133.1) (fig. 55), the coracoid is inflected medially from the scapula forming an L-shaped scapulocoracoid in lateral view (char. 137.1) and the glenoid fossa faces laterally (char. 138.1) as opposed to the plesiomorphic posterior orientation (fig. 50). Additionally, the furcula is nearly symmetrical in shape as opposed to the asymmetry present in the furcula of more basal taxa (char. 474.1). The scapular acromion is the origin site of the m. deltoideus clavicularis ( =  m. propatagialis in Aves) (Meers, 2003; Baumel and Witmer, 1993; Jasinoski et al., 2006). Ancestrally this muscle served to protract and abduct the humerus. This eversion of the acromion in paravians may relate to a partial reorganization of the muscle orientation or served to increase the surface area of attachment. The L-shaped scapulocoracoid represents an incipient stage of the enhanced bracing of the shoulder girdle where the coracoids have an extensive anterior contact with the ossified sternum, while the scapulae take a more dorsally displaced position on the back, paralleling the vertebral column. This modification also results in a partial reorientation of the glenoid such that the short axis is inclined vertically (although not completely vertical) and that ancestral protraction-retraction movement of the humerus across this joint results in partial elevation-depression (Jenkins, 1993). Along with this modification, the laterally oriented glenoid allows the humerus to be abducted to a greater degree than in more basal coelurosaurs, although still probably not greatly above horizontal. Early analyses of these morphological modifications and/or coelurosaur phylogeny placed these changes at the avialan node (Sereno, 1997, 1999; Jenkins, 1993), but it is now clear that they are paravian synapomorphies.

Troodontids, dromaeosaurids, and avialans all share a dentary symphyseal region that is in line with the main part of the buccal margin (char. 66.0), i.e., the symphysis is not downturned as in oviraptorosaurs, therizinosaurs, and Epidexipteryx. In paravians, but unknown in Epidexipteryx, the proximal surface of ulna is divided into two distinct fossae separated by a median ridge (char. 144.1) (fig. 46). This feature may in fact have a broader distribution, but the proximal surface of the ulna is poorly known in oviraptorosaurs and is heavily modified in alvarezsaurids. Ancestrally in paravians, the anterior end of the ilium is strongly convex or lobate (char. 156.1) (fig. 55). This is in contrast to the slightly rounded to straight anterior edge present in non-tyrannosaurid coelurosaurs, although some derived euornithines revert to the plesiomorphic rounded anterior edge whereas Laurasian dromaeosaurids (i.e., Velociraptorinae + Dromaeosaurinae) have a pointed anterodorsal corner with a concave anteroventral edge.

Along the dorsal edge of the ilium, paravians possess a distinct tuber (or processus supratrochantericus) (char. 160.1) (figs. 18, 55). This process is often associated with an oblique ridge dividing the lateral surface of the ilium (Hutchinson, 2001a; Vanden Berge and Zweers, 1993). This ridge marks the division between the origin of m. iliofemoralis externus and the m. iliofibularis. The oblique ridge serves a role similar to that of the “vertical ridge” present in tyrannosauroid ilium—namely the division of the preacetabular concavity from the postacetabular concavity. We follow the conclusion of Hutchinson (2001a) and do not consider the “vertical ridge” and the oblique ridge below the supratrochanteric process homologous. Numerous tests of congruence reject this hypothesis as well as incomplete satisfaction of the connectivity criterion (e.g., no supratrochanteric process in Tyrannosauroidea).

The femora of paravians possess a distinct posterior trochanter (char. 414.0/1) (fig. 18). Ostrom (1976a) described this feature in Deinonychus antirrhopus noting that it coincided with the insertion of the m. ischiofemoralis. This trochanter is a rounded prominent tubercle among basal paravians, while in euornithines it is further derived into a hypertrophied shelflike structure (Hutchinson, 2001b).

Deinonychosauria Colbert and Russell, 1969

Definition

A node-based monophyletic group containing the last common ancestor of Troodon formosus Leidy, 1856, and Velociraptor mongoliensis 262263Osborn, 1924, and all of its descendants (sensu Sereno, 1998).

Since Deinonychosauria was erected by Colbert and Russell (1969) the group has been primarily characterized by a distinctive foot morphology comprised of a highly modified, raptorial digit II (fig. 2), exhibited to various degrees in both dromaeosaurids and troodontids. Both Colbert and Russell (1969) and Ostrom (1969a) discussed the great similarity between the dromaeosaurids and troodontids known at the time, but neither explicitly included troodontids in Deinonychosauria. Gauthier (1986) explicitly discussed Deinonychosauria in terms of including both Dromaeosauridae and Troodontidae.

Numerous authors have raised doubts regarding Deinonychosaurian monophyly. These include Barsbold (1983a), Currie (1987), Osmólska (1981), Osmólska and Barsbold (1990), Currie and Zhao (1993), Currie (1995), and even Gauthier (1986) in the addendum at the end of the paper (Gauthier, 1986: 48). With subsequent discoveries of new dromaeosaurids and troodontids, coupled with newer and more comprehensive phylogenetic analyses, Deinonychosauria is now a well-supported clade.

Beyond the characteristic pedal morphology, eight synapomorphies are present throughout the skeleton. In deinonychosaurs the anterodorsal process of the lacrimal is much longer than the posterior process (char. 39.2). This feature in reversed in microraptorine, dromaeosaurine, and velociraptorine dromaeosaurids. The occurrence of this feature in Austroraptor is what results in this feature optimizing as a deinonychosaur synapomorphy as opposed to a troodontid synapomorphy. The pterygoid flange is well developed (char. 61.0) unlike the poorly developed pterygoid flange seen in all other maniraptorans. Nutrient foramina on the external surface of the dentary lie within a deep groove ancestrally for deinonychosaurs (char. 71.1). This feature was previously interpreted as convergently shared between unenlagiine dromaeosaurids and troodontids but currently optimizes at Deinonychosauria. This feature is reversed in dromaeosaurids more derived than Unenlagiinae. A large surangular foramen is present in deinonychosaurs (char. 74.1) (figs. 10, 38). This feature is present primitively for coelurosaurs, but is absent in Proceratosaurus bradleyi, Dilong paradoxus, and Ornitholestes hermanni and small in ornithomimosaurs. A surangular foramen is present in Compsognathus longipes, but absent in alvarezsaurids (Shuvuuia deserti IGM 100/977), Oviraptorosauria + Therizinosauria, and most avialans—Sapeornis chaoyangensis IVPP V13396 and Confuciusornis sanctus IVPP V11374 convergently possess a surangular foramen.

The splenial in deinonychosaurs is uniquely exposed as a broad triangle between dentary and angular visible on the lateral surface of the mandible (char. 75.1) (fig. 38). Currently, only in deinonychosaurs (and convergently in Conchoraptor gracilis) do the scars for the interspinous ligaments terminate below the apex of the neural spine (char. 109.1) as opposed to the plesiomorphic condition where the scars are present up to the apices of the neural spine. The distribution of this plesiomorphic condition, however, is poorly known among nonparavian coelurosaurians, particularly in avialans, compsognathids, and alvarezsaurids. The plesiomorphic state is known to be present in Ornitholestes hermanni AMNH FARB 619 and Coelurus fragilis YPM 2010.

The characteristic ungual and penultimate phalanx of pedal digit II that is highly modified for extreme hyperextension (char. 204.1) (fig. 40), not unexpectedly, continues to optimize as a Deinonychosauria synapomorphy. This complex consists of a shortened phalanx II-2 with a prominent proximal ventral flexor heel and a distal end with a large and deeply grooved ginglymoid articular facet. This surface extends ventrally far past the proximal extent of the dorsal limit. The ungual for this digit is more strongly curved and significantly larger than that of digit III (see Discussion below for an analysis of this supposed condition in Archaeopteryx). Lastly, in deinonychosaurs the bicipital scar is developed as a slightly raised scar (char. 384.1) (fig. 46). A bicipital scar is convergently shared with pygostylian avialans and is particularly well developed in Ornithurae.

Troodontidae Gilmore, 1924

Definition

A stem-based monophyletic group containing Troodon formosus Leidy, 1856, and all coelurosaurs closer to it than to Velociraptor mongoliensis 262263Osborn, 1924, or Passer domesticus (Linnaeus, 1758) (sensu Sereno, 1998).

Troodontidae (fig. 63) is moderately well supported in this analysis. Because of the increased taxon sampling and addition of five new basal troodontids, most of the characters identified by Makovicky et al. (2003) as synapomorphic for Troodontidae are here found to characterize less inclusive troodontid clades. The phylogeny of Troodontidae is currently under investigation (R. Pei, personal commun.), so conclusions reached here are preliminary.

Fig. 64

Reduced strict consensus cladogram illustrating the dromaeosaurid relationships when the alternate positions of Pyroraptor are ignored. Troodontid taxa have been collapsed into a single terminal.

i0003-0090-371-1-1-f64.tif

Currently, three unambiguous synapomorphies support Troodontidae with Anchiornis huxleyi + Xiaotingia zhengi as its basalmost clade. The internarial bar is flat in cross section (char. 21.1). The quadrate is strongly inclined anteroventrally, so that the distal end lies far forward of the proximal end (char. 51.1), a feature shared convergently with Sinosauropteryx, Struthiomimus, Gallimimus, Ornithomimus, Erlikosaurus, and Incisivosaurus.

The discovery of Sinovenator changii (Xu et al., 2002a) appeared to provide a transitional morphology of a partially arctometatarsalian condition (like that in Sinovenator) to the more extreme constriction seen in all other troodontids (except Sinornithoides). However, with the inclusion of Anchiornis, IGM 100/1323, and IGM 100/1126 and recognition of these specimens as basal troodontid taxa, the subarctometatarsalian condition in Sinovenator seems to be a partial reversal from an ancestral metatarsal III proximal shaft that is very pinched and not exposed along the proximal section of the metapodium (arctometatarsal: char. 203.2).

Unnamed Clade (Anchiornis huxleyi + Xiaotingia zhengi)

Five characters unambiguously support this novel clade of Late Jurassic Chinese troodontids. Two features in the maxilla unite this clade. These include the presence of a tertiary antorbital fenestra (promaxillary fenestra) (char. 29.1) and the lateral lamina of the ventral ramus of the nasal process of the maxilla is reduced to a small triangular exposure (char. 244.1). The former feature is shared convergently with a number of other paravians including Xixiasaurus, Sinovenator, non-unenlagiine dromaeosaurids, and Archaeopteryx. The latter feature is also present among other paravians, including Jinfengopteryx, IGM 100/1126, Sinovenator, Microraptor, Sinornithosaurus, Bambiraptor, and Shanag.

The ridge bounding the cuppedicus fossa extends far posteriorly and is almost confluent with the acetabular rim (char. 163.1) in Anchiornis and Xiaotingia and is shared convergently with the microraptorine Tianyuraptor and the unenlagiines Rahonavis and Unenlagia. Additionally, metatarsal I articulates to the posterior surface of the distal quarter of metatarsal II (char. 205.1) and the neural spines on the posterior dorsal vertebrae are anteroposteriorly expanded distally in lateral view (char. 209.1).

Unnamed Clade (Jinfengopteryginae + Sinovenator + Mei + Xixiasaurus + IGM 100/44 + Byronosaurus + Sinornithoides + Troodon + Saurornithoides + Zanabazar)

Nine characters unambiguously support the clade along with a moderate jackknife value. Troodontids more derived than Anchiornis have a well-developed supraorbital crest on the lacrimal that takes the form of a large lateral expansion anterior and dorsal to the orbit (char. 37.2). This feature is unique to the clade among nonavian theropods although it is convergently present in some paleognaths and galloanseriforms.

The foramen magnum is oval shaped in troodontids being taller than wide (char. 54.1). This is reversed, however, in Troodon formosus. Numerous dentary teeth diagnose this clade (char. 84.1). Dentary tooth size is a very homoplastic character. A number of other maniraptorans have numerous small teeth (e.g., therizinosaurs, Haplocheirus, Shuvuuia, Pelecanimimus, and unenlagiine dromaeosaurids).

The neural spines on the distal caudals are absent and midline sulci are instead present centered on the neural arches (char. 119.2)—a feature completely unique to troodontids. Troodontids more derived than Anchiornis have a scapula that is longer than the humerus (139.0), but this feature is unknown in many derived troodontids. The asymmetrical foot discussed by Makovicky et al. (2003) with slender metatarsal II and very robust metatarsal IV (chars. 208.1 and 434.2) now diagnoses this clade of troodontids although it is paralleled in Microraptor. Additional synapomorphies of the pes include a metatarsal II that is shorter than metatarsal IV (char. 438.1/2) and a pedal phalanx II-2 with a distal articular surface less than half the size of the proximal surface (char. 456.1—reversed in Sinovenator changii).

Jinfengopteryginae, new clade name

Definition

A stem-based monophyletic group containing Jinfengopteryx elegans Ji et al., 2005, and all coelurosaurs closer to it than to Troodon formosus Leidy, 1856, Passer domesticus (Linnaeus, 1758), and Sinovenator changii 367Xu et al., 2002.

We name this new clade of troodontids for two reasons. First, the clade represents a distinct group that is the sister taxon to nearly all other troodontids, therefore having a specific name for this group serves a practical purpose in discussions of troodontid relationships and evolution. Second, the clade possesses strong character support (six unambiguous synapomorphies) and high jackknife support, rendering it relatively stable. A stem-based clade name was chosen in order to provide a taxonomic framework for additional troodontid taxa that may be found or are undescribed and prove to be close relatives to this basal troodontid clade.

Jinfengopterygines are diagnosed by an enlarged and pronounced round accessory antorbital fenestra (maxillary fenestra) that takes up most of the space between the anterior margins of the antorbital fenestra and fossa (char. 27.2). The presence of a maxillary fenestra is common to most coelurosaurs, although pygostylians represent a derived reversal to an absence of this structure. This enlarged and somewhat triangular maxillary fenestra is unique to jinfengopterygines. In addition to its shape, the maxillary fenestra is also situated at the rostral border of the antorbital fossa in jinfengopterygines (char. 28.0). This is the ancestral condition for Maniraptora; however, paravians show a shift in the location of the maxillary fenestra away from the anterior border of the antorbital fossa.

The lacrimal in jinfengopterygines shows an apomorphic reversal to a T-shaped outline in lateral view because an anterodorsal process that is equal in length to the posterior process (char. 39.1). This feature is shared convergently with Archaeopteryx, Pengornis, Mei, Incisivosaurus, and Laurasian dromaeosaurids.

Ancestrally for paravians the ischium is considerably shorter than the pubis. This is the case for non-euornithine avialans, all non-jinfengopterygine troodontids, and dromaeosaurids (except Achillobator). In Jinfengopteryginae the ischium secondarily reverts to being more than 2/3 the length of the pubis (char. 173.0). The pubic apron is less than 1/3 shaft length (char. 181.1) in this clade, a feature shared convergently with avialans and derived oviraptorosaurs and Patagonykus puertai.

Completely unique to this subclade of troodontids is a jugal with the sublacrimal part bifurcated anteriorly (char. 262.3). In the pes of jinfengopterygines, metatarsal III is displaced plantarly at its proximal end relative to the position of metatarsals II and IV (char. 428.1). This feature is shared convergently with all euornithines more derived than Patagopteryx and Hongshanornis.

Unnamed Clade (IGM 100/1126 + Jinfengopteryx)

The sister-group relation of these two taxa to the exclusion of IGM 100/1323 is supported by the reduction of the lateral lamina of the ventral ramus of the nasal process of the maxilla to a small triangular exposure (char. 244.1). This is paralleled in Sinovenator changii and Sinornithosaurus millenii, Microraptor zhaoianus, Shanag ashile, and the basal avialan Jeholornis prima.

Unnamed Clade (Sinovenator + Mei + Xixiasaurus + IGM 100/44 + Byronosaurus + Sinornithoides + Troodon + Saurornithoides + Zanabazar)

The membership of this clade (with the exclusion of Xixiasaurus) is what has previously constituted Troodontidae. As such, many of the characters that optimize as synapomorphic for this clade are features previously identified as troodontid synapomorphies (Makovicky et al., 2003). In this clade a depression (possibly pneumatic) is present on the ventral surface of the postorbital process of the laterosphenoid (char. 224.1). In members of this clade, the quadrate is pneumatic (char. 299.1) and bears a single pneumatic foramen on the posteromedial surface of the corpus of the quadrate (char. 301.1). Lastly, on the pes a large longitudinal flange along the caudal or lateral face of the metatarsal IV is present (char. 229.1).

Unnamed Clade (Mei + Sinovenator + Xixiasaurus)

This small clade of Early Cretaceous Chinese troodontids, all from the lower part of the Yixian Formation, is supported by four unambiguous synapomorphies. Because of the fragmentary nature of Xixiasaurus none of four synapomorphies are known in this taxon. A single character (a maxillary process of the premaxilla that extends posteriorly to separate the maxilla from the nasal posterior to the nares: char. 20.0) unites Xixiasaurus with Sinovenator and thus places it within this clade in all the most parsimonious trees.

Sinusonasus magnodens (also from the lower Yixian Formation) is very similar to Sinovenator changii and likely belongs to this clade or may even be synonymous with Sinovenator changii. The close group relationship between Sinovenator and Mei is supported by cervical neural spines that are anteroposteriorly long (char. 99.0). Also in these taxa, the posterior edge of ischium has a proximal median posterior process (char. 165.1).

Whereas a subarctometatarsalian pes was used to diagnosis the base of Troodontidae (Xu et al., 2002a), the subarctometatarsalian condition is now synapomorphic for this restricted troodontid clade. The condition is formed by the proximal shaft of metatarsal III being constricted and much narrower than either II or IV, but still exposed along most of the metapodium (char. 203.1). Also regarding the metapodium, Sinovenator and Mei share the presence of a mediolaterally widened metatarsal IV shaft that is flat in cross section (char. 207.1).

Unnamed Clade (IGM 100/44 + Byronosaurus + Sinornithoides + Troodon + Saurornithoides + Zanabazar)

A single synapomorphy supports this clade, which is the presence of a subotic recess (char. 8.1). A subotic recess is primitively absent in theropods. Struthiomimus altus, Gallimimus bullatus, Ornithomimus edmonticus, Troodon formosus, Saurornithoides mongoliensis, Zanabazar junior, Byronosaurus jaffei, and the Early Cretaceous troodontid IGM 100/44 possess a subotic recess. This was used by some authors (Holtz, 1998; Senter et al., 2004) to suggest a sister-group relationship between troodontids and ornithomimosaurs. It is clear that the structures in the two groups are convergent given that basal troodontids such as Sinovenator changii, Mei long, and two jinfengopterygine troodontids (IGM 100/1126 and IGM 100/1323) lack a subotic recess.

Unnamed Clade (Byronosaurus + Sinornithoides + Troodon + Saurornithoides + Zanabazar)

A single synapomorphy supports this clade. The ala parasphenoidalis is present, well developed, and crest shaped, forming the anterior edge of enlarged pneumatic recess with the ala continuous with the anterior tympanic crista (char. 468.1). This character is largely a rewording of character 6 from older TWiG matrices. The rewording reflects nomenclatural preferences as well as a slight revision to the homology concept for the character. Nevertheless, the feature it references is the enlarged pneumatic chamber associated with the inflation of the anterior tympanic recess and the full development of an ala parasphenoidalis (otosphenoidal crest of previous authors).

Unnamed Clade (Sinornithoides + Troodon + Saurornithoides + Zanabazar)

This clade is supported by maxillary and dentary teeth that lack serrations anteriorly (char. 83.1).

Unnamed Clade (Troodon + Saurornithoides + Zanabazar)

A slightly medially recurved symphyseal region of the dentary (char. 65.1) is an unambiguous synapomorphy for this clade. An enlarged anterior tympanic recess confluent with the subotic recess (forming the Lateral Depression sensu Currie, 1985) is an ambiguous synapomorphy for the group. This is ambiguously optimized at this node because the condition in Sinornithoides youngi is unknown.

Unnamed Clade (Saurornithoides +Zanabazar)

An accessory antorbital fenestra (maxillary fenestra) situated at rostral border of antorbital fossa (char. 28.0) diagnoses this clade.

Dromaeosauridae Matthew and Brown, 1922

Definition

A stem-based monophyletic group containing Dromaeosaurus albertensis Matthew and Brown, 1922, and all deinonychosaurs closer to it than to Troodon formosus Leidy, 1856, or Passer domesticus (Linnaeus, 1758) (sensu Sereno, 1998).

The exact set of Dromaeosauridae synapomorphies is ambiguous, because of the labile position of Pyroraptor olympius (fig. 59) due to the extreme paucity of data for this taxon. Considering all topologies, the single consistent dromaeosaurid synapomorphy is the presence on the distal end of metatarsal II of a distinct and developed ginglymus (char. 201.1)—shared convergently with some avialans like Vorona berivotrensis UA 8651 and Ichthyornis dispar (Marsh, 1872, 1880; Clarke, 2004). The presence of only one synapomorphy for Dromaeosauridae occurs on the MPT where Pyroraptor olympius is depicted as the basalmost dromaeosaurid. When this topology is ignored (or when Pyroraptor is excluded from the analysis) seven additional characters are shown to be synapomorphic for the clade (fig. 64).

In alvarezsaurids (Shuvuuia deserti IGM 100/977 and Mononykus olecranus IGM 107/6) and Archaeopteryx lithographica BMNH 37001 there is an accessory tympanic recess located dorsal to the crista interfenestralis. This accessory recess is absent in dromaeosaurids, a condition that optimizes as a synapomorphy for the group (char. 17.0). All maniraptorans (except Erlikosaurus andrewsi [Clark et al., 1994], Citipati osmolskae IGM 100/978, and Chirostenotes pergracilis [Currie and Russell, 1988]) have short paroccipital process with a convex distal end. Dromaeosaurids share a derived morphology of the paroccipital process in which it is elongate and slender, with dorsal and ventral edges nearly parallel (char. 56.0) (fig. 39). Additionally, on the paroccipital process of dromaeosaurids the dorsal edge twists anterolaterally at its distal end (char. 58.1) (fig. 39).

The teeth in dromaeosaurids lack a root-crown constriction (char. 88.1), although this is reversed to a constricted morphology in Microraptor zhaoianus (Hwang et al., 2002). In nearly all non-dromaeosaurid maniraptoriforms the anterior cervical centrum extends beyond the posterior limit of the neural arch. In Dromaeosauridae, the anterior cervical centrum is level with or shorter than the posterior extent of the neural arch (char. 96.0), which ultimately is a reversal of the plesiomorphic tetanuran morphology. In their detailed analysis of dromaeosaurids and, in particular, Velociraptor mongoliensis morphology, Norell and Makovicky (1999) showed that parapophyses on the posterior trunk vertebrae were distinctly projected on pedicels (char. 103.1) (fig. 31). This feature remains a consistent dromaeosaurid synapomorphy; however, it does occur convergently in Mononykus olecranus IGM 107/6, Shuvuuia deserti IGM 100/977 and Confuciusornis sanctus IVPP V11370, and in a slightly different manifestation in some ceratosaurian theropods.

A newly identified synapomorphic character from this study deals with the nature of the relatively reduced pneumaticity of the dromaeosaurid braincase (i.e., relative to most derived maniraptorans). In extant avians (Witmer, 1990), Archaeopteryx lithographica BMNH 37001, Incisivosaurus gauthieri IVPP V13326, and basal troodontids (IGM 100/1126 and IGM 100/1323) the anterior tympanic recess has migrated caudally to be located below the exit of cranial nerves VII and V. In dromaeosaurids, the lateral braincase wall is generally considered less pneumatic (Currie, 1995; Norell et al., 2006). The “basipterygoid recess” identified by Norell et al. (2006) is in fact a very anteriorly located anterior tympanic recess. Therefore, the anterior tympanic recess located anteriorly with little or no development posterior to the basipterygoid processes (char. 452.1) optimizes as a dromaeosaurid synapomorphy.

Unnamed Clade (Unenlagiinae + Microraptorinae + Dromaeosaurinae + Velociraptorinae)

Only two synapomorphies unite the four major clades of Dromaeosauridae. The two characters are the presence of pleurocoels on anterior sacrals only (char. 113.1), and a frontoparietal suture positioned well posterior to the postorbital process (char. 464.0).

Unenlagiinae Bonaparte, 1999

Definition

A stem-based monophyletic group containing Unenlagia comahuensis Novas and Puerta, 1997, and all coelurosaurs closer to it than to Velociraptor mongoliensis 262263Osborn, 1924, Dromaeosaurus albertensis Matthew and Brown, 1922, Microraptor zhaoianus Xu et al., 2000, and Passer domesticus (Linnaeus, 1758) (sensu Sereno, 2005).

This clade was initially discovered and diagnosed by Makovicky et al. (2005). In this initial study, a reduced supraacetabular crest, a vertically oriented pubis, and a concave dorsocaudal edge of the ilium were synapomorphic for the group.

In the present study, the ridge bounding the cuppedicus fossa that extends far posteriorly and is confluent or almost confluent with the acetabular rim (char. 163.1) and a proximal median posterior process on the posterior edge of the ischium (char. 165.1) optimize as Unenlagiinae synapomorphies.

Also in the most parsimonious topologies the three characters found by Makovicky et al. (2005) as synapomorphic for Unenlagiinae also optimize at this node—supraacetabular crest on ilium reduced so as to not form a hood (char. 157.1), a vertically oriented pubis (char. 177.1), and a concave dorsocaudal edge of the ilium (char. 226.1). Both Austroraptor and Buitreraptor have well-defined, parallel ventrolateral ridges along the posterior cervical centra, which may turn out to be synapomorphic of the clade.

Unnamed Clade (Buitreraptor + Unenlagia + Austroraptor)

In both Makovicky et al. (2005) and Turner et al. (2007b) Unenlagia was found as the sister taxon to Rahonavis ostromi to the exclusion of Buitreraptor gonzalezorum. In the present analysis, this topology was not recovered. Five synapomorphies unite Buitreraptor gonzalezorum, Austroraptor cabazai, and Unenlagia. These include a metatarsal III with a constricted proximal shaft that is much narrower than either metatarsals II or IV, but still exposed along most of metapodium—the so-called subarctometatarsal condition (char. 203.1), a large longitudinal flange along the caudal or lateral face of metatarsal IV (char. 229.1), the presence of a postacetabular end of the ilium with a lobate brevis shelf projecting from the ilium beyond the end of postacetabular lamina (char. 227.1), thoracic vertebral centra with a length markedly greater than midpoint width (char. 317.0), and a convex scapular articular surface on the coracoid (char. 339.1).

The subarctometatarsalian condition is present in a number of other paravians including most microraptorines and the troodontids Mei long, Sinovenator changii, and Sinornithoides youngi. A lobate brevis shelf is present convergently in Microraptor zhaoianus (Hwang et al., 2002). This feature is not present in Rahonavis ostromi (UA 8656), which instead exhibits an ilium with a rounded posterior margin in dorsal view. A convex scapular articular surface on the coracoid is a feature seen in derived enantiornithines, but is also present in these two unenlagiine taxa. Rahonavis ostromi (UA 8656) lacks a coracoid, but the coracoid facet on the scapula is flat to weakly concave, suggesting the potential for an intermediate morphology in the sister taxon to Buitreraptor + Unenlagia.

Unnamed Clade (Unenlagia + Austroraptor)

Austroraptor cabazai is found here as the sister taxon of Unenlagia. This position is consistent with the results of Novas et al. (2009), as they found an unresolved clade containing Unenlagia, Austroraptor, and Rahonavis. Our analysis departs from the results of Novas et al (2009) in that we find Buitreraptor gonzalezorum as the sister taxon to Unenlagia + Austroraptor, with Rahonavis being the basalmost unenlagiine dromaeosaurid. Three characters support the sister-group status of Unenlagia and Austroraptor. These synapomorphies are neural spines on the dorsal vertebrae that expand to form a spine table (char. 108.1), a calcaneum and astragalus that are unfused to each other or to the tibia (char. 198.0), and the presence of pleurocoels in the dorsal vertebrae (char. 265.1/2).

Unnamed Clade (Shanag + Microraptorinae + Velociraptorinae + Dromaeosaurinae)

Two characters unite the small Asian dromaeosaurids Shanag ashile with dromaeosaurids more derived than unenlagiines. These are maxillary and dentary teeth that lack serrations anteriorly (char. 83.1) and the dorsal displacement of the accessory antorbital (maxillary) fenestra (char. 237.1).

Unnamed Clade (Microraptorinae + Velociraptorinae + Dromaeosaurinae)

Two characters unite microraptorine, velociraptorine, and dromaeosaurine taxa. Members of this clade possess a tertiary antorbital fenestra (promaxillary fenestra) (char. 29.1) and the nutrient foramina are superficial on the external surface of dentary (char. 71.1).

Microraptorinae Senter et al., 2004

Definition

A stem-based monophyletic group containing Microraptor zhaoianus Xu et al., 2000, and all coelurosaurs closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, Velociraptor mongoliensis 262263Osborn, 1924, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus (Linnaeus, 1758) (sensu Sereno, 2005).

Five synapomorphies unite Tianyuraptor, Microraptor, Graciliraptor, Hesperonychus, and Sinornithosaurus in Microraptorinae. These are a semilunate distal carpal that is small and covers about half the base of metacarpals I and II (char. 148.1), a proximal shaft of metatarsal III that is constricted and much narrower than either metatarsal II or IV, but still exposed along most of the metapodium (subarctometatarsal: char. 203.1), the lateral face of pubic shaft possesses a prominent lateral tubercle about halfway down the shaft (char. 231.1), the caudal chevrons have very elongated posterior extensions (char. 442.1), and the combined length of metacarpal I plus phalanx I-1 is equal to or less than the length of metacarpal II (char. 444.1).

Eudromaeosauria (i.e., Velociraptorinae + Dromaeosaurine) Longrich and Currie, 2009

Definition

The node-based monophyletic group containing the last common ancestor of Saurornitholestes langstoni Sues, 1978, Deinonychus antirrhopus 270Ostrom, 1969, Dromaeosaurus albertensis Matthew and Brown, 1922, and Velociraptor mongoliensis 262263Osborn, 1924, and all its descendants.

Like many of the other larger clades within Dromaeosauridae, the many alternate positions of Pyroraptor olympius result in ambiguous character optimizations. For the clade composed of Velociraptorinae and Dromaeosaurinae only one character optimizes at this node in all trees. The apomorphic character is a flexor heel on phalanx II-2 that is long and lobate, with an extension of the midline ridge onto the dorsal surface of the heel (char. 228.1). More basally positioned dromaeosaurids display a small and typically asymmetrically developed heel present primarily on the medial side of the vertical ridge on the proximal articulation surface.

When Pyroraptor olympius is not the sister taxon to this clade then eight additional characters are found to be synapomorphies. This includes a maxillary process of the premaxilla that extends posteriorly to separate the maxilla from the nasal posterior to nares (char. 20.2)—a feature otherwise only seen in ornithomimosaurs. Two characteristics of the frontal diagnose this clade of dromaeosaurids: the postorbital process of the frontal is sharply demarcated from the orbital margin (char. 43.1—a trait discussed by Currie, 1995); and the frontal edge is notched in the region of the lacrimal suture (char. 44.1). In this clade the internal mandibular fenestra is large and rounded (char. 73.1). Typically in coelurosaurs the internal mandibular fenestra is small and slitlike. Apart from this clade of dromaeosaurids, only some tyrannosaurids (e.g., Tyrannosaurus rex FMNH PR 2081) and Incisivosaurus gauthieri (IVPP V13326) have a large and rounded internal mandibular fenestra. On the lateral braincase wall of velociraptorines and dromaeosaurines, a shallow prootic recess is present (char. 450.1). This optimizes as a synapomorphy for the clade because it is observable in Dromaeosaurus albertensis (AMNH FARB 5356), Bambiraptor feinbergorum (AMNH FARB 30556), and Tsaagan mangas (IGM 100/1015). Among other members of Dromaeosaurinae and Velociraptorinae the presence of this morphology is unknown. Among more basal dromaeosaurids, only Microraptor zhaoianus (IVPP V uncataloged 3) is known to lack a prootic recess, so it is possible that this feature has a broader distribution within Dromaeosauridae, but the plesiomorphic lack of a prootic recess in basal dromaeosaurids is entirely consistent with the absence of this recess in all troodontids and avialans with known and observable braincase material (e.g., IGM 100/1126 and Archaeopteryx lithographica BMNH 37001, respectively).

Two vertebral synapomorphies characterize this clade: no carotid processes on the posterior cervical vertebrae (char. 97.0) and thoracic vertebral centra with lengths markedly greater than midpoint widths (char. 317.0). Additionally, the acromion process of the scapula does not project anteriorly past the articular surface for the coracoid (char. 354.1).

Dromaeosaurinae Matthew and Brown, 1922

Definition

A stem-based monophyletic group containing Dromaeosaurus albertensis Matthew and Brown, 1922, and all coelurosaurs closer to it than to Velociraptor mongoliensis 262263Osborn, 1924, Microraptor zhaoianus Xu et al., 2000, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus (Linnaeus, 1758) (sensu Sereno, 2005).

Dromaeosaurinae was diagnosed by Currie (1995) as identical to Dromaeosaurus albertensis because at the time he considered it the only clear member of Dromaeosaurinae (although he considered Adasaurus mongoliensis as a potential member). Currie's (2005) diagnosis included an anterior carina of maxillary or mandibular teeth that twist toward the lingual surface.

Because early phylogenetic analyses of coelurosaur relationships considered Dromaeosauridae at a supraspecific level (Sereno, 1997, 1999; Holtz, 1998), interrelationships among dromaeosaurids were not considered, and so content of and/or monophyly of the Dromaeosaurinae/Velociraptorinae dichotomy were not tested. Early analyses utilizing the versions of the TWiG matrix recovered little or no consensus on dromaeosaurid interrelationships (e.g., Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003) (fig. 3). Later analyses revealed that underlying structure existed within Dromaeosauridae by increasing character sampling (Makovicky et al., 2005) or looking at reduced strict and Adams consensuses (Novas and Pol, 2005; Norell et al., 2006; Turner et al., 2007a). These analyses showed that distinct Dromaeosaurinae and Velociraptorinae clades exist. In these analyses Dromaeosaurinae consisted of Dromaeosaurus albertensis, Utahraptor ostrommaysorum, Achillobator giganticus, and Adasaurus mongoliensis with Saurornitholestes langstoni variably resolving as a member of this group.

Our study identifies four unambiguous synapomorphies for Dromaeosaurinae. These include maxillary and dentary teeth with serrations on both anterior and posterior margins (char. 83.0)—a particularly homoplastic character when considered across all coelurosaur diversity, but one that provides strong support for lower level clades. The pubis in dromaeosaurines is vertically oriented (char. 177.1) and the pubic boot projects anteriorly and posteriorly (char. 178.0). In the skull, the jugal process of the maxilla, ventral to the external antorbital fenestra, is dorsoventrally wide (char. 238.1). The twisting carinae noted by Currie (1995) remain an autapomorphy for Dromaeosaurus albertensis and is not indicative of a more inclusive clade.

Dromaeosaurus albertensis, Utahraptor ostrommaysorum, Achillobator giganticus and the poorly known Atrociraptor marshalli are the only dromaeosaurine taxa in the present study. Firsthand reexamination of Adasaurus mongoliensis resulted in 102 changes to the character scoring of taxon (table 3), which resulted in the repositioning of it as a velociraptorine and, further, in the clarification of the position of Saurornitholestes langstoni.

Velociraptorinae 12Barsbold, 1983

Definition

A stem-based monophyletic group containing Velociraptor mongoliensis 262263Osborn, 1924, and all coelurosaurs closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, Microraptor zhaoianus Xu et al., 2000, Unenlagia comahuensis Novas and Puerta, 1997, and Passer domesticus (Linnaeus, 1758) (sensu Sereno, 2005).

Velociraptorinae was diagnosed by Currie (1995) as dromaeosaurids with maxillary and dentary teeth possessing denticles on the anterior carinae that are significantly smaller than the posterior denticles, and which have a second premaxillary tooth that is significantly larger than the third and fourth premaxillary teeth. Currie (1995) also considered nasals that are depressed in lateral view (an observation strongly supported by Paul, 1988) as diagnostic for velociraptorines although he did note that that element was unknown in Dromaeosaurus, so the character was equivocally diagnostic of dromaeosaurids he considered Velociraptorinae (Deinonychus antirrhopus, Saurornitholestes langstoni, Velociraptor mongoliensis, and Utahraptor ostrommaysorum).

Similar to Dromaeosaurinae, cladistic tests of Velociraptorinae monophyly did not happen until the first species-level phylogenies were made (e.g., Norell et al., 2001) and, as was the case with Dromaeosaurinae, these early analyses found little consensus on interrelationships among dromaeosaurids. The reduced strict consensus of Novas and Pol (2005) recovered a distinct velociraptorine clade, which included Velociraptor mongoliensis, Deinonychus antirrhopus, and Tsaagan mangas (unnamed at the time). The position of Saurornitholestes was labile and therefore unclear whether it belonged to Velociraptorinae. Similar resolution and membership was found by Makovicky et al. (2005), Norell et al. (2006), and Turner et al. (2007a, 2007b). Neither Senter et al. (2004) nor Senter (2007) recovered a Velociraptorinae clade. In his most recent analysis (Senter, 2007), Deinonychus antirrhopus, Saurornitholestes langstoni, Velociraptor mongoliensis, Adasaurus mongoliensis, and Tsaagan mangas (labeled IGM 100/1015) were found as successive sister taxa to a Dromaeosaurinae clade identical in composition to that recovered in our study.

The phylogenetic analysis from our study recovers a monophyletic and well-supported Velociraptorinae clade composed of Bambiraptor feinbergorum, Tsaagan mangas, Saurornitholestes langstoni, Deinonychus antirrhopus, Velociraptor mongoliensis, and Adasaurus mongoliensis. The character-scoring changes for Adasaurus mongoliensis resulted in repositioning of it as a derived velociraptorine as opposed to the dromaeosaurine position recovered in previous analyses (e.g., Turner et al., 2007a, 2007b).

This clade is supported by three unambiguous synapomorphies. The posterior opening of the basisphenoid recess is divided into two small, circular foramina by a thin bar of bone (char. 10.1), and the dorsal tympanic recess is present as a deep, posterolaterally directed concavity (char. 16.2). Pleurocoels are present in all dorsal vertebrae (char. 265.2). An additional feature, an accessory depression in the supratemporal fossa (char. 466.1), may serve as a future synapomorphy for this clade. Due to the unknown presence or absence of this feature in Shanag, it is currently optimized as an ambiguous synapomorphy for the group.

Unnamed Clade (Tsaagan + Saurornitholestes + Deinonychus + Velociraptor + Adasaurus + Balaur)

A single synapomorphy supports this group of velociraptorine dromaeosaurids. Exits of cranial nerves X–XII that are located together in a bowllike depression (char. 19.1) optimizes as synapomorphic for this clade, however, this character is preserved only in Tsaagan mangas and Velociraptor mongoliensis.

Unnamed Clade (Saurornitholestes + Deinonychus + Velociraptor + Adasaurus + Balaur)

This clade is supported by the presence of a dorsal recess on the ectopterygoid (char. 60.1)—unknown in Adasaurus mongoliensis and paralleled in Archaeopteryx lithographica. Additionally, the first premaxillary tooth in these taxa (unknown in Adasaurus mongoliensis) are much smaller than crowns of premaxillary teeth 2 and 3 (char. 251.1).

Unnamed Clade (Deinonychus + Velociraptor + Adasaurus + Balaur)

This clade is supported by a single feature—the shaft of metatarsal IV mediolaterally wide and flat in cross section (char. 207.1). This trait is paralleled in Utahraptor ostrommaysorum and the troodontid clade of Mei + Sinovenator.

The uncertain position of Balaur within this clade collapses resolution within the group. When Balaur is excluded from the analysis an additional synapomorphy unites Deinonychus, Velociraptor, and Adasaurus. Basal dromaeosaurids like Sinornithosaurus millenii, Microraptor zhaoianus, Rahonavis ostromi, Buitreraptor gonzalezorum, basal velociraptorines, and avialans possess scapulae that are shorter than the humerus. Because of missing data in Mahakala omnogovae, Pyroraptor olympius, and dromaeosaurines, and because of the long scapulae of troodontids, the ancestral morphology for Dromaeosauridae, Deinonychosauria, and Paraves is ambiguous. However, because of the short scapula relative to humeral length in basal dromaeosaurids, the reversal to a long scapula relative to the humerus (char. 139.0) is optimized as a synapomorphy for the Deinonychus + Velociraptor + Adasaurus clade.

Likewise in the absence of Balaur, Velociraptor and Adasaurus are united as sister taxa. A fused scapulocoracoid (char. 135.1), a calcaneum and astragalus fused to each other but not to the tibia (char. 198.1), and distal tarsals fused to metatarsals (char. 199.1) support the monophyly of these two taxa. Because all the synapomorphies of this clade relate to increased fusion of skeletal elements, the shared similarity between them could reflect ontogenetic variability. Clarifying whether these synapomorphies reflect common descent or senescene will require additional work assessing the ontogenetic stage of the specimens for these taxa as well as additional specimens of Adasaurus.

Avialae Gauthier, 1986

Definition

A stem-based monophyletic group containing Passer domesticus (Linnaeus, 1758) and all coelurosaurs closer to it than to Dromaeosaurus albertensis Matthew and Brown, 1922, or Troodon formosus Leidy, 1856 (sensu Maryanska et al., 2002).

Sixteen unambiguous synapomorphies support the monophyly of Avialae (fig. 65). Some of these synapomorphic characters relate to modifications of the skull associated with increased pneumaticity as well as more extensive coossification. A number of the characters are associated with flight-related morphology such as forelimb elongation, hindlimb reduction, tail reduction, and modifications to the pelvis likely associated with restructuring of the hip and femoral musculature. These characters are: asymmetric vaned feathers on forelimb (char. 1.1); caudal (posterior) tympanic recess extends into opisthotic posterodorsal to fenestra ovalis, confluent with this fenestra (char. 18.2); parietals separate (char. 46.0); lateral border of quadrate shaft straight (char. 53.0); preacetabular portion of ilium markedly longer (more than 2/3 of total ilium length) than postacetabular part (char. 155.1); posterior edge of ischium with proximal median posterior process (char. 165.1); pubic apron less than 1/3 of shaft length (char. 181.1); metatarsal I articulates with the medial surface of metatarsal II at the distal end (char. 205.3); ulna/femoral length ratio equal to or greater than one (char. 236.1); in lateral view, dorsal border of the antorbital fossa formed by the lacrimal and nasal (char. 243.1); ratio of femur to humerus less than 1 (char. 266.2); basisphenoid/pterygoid articulation oriented mediolaterally (char. 283.1); pterygoid, articular surface for basisphenoid flat to convex (char. 284.1); thoracic vertebral centra approximately equal in length and midpoint width (char. 317.0); distalmost mediolateral width of tibia approximately equal to shaft width, no distal expansion of whole shaft, although condyles may be variably splayed mediolaterally (char. 426.1); combined length of metacarpal I plus phalanx I-1 equal to or less than length of metacarpal II (char. 444.1).

Fig. 65

Reduced strict consensus cladogram illustrating avialan relationships when the alternate positions of Limenavis are ignored. Deinonychosaurian taxa have been collapsed into a single terminal.

i0003-0090-371-1-1-f65.tif

Unnamed Clade (Sapeornis + Jeholornis + Jixiangornis + Pygostylia)

This clade of avialans is extremely well supported (GC  =  89) with little conflicting data and Bremer support of 2. Thirteen unambiguous synapomorphies support its monophyly. The dorsal surfaces of the parietals are flat and a lateral ridge borders the supratemporal fenestra (char. 45.0), although this character reverses apomorphically in Euornithes to dorsally convex with a very low sagittal crest. This node also marks the beginning of increased sacralization of vertebrae (6 to 7 sacral vertebrae: char. 110.1/2). In these taxa, the prezygapophyses of distal caudal vertebrae are between 1/3 and whole centrum length (char. 120.0), and the obturator process of the ischium is absent (char. 169.0).

At this level of the tree, the fibula is short and not in contact with the proximal tarsals (char. 191.1), the distal end of astragalus and calcaneum form distinct condyles separated by a prominent tendoneal groove on the anterior surface (char. 194.1), and the distal tarsals fuse to the metatarsals forming a tarsometatarsus (char. 199.1). The proximodorsal process of the ischium (when present in the basal taxa) is large, proximodorsally hooked and separated from the iliac peduncle of the ischium by a notch (char. 230.1). Pleurocoels are present in all dorsals (char. 265.2) and the lateral surfaces of thoracic vertebral centra bear deeply emarginated fossae (char. 318.1).

The ulna and humerus are approximately equal in length (char. 356.1) although Confuciusornis sanctus IVPP V11374, Patagopteryx deferrariisi MACN N 11, Hongshanornis longicresta IVPP V14533, Hesperornis regalis (Marsh, 1880), Baptornis advenus (Marsh, 1877a), and Anas platyrhynchus (AMNH 27496) reverse to the condition of having a humerus longer than the ulna. The semilunate carpal and metacarpals exhibit incomplete proximal fusion (char. 390.1) and metatarsal I is curved or distally deflected, but not twisted with the ventral surface convex and J-shaped (char. 432.1).

Unnamed Clade (Jeholornis + Jixiangornis + Pygostylia)

Like the previous node, this node is extremely well supported with 11 synapomorphies and few conflicting topologies in the jackknife analysis (GC  =  84). Many of the synapomorphies for this clade mark the first occurrence of very “avian”-like characteristics. For example, it is at this node that both an edentulous premaxilla (char. 80.1) and maxilla (char. 82.1) evolved and the nasal (frontal) process of premaxilla becomes long and closely approaches the frontal (char. 273.1).

The shoulder girdle also begins to take on a more modern aspect with the coracoids possessing a strutlike morphology (in lateral view the coracoid is more than twice as tall as wide: char. 136.3). The scapula and coracoid articulation takes on a “ball and socket” conformation with a pit-shaped scapular cotyla developed on the coracoid and coracoidal tubercle developed on the scapula (char. 339.0). The glenoid facet of the coracoid migrates ventral to the acrocoracoid process (char. 347.1). The angle between the coracoid and scapula at the glenoid is 90° or less (char. 351.1) and the posterior end of the scapula tapers distally (char. 352.1).

The arm and the metatarsus also assume a more “modern” aspect. The distal articular surface of the ulna (dorsal condyle and dorsal trochlea in birds) becomes a convex semilunate surface (char. 143.1) and metacarpal III is bowed (char. 445.1). In the metatarsus, coossification of metatarsals begins proximally (char. 200.1).

Unnamed Clade (Jixiangornis + Pygostylia)

The sister-group relationship of Jixiangornis to Pygostylia has high jackknife support and moderate Bremer support. Five unambiguous morphological changes are synapomorphic for this node. The sacral vertebrae in these taxa have unfused zygapophyses (char. 111.0) and the ossified uncinate processes remain unfused to the ribs (char. 125.1). It is at this level of the paravian tree that the ossified sternal plates fuse to each other (char. 128.1). Members of this clade have thoracic vertebrae with centra markedly longer than midpoint width (char. 317.1), although two apparently independent reversals to more boxlike thoracic vertebrae occur in Patagopteryx deferrariisi and Hongshanornis longicresta. The dorsal process of the ischium contacts the ilium (char. 404.1) in basal members of this clade (e.g., Jixiangornis orientalis, Confuciusornis sanctus, Cathayornis yandica), but a reversal to the plesiomorphic noncontacting relationship occurs in Euornithes. Metatarsal V is absent (char. 427.1) in Jixiangornis orientalis and all more derived avialans; however, retention of the metatarsal in Confuciusornis and Vorona makes the optimization of this character state at this node ambiguous.

Pygostylia Chatterjee, 1997

Definition

A node-based monophyletic group including the last common ancestor of Confuciusornis sanctus 153Hou et al., 1995, and Passer domesticus (Linnaeus, 1758) and all of its descendants (sensu Chiappe, 2001).

Expectedly, an abbreviated tail with less than eight free caudal vertebrae (char. 121.3) and distal caudals that are fused into a pygostyle (char. 323.1), optimize as synapomorphic for this node. The pubic boot is absent in pygostylians with no anteroposterior projections (char. 178.2) except for Cathayornis yandica, which shows a reversal to the condition of a slightly posteriorly projecting pubic boot.

In the skull, the dentary is subtriangular in lateral view (char. 70.0), symphyseal foramina are present at the mandibular symphysis (char. 306.1)—a feature completely unique among coelurosaurs, and the Meckelian groove is covered by the splenial and is not exposed medially (char. 309.1). The proximoposterior surface of the deltopectoral crest of the humerus is concave (char. 365.1) and the distal condyles of the humerus are developed on the anterior surface of the humerus (char. 371.1).

It is at this node within Avialae that the pelvis shows signs of increased fusion (i.e., the ilium, ischium, and pubis are partially fused proximally: char. 402.1). On the hindlimb, a laterally projected fibular trochlea developed as a small notch (char. 418.1) is present, although this character shows further transformation into a shelflike conformation in ornithurines. Distally on the tibia, the articular surface for the distal tarsals/tarsometatarsus is well developed posteriorly forming the sulcus cartilaginis tibialis of Aves (Baumel and Witmer, 1993) and a distinct articular surface that extends up the posterior surface of the tibiotarsus (char. 425.1) is present. Lastly, a proximal vascular foramen on the tarsometatarsus is present as a single foramen between metatarsals III and IV (char. 431.1).

Ornithothoraces (i.e., Enantiornithes + Euornithes) Chiappe and Calvo, 1994

Definition

The node-based monophyletic group containing the last common ancestor of Iberomesornis romerali Sanz and Bonaparte, 1992, and Passer domesticus (Linnaeus, 1758) and all of its descendents (sensu Chiappe, 1995).

Eight unambiguous synapomorphies support the monophyly of this clade. Ornithothoracines have nutrient foramina on the external surface of dentary that lie within a deep groove (char. 71.1). The posterior margin of the sternum has distinct posteriorly projected medial and/or lateral processes (char. 333.1) and the interclavicular angle of the furcula is less than 90° (char. 335.1).

A number of changes on the humerus and wrist optimize here and are further modified toward what can be considered a modern avian morphology. These include a ventral tubercle and capital incisure present on the proximal part of the humeral head (char. 359.1), a transverse groove present and developed as a discreet, depressed scar on the proximal surface of the bicipital crest or as a slight transverse groove (char. 362.1), the semilunate carpal and metacarpal exhibiting complete proximal fusion (char. 390.2), and the presence of a pisiform process (char. 394.1). At the ankle, the tibiotarsus-formed condyles are equal in anterior projection (char. 419.1) as opposed to the plesiomorphic condition where the medial condyle typically projects further anteriorly.

Enantiornithes Walker, 1981

Definition

A stem-based monophyletic group containing Cathayornis yandica Zhou et al., 1992, and all coelurosaurs closer to it Passer domesticus (Linnaeus, 1758) (sensu Sereno, 2005).

Only two synapomorphies support the monophyly of Enantiornithes with Liaoningornis and Vorona as the basalmost members. These features are a toothed premaxilla (char. 80.0) and a toothed maxilla (char. 82.0).

Euornithes Cope, 1889

Definition

A stem-based monophyletic group containing Passer domesticus (Linnaeus, 1758) and all coelurosaurs closer to it than to Cathayornis yandica Zhou et al., 1992 (sensu Sereno, 1998).

Euornithes is a strongly supported clade in our analysis. The dorsal surfaces of the parietals are dorsally convex with a low sagittal crest along the midline (char. 45.1). The metatarsals show increased fusion relative to outgroups with metatarsals fusing to each other proximally and distally (char. 200.3). Metatarsal I articulates on the posterior surface of the distal quarter of metatarsal II (char. 205.1). The scapula in euornithines is dorsoventrally curved (char. 353.1). The ilium, ischium, and pubis show complete fusion proximally (char. 402.2) and the dorsal process of the ischium fails to contact the ilium (char. 404.0). The posterior trochanter, present ancestrally in paravians, is lost on the femur (char. 414.2) within Euornithes. On the tarsometatarsus, a characteristically “avian” feature—the hypotarsus—first arises in this clade. The hypotarsus is a structure associated with the passage of tendons of the pedal flexors in living birds. In Euornithes the hypotarsus takes the form of a projected surface or grooves on the proximoposterior surface developed as a posterior projection with a flat posterior surface (char. 430.1).

Unnamed Clade (Hongshanornis + Yixianornis + Songlingornis + Yanornis + Apsaravis + Ornithurae)

This clade of euornithines has moderate to low support. Four synapomorphies unite these taxa. In this clade, the procoracoid process is present on the coracoid (char. 340.1) and the proximal end of the humeral head is domed proximally (char. 357.1). In the manus, phalanx 1 of digit II is strongly dorsoventrally compressed with a flat caudal surface (char. 399.1). Lastly, at the ankle the condyles of the tibiotarsus are approximately equal in mediolateral width (char. 422.1) as opposed to the plesiomorphic condition where the medial condyle is wider.

Unnamed Clade (Yixianornis + Songlingornis + Yanornis + Apsaravis + Ornithurae)

Eight unambiguous synapomorphies support this clade. These include a dentary with subparallel dorsal and ventral edges (char. 70.1) and a toothed premaxilla (char. 80.1). In the pelvis, a preacetabular part of the ilium that is roughly as long as the postacetabular part of the ilium (char. 155.0) and an anterior end of the ilium that is gently rounded or straight (char. 156.0) unite these taxa.

Additional synapomorphies include a lateral process on the coracoid (char. 345.1), a medially hooked acrocoracoid (char. 349.1), an anteroposterior diameter of metacarpal III that is approximately equal to or greater than 50% of the anteroposterior diameter of metacarpal II (char. 391.1), and a metatarsal III that is proximally displaced plantarly relative to metatarsals II and IV (char. 428.1).

Unnamed Clade (Songlingornis + Yanornis + Apsaravis + Ornithurae)

Recent analysis examining basal euornithine relationships have typically recovered a Yixianornis + Songlingornis + Yanornis clade. The clade was not recovered in our analysis. Instead, two unambiguous synapomorphies united Songlingornis and Yanornis with more derived avialans—a fully toothed maxilla (char. 82.0) and dentary (char. 220.0). These are very homoplastic characters in this part of the tree. As such, this should not be considered a strong contradiction of the possible Yixianornis + Songlingornis + Yanornis clade.

Unnamed Clade (Yanornis + Apsaravis + Ornithurae)

A single synapomorphy unites Yanornis with Apsaravis and Ornithurae. The presence of small but numerous (25–30) teeth in the maxilla and dentary (char. 84.1) optimizes at this node.

Unnamed Clade (Apsaravis + Ornithurae)

This clade is diagnosed by possessing cervical and anterior trunk vertebrae that are at least partially heterocoelous (char. 101.2), the presence of 10 or more sacral vertebrae (char. 110.5), distal articular end of metacarpal II that is ginglymoid and a metacarpal I that is shelflike (char. 213.2), and pubes that are compressed mediolaterally (char. 412.1), and do not contact each other distally (char. 413.1). The tarsometatarsus within members of this clade bears a distinct, well-developed, and globose intercotylar eminence (char. 429.1) and metatarsal I is absent (char. 447.1).

Ornithurae Haeckel, 1866

Definition

A node-based monophyletic group including the last common ancestor of Hesperornis regalis Marsh, 1880, and Passer domesticus (Linnaeus, 1758) and all of its descendents (sensu Chiappe, 1995).

Eight synapomorphies diagnose this clade of derived avialans with a Recent avian aspect (although Hesperornis and Ichthyornis still retain teeth). In these taxa, the anterior trunk vertebrae have large hypapophyses (char. 102.1) and the acromion margin of the scapula is laterally everted (char. 133.1). On the humerus, a brachial fossa is present and developed as a flat scar or as a scarred fossa (char. 376.1) and demarcation of muscle origins (e.g., m. extensor metacarpi radialis in Aves) on the dorsal edge of the distal humerus are present as pit-shaped scars or as variably projected scar-bearing tubercles or facets (char. 378.1). The ulnare in ornithurines is V-shaped with well-developed dorsal and ventral rami (char. 388.1) and the tibiotarsal condyles bear an extensor canal present as an emarginated groove (char. 420.1). There are two proximal vascular foramina on the tarsometatarsus (char. 431.2) and the distal plantar surface of metatarsal II possesses a fossa (in the form of a shallow notch) for metatarsal I (char. 433.1).

Aves Linnaeus, 1758

Definition

A node-based monophyletic group containing the last common ancestor of Struthio camelus (Linnaeus, 1758), Tinamus major (Gmelin 1789), and Passer domesticus (Linnaeus, 1758) and all of its descendents (sensu Gauthier, 1986, and Gauthier and de Queiroz, 2001).

Due to the lability of some taxa closely related to Aves (e.g., Limenavis patagonicus, and Iaceornis marshi) the characters diagnosing Aves vary among topologies. In a reduced taxon-sampling analysis, a subset of these potential synapomorphies stabilize and that set is used here for the purposes of diagnosing this clade. Nineteen unambiguous synapomorphies diagnose Aves. This will be listed below but not discussed further: supraorbital crests on lacrimal with lateral expansion anterior and dorsal to orbit (char. 37.2); dorsal surface of parietals flat, lateral ridge borders supratemporal fenestra (char. 45.0); maxilla edentulous (char. 82.1); 11 or more sacral vertebrae (char. 110.6/7); obturator process of ischium proximal in position (char. 169.1); dentary edentulous (char. 220.2); in lateral view no dorsal projection of maxilla participates in the anterior margin of the internal antorbital fenestra (char. 241.2); dentaries fused anteriorly (char. 270.1); maxillary process of the premaxilla extending for at least half the length of the facial margin (char. 272.1); kinked pterygoid absent, basipterygoid articulation in line with axis of pterygoid (char. 285.1); mandibular articulation of quadrate tricondylar due to presence of additional posterior condyle or broad articular surface (char. 298.1); sternal pneumatic foramina present in the depressions (loculi costalis; Baumel and Witmer, 1993) between rib articulations (char. 329.1); paired intermuscular ridges (linea intermuscularis; Baumel and Witmer, 1993) parallel to the sternal midline (char. 332.1); a pneumatized coracoid (char. 343.1); proximal end of the humerus with one or more pneumatic foramina (char. 370.1); preacetabular blade of the ilium extending anterior to the first sacral vertebrae and overlapping one or more ribs (char. 408.1); medial condyle of the tibiotarsus that projects further anteriorly than laterally (char. 419.0); projected surface or grooves on proximoposterior surface of tarsometatarsus (associated with the passage of tendons of the pes flexors in Aves; hypotarsus) (char. 430.2); distal vascular foramen forked, two exits (plantar and distal) between metatarsals III and IV (char. 436.1).

Nodal Support

Character support for the nodes present in the most parsimonious reconstructions was calculated using two methods. The first is a statistical resampling technique, the jackknife applied to character resampling (Farris et al., 1996). Farris et al. and others (e.g., Goloboff et al., 2003) have given detailed reasons for preferring this measure over the more commonly employed nonparametric bootstrap resampling (Felsenstein, 1985), which relies on assumptions not met by our dataset (or most other morphological datasets). The second method used is Bremer support (Bremer, 1988, 1994), which evaluates node stability/sensitivity by exploring suboptimal tree solutions in order to determine how many additional steps must be allowed in searching for topologies before the hypothesized clade is no longer recovered. Bremer support was calculated using negative constraints through the use of the BREMER.RUN script supplied with TNT.

The jackknife support analysis was calculated using TNT (Goloboff et al., 2008a, 2008b). The analysis was performed using 1000 replicates for which the probability of independent character removal was set to 0.20. Each jackknife replicate was analyzed using a tree search strategy consisting of 10 replicates of RAS followed by TBR branch swapping (saving 10 trees per replicate). The topologies obtained during the jackknife replicates are summarized using GC frequencies. This follows the recommendations of Goloboff et al. (2008a, 2008b). These frequencies differ from the raw clade frequencies, because they measure the difference in frequency between the analyzed group and the most frequent contradictory group. Using raw frequencies is not recommended because there are cases in which groups lacking support have a frequency of 0.5 (Goloboff et al., 2003). GC frequencies are preferable because they reflect the balance between the amount of evidence that corroborates a given clade with the amount that falsifies that group.

The results from the entire dataset reflect a wide range of support for nodes across the entire tree (figs. 66Fig. 6768). Unsurprisingly, coelurosaurian monophyly is extremely well supported (GC  =  95) with little contradictory evidence. The basal Tyrannosauroidea clade is also well supported as is the less inclusive Tyrannosauridae node (GC  =  79 and 73, respectively).

Fig. 66

Jackknife analysis. Strict consensus cladogram of basal coelurosaurs. Values indicate jackknife support reflected as GC frequencies derived from 1000 jackknife replicates with a character-removal probability of 0.20.

i0003-0090-371-1-1-f66.tif

Fig. 67

Jackknife analysis. Strict consensus cladogram of paravians. Values indicate jackknife support reflected as GC frequencies derived from 1000 jackknife replicates with a character-removal probability of 0.20.

i0003-0090-371-1-1-f67.tif

Fig. 68

Jackknife analysis. Reduced strict consensus cladogram of paravians. Values indicate jackknife support reflected as GC frequencies derived from 1000 jackknife replicates with a character-removal probability of 0.20.

i0003-0090-371-1-1-f68.tif

The node containing all coelurosaurs more derived than tyrannosauroids possesses little contradictory evidence (GC  =  60). Most of the intervening nodes between Proceratosaurus bradleyi, Ornithomimosauria, and derived maniraptorans have extremely low support (GC values between 2 and 12). This is neither surprising nor very informative given that most of these nodes collapse in the strict consensus topology of the phylogenetic analysis due to the labile positions of Proceratosaurus bradleyi, Dilong paradoxus, and Coelurus fragilis. Compsognathidae monophyly has relatively high levels of contradictory character data (GC  =  11), but Ornithomimosauria monophyly shows high jackknife support with little contradictory data (GC  =  91). Most of the constituent ornithomimosaur clades are moderately supported (GC values in the 40s).

Maniraptora is poorly supported in the analysis (GC  =  5). Most other derived maniraptoran clades, however, show surprisingly high levels of jackknife support. Alvarezsauroidea is moderate to weakly supported (GC  =  25), but the less inclusive Alvarezsauridae and its constituent clades have high jackknife support (ranging from 70 to 83). The sister taxon relationship of Patagonykus puertai with Shuvuuia deserti and Mononykus olecranus is strongly supported (GC  =  80).

The node containing the common ancestor of Alvarezsauroidea plus Paraves is well supported (GC  =  67) whereas the next most derived node (i.e., Therizinosauria + Oviraptorosauria + Paraves) has only moderate-to-low jackknife support (GC  =  30). Therizinosauria and Oviraptorosauria monophyly have high GC frequencies, 62 and 81 respectively. Less inclusive clades within Oviraptorosauria show moderate to low support. Avimimus + all more derived oviraptorosaurs has a GC value of 45, Microvenator + all more derived oviraptorsaurs has a GC value of 48, but clades more deeply nested than these nodes show higher levels of contradictory character data with GC values as low as 16 and 11.

The monophyly of Paraves is poorly supported (GC  =  3) in part because of the placement of Epidexipteryx at the base of the clade. Analyses excluding Epidexipteryx find less contradictory data for the clade (GC  =  54). The Avialae + Deinonychosauria node is also weakly supported with a GC value of 22. The Avialae node has moderate jackknife support (GC  =  47). Deinonychosauria monophyly has significantly less support (GC  =  3). The Troodontidae node has high levels of contradictory data (GC  =  9), but most clades within Troodontidae have GC values between 50 and 80. Indeed, the new basal troodontid clade—Jinfengopteryginae—possesses some of the strongest jackknife support among troodontids, with a GC value of 80 and the Xiaotingia + Anchiornis clade has a GC value of 34.

Dromaeosauridae monophyly has weak jackknife support (GC  =  1), and support for most clades within Dromaeosauridae is also quite weak. Only the Laurasian dromaeosaurid clade (GC  =  20), Dromaeosaurinae (GC  =  24), and the clade comprised of Austroraptor + Unenlagia (GC  =  21) have jackknife frequencies at or above 20. Due to the conflicting position of several of the dromaeosaurid taxa in this analysis, the jackknife support was also explored analyzing the entire dataset, but summarizing the node stability while ignoring the position of the most unstable taxa (i.e., Pyroraptor olympius and Limenavis patagonicus) (fig. 68). This result therefore evaluates the node stability ignoring the poorly known and problematic taxa mentioned above. As the procedure matched the elaboration of the agreement subtree (fig. 58) above (i.e., considering the information of these taxa for the analysis, allowing their information to influence the interrelationships of all taxa, but excluding them from the consensus to see the underlying structure of the data), the support values obtained through this method are directly comparable with the tree in figure 58.

This reduced jackknife analysis shows higher GC values for Deinonychosauria (GC  =  25) and both Troodontidae (GC  =  25) and Dromaeosauridae nodes (GC  =  16). The node containing all dromaeosaurids more derived than Mahakala omnogovae is very weakly supported and does not show up in the jackknife analysis. However, other dromaeosaurid nodes that were poorly supported in the total analysis show higher support values. The Laurasian dromaeosaurid clade (GC  =  50), Unenlagiinae (GC  =  33), and the clade composed of Unenlagia + Austroraptor (GC  =  57) all show moderate GC values. Velociraptorinae continues to have low support (GC  =  19), but Dromaeosaurinae monophyly continues to have generally low levels of contradictory data (GC  =  56).

Within Avialae, basal nodes show extremely high support (GC values between 92 and 75) with little contradictory data present. In the total analysis, Pygostylia has moderate to high support (GC  =  73), but in the reduced jackknife analysis this node also has almost no contradictory data (GC  =  92). Ornithothoraces and Enantiornithes are weakly supported in the total analysis (GC values of 20 and 5, respectively), but support in the reduced analysis is a bit higher (23 and 11). The subclade of enantiornithines including Neuquenornis + Gobipteryx + Concornis + Cathayornis + Pengornis is highly supported (GC  =  79) and the Liaoningornis + Vorona clade has weak, but not terrible support (GC  =  25). Euornithes is highly supported with little contradictory data (GC  =  86) as are the first few more derived euornithine nodes. The Apsaravis + all more derived euornithines node has very little data contradicting this relationship (GC  =  89). Unsurprisingly, the jackknife support of many of the most derived avialan nodes is weak for both the total and reduced jackknife analyses because of the various positions taken by Limenavis and Iaceornis, and failure to consistently depict Lithornis and Crypturellus as a monophyletic Paleognathae. Neognathae monophyly, however, is supported with a GC value of 94.

Bremer support was calculated using negative constraints as employed by the BREMER.RUN script supplied with TNT. As in the jackknife analysis, two consensus trees were estimated from the set of 100,000 suboptimal trees. The first was a strict consensus including all the analyzed taxa, as is typically done in this sort of analysis (Bremer, 1994). The second was also a strict consensus, but now ignoring the alternative position of the conflictive and incompletely known taxa discussed above. As in the case of the reduced jackknife support, the 100,000 suboptimal trees were found analyzing the entire dataset allowing their information to influence the results. As in the previous case, their exclusion from the strict consensus used to summarize the Bremer support reveals some structure to the data that otherwise remains hidden.

The Bremer support analysis including the entire set of coelurosaurian taxa results in a consensus tree with minimal support for most of the nodes of the tree. Most nodes have extremely low Bremer values, ranging between 1 and 2 (figs. 69Fig. 7071). The exceptions to this are the Ornithomimosauria node (Bremer value  =  4), the Oviraptorosauria node (Bremer value  =  3), the Patagopteryx + derived euornithine node (Bremer value  =  3), the Yixianornis + derived euornithine node (Bremer value  =  4), and the three Neognathae nodes. All of these nodes with high Bremer support also showed very little contradictory data in the jackknife analysis discussed above. Within Deinonychosauria, no node has a Bremer value higher than one, which is not altogether surprising given the uncertainty in the position of Pyroraptor olympius.

Fig. 69

Bremer analysis. Strict consensus cladogram of basal coelurosaurs. Values indicate Bremer support derived from the BREMER.RUN script supplied by TNT.

i0003-0090-371-1-1-f69.tif

Fig. 70

Bremer analysis. Strict consensus cladogram of paravians. Values indicate Bremer support derived from the BREMER.RUN script supplied by TNT.

i0003-0090-371-1-1-f70.tif

Fig. 71

Bremer analysis. Reduced strict consensus cladogram of paravians ignoring the alternate positions of Pyroraptor. Values indicate Bremer support derived from the BREMER.RUN script supplied by TNT.

i0003-0090-371-1-1-f71.tif

The Bremer support values of the nodes described and diagnosed above are generally not much higher when the alternative positions of the uncertain taxa, excluded in the reduced strict consensus, are ignored (i.e., Pyroraptor olympius and Limenavis patagonicus). The exception to this is for nodes within Paraves, which again is not altogether unsurprising, because this is the clade within which these most labile taxa have uncertain positions. As in the case of the jackknife analysis and the Bremer analysis of the complete dataset, the avialan nodes of Paraves are most strongly supported (fig. 71) relative to other paravians, suggesting the robustness of the evidence favoring Avialae monophyly and successively more derived positions of Sapeornis chaoyangensis, Jeholornis prima, Jixiangornis orientalis to the Confuciusornis + all more derived avialan node (all with Bremer values of 2).

The reduced Bremer support analysis also indicates that the most poorly supported nodes in the Bremer analysis are generally the same as those in the reduced jackknife analysis. As in the reduced jackknife analysis, Bremer support for the basal nodes of Troodontidae and Dromaeosauridae show little improvement over the total analysis. Troodontidae shows a Bremer value of 1 as does Dromaeosauridae. The monophyly of the basal troodontids composing Jinfengopteryginae is supported in the reduced Bremer analysis with Bremer value of 2, which is in contrast to the high jackknife support (GC  =  90). On the dromaeosaurid side, the dromaeosaurid node excluding Mahakala omnogovae has a Bremer value of 1, as does the Laurasian dromaeosaurid node. The reduced jackknife analysis indicated generally weak support for Dromaeosaurinae and Velociraptorinae and the same is true for the reduced Bremer analysis. This suggests that whereas relationships among deinonychosaurs generally and dromaeosaurids in particular have begun to stabilize, the character support for these nodes is weak and the addition of new data in the form of new fossils or new characters has the potential to overturn some of the clades discussed in the present study. The exception to this within Dromaeosauridae appears to be the unenlagiines, which have comparatively high Bremer support values. Unenlagiinae has a Bremer value of 2 the Buitreraptor + Austroraptor + Unenlagia node has a Bremer value of 3, and the Austroraptor + Unenlagia node has a Bremer value of 4.

DISCUSSION

Deinonychosaurian Monophyly: Strength and Sensitivity

Deinonychosaurian monophyly is well supported in the present cladistic analysis and has been consistently recovered in all TWiG analyses after the original Norell et al. (2001) analysis. The few analyses, such as Senter (2004), that failed to find deinonychosaur monophyly tended to have poor taxonomic and character sampling relevant to the basal nodes of Troodontidae and Dromaeosauridae. When these deficiencies are accounted for (e.g., Holtz, 2001; Senter, 2007) deinonychosaur monophyly is recovered. Therefore, Deinonychosauria has proven to be one of the more consistently robust coelurosaur clades possessing clear, relatively nonhomoplastic synapomorphies.

The jackknife analysis conducted in the above chapter showed that there is only moderate level of evidence contradicting Deinonychosauria monophyly (GC  =  25). However, the Bremer analysis showed that collapsing the Deinonychosaurian node requires accepting trees only 1 step longer than the most parsimonious trees. Exploring sensitivity to longer trees and the topologies they may entail illustrates possible topologies other than deinonychosaur monophyly (figs. 72, 73). Splitting Deinonychosauria, but retaining the division and monophyly of the three constituent paravian clades (Dromaeosauridae, Troodontidae, and Avialae) requires only 1 or 2 additional steps depending on the resulting topology. If Dromaeosauridae is constrained to be the sister taxon to Avialae, the resulting topology requires 4 additional steps beyond the most parsimonious reconstruction (fig. 72). On the other hand, only 1 additional step is required to place Troodontidae as the sister taxon to Avialae (fig. 73). These only slightly less parsimonious topologies could be interpreted as an indication of weakness in deinonychosaurian monophyly. We are inclined to interpret (and think it is more readily borne out by the data) that this is instead a reflection of the overall morphological similarity of the basal members of each paravian clade (e.g., compare Mahakala to IGM 100/1126 or Archaeopteryx).

Fig. 72

Simplified cladogram showing manipulated paravian relationships. Constraining Dromaeosauridae to be sister group of Avialae requires 4 additional steps.

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Fig. 73

Simplified cladogram showing manipulated paravian relationships. Constraining Troodontidae to be sister group of Avialae requires 1 additional step.

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Nonmonophyly of any of the three primary paravian groups is strongly unparsimonious. As an example, troodontid nonmonophyly is particularly illustrative. As context, troodontid similarities to basal avialans have been discussed in the past (Currie, 1987, 1995). Furthermore, Hwang et al. (2004a) raised the possibility of troodontid paraphyly relative to Avialae due to many potentially derived similarities among basal taxa (recovered by our analysis as members of the Jinfengopteryginae) and basal avialans. As illustrated in figure 74 troodontid paraphyly and particularly a Jinfengopteryginae + Avialae relationship is strongly unparsimonious requiring 12 steps more than the most parsimonious topology. This speaks to the robust nature of Troodontidae monophyly, even though there is low jackknife (GC  =  25) and Bremer (Bremer value  =  1) support.

Fig. 74

Modified cladogram showing manipulated paravian relationships. Constraining Troodontidae to be paraphyletic relative to Avialae with Xiaotingia + Anchiornis and Jinfengopteryginae as successive sister taxa to Avialae requires 12 additional steps.

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Inclusion of Problematic Paravian Taxa

The Middle to Late Jurassic Chinese taxa Epidendrosaurus ningchengensis, Epidexipteryx hui, and Pedopenna daohugouensis have occupied various positions within Paraves depending on the analysis. Epidendrosaurus has typically been recovered as a basal avialan either just outside the Archaeopteryx node (Senter, 2007; Zhang et al., 2008) or just inside the Archaeopteryx node (Choiniere et al., 2010), or in an unresolved position basally in Avialae (Xu and Zhang, 2005). Epidexipteryx has typically been recovered as the sister taxon to Epidendrosaurus forming a clade called Scansoriopterygidae (Zhang et al., 2008). The phylogenetic position of Pedopenna has been tested only by Xu and Zhang (2005) and was found in a polytomy with an unresolved avialan clade and deinonychosaurs.

Epidexipteryx

Inclusion of Epidexipteryx in our analysis resulted in a novel phylogenetic position for this taxon as a basal paravian outside of the split between deinonychosaurs and avialans, as was discussed above. Epidexipteryx resembles basal oviraptorosaurs in several respects, particularly in its cranial morphology. Zhang et al. (2008) noted some of these, drawing attention to the anteroposteriorly short but dorsoventrally tall skull, the posterodorsally displaced naris and anteroposteriorly long parietals. Likewise, the highly procumbent anterior dentition and the slightly downturned mandible compares favorably to basal oviraptorosaurs like Incisivosaurus, Caudipteryx, and putatively Protarchaeopteryx. Constraining Epidexipteryx as a basal oviraptorosaur requires only one additional step in our dataset (fig. 75). Three features support the inclusion of Epidexipteryx in Oviraptorosauria, which are caudal vertebrae without a transition point (char. 115.1), a dentary that has teeth only anteriorly (char. 220.1), and a first premaxillary tooth much larger than the succeeding teeth (char. 251.2). A tail without a transition point is unique to oviraptorosaurs and Epidexipteryx and premaxillary teeth greatly enlarged relative to other premaxillary teeth is unique to Incisivosaurus, Protarchaeopteryx, and Epidexipteryx.

Fig. 75

Reduced strict consensus cladogram of Paraves + Oviraptorosauria showing the number of extra steps required to place Epidexipteryx as a basal avialan (2 steps longer), or to place it at the base of Oviraptorosauria (1 step longer). Analysis conducted in TNT using positive constraints.

i0003-0090-371-1-1-f75.tif

Because all previous analyses found Epidexipteryx as a basal avialan, we tested this position using our dataset but constraining Epidexipteryx to this position. This constraint analysis was only two steps longer than the most parsimonious solution (fig. 75) indicating that there is some signal there. A single synapomorphy (less than 25 caudal vertebra: char. 121.2) supports this position. Therefore, moving the phylogenetic position of Epidexipteryx among paravian and the closely related oviraptorosaurs requires accepting only slightly less parsimonious topologies. The great similarity that exists among basal paravians, basal oviraptorosaurs, and Epidexipteryx leads us to caution that the precise phylogenetic position of Epidexipteryx requires additional work to understand the interesting, and highly derived, anatomy of this taxon as well as a better understanding of the character changes taking place near the split between oviraptorosaurs and paravians.

Epidendrosaurus

Exploratory analysis including Epidendrosaurus finds it a basal avialan (fig. 76A). A metatarsal I that articulates with the medial surface of the distal end of metatarsal II (char. 205.3) and a femur that is equal in length to or shorter than the ulna (char. 236.1) and the humerus (char. 266.2) support this relationship. As we have discussed earlier, we view it as problematic to include Epidendrosaurus in a phylogenetic analysis because of the extremely poor level of preservation, which results in much missing data, and the likely juvenile status of the holotype specimen. Our exploratory analysis reinforces this latter point. All three recovered synapomorphies are subject to ontogenetic changes and the observed morphology and proportions of the holotype of Epidendrosaurus may very well be different from its adult phenotype. We advise exclusion of Epidendrosaurus from primary phylogenetic analyses of paravians until more information is known about its adult morphology and a significant number of more cells in the data matrix can be populated.

Fig. 76

Results from exploratory phylogenetic analysis including Epidendrosaurus ningchengensis. A, reduced strict consensus cladogram. B, detail of the base of Avialae showing the number of additional steps required to constrain Epidendrosaurus and Epidexipteryx are sister taxa.

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It is important to note that inclusion of Epidendrosaurus in our analysis did not result in an Epidendrosaurus plus Epidexipteryx clade (i.e., Scansoriopterygidae). Constraining the monophyly of Scansoriopterygidae requires four additional steps and draws Epidexipteryx uptree, with the clade positioned basally among avialans (fig. 76B). A single feature, the absence of metatarsal V (char. 506.1), unites Epidexipteryx and Epidendrosaurus in our dataset.

Pedopenna

An exploratory analysis including Pedopenna results in over 100,000 most parsimonious trees of length 2031—certainly due to the fragmentary nature of the type, a unique specimen. The strict consensus of these topologies is largely unresolved above the alvarezsauroid node (fig. 77). Examination of the fundamental trees shows that Pedopenna is recovered in numerous places among oviraptorosaurs, therizinosaurs, and outside the paravian node near Epidexipteryx or sister to Epidexipteryx. Support for these various positions are typically weak (one synapomorphy) or nonexistentent (no unambiguous synapomorphies). The relatively global lability of Pedopenna among maniraptorans results in the reduced resolution of the strict consensus, but the underlying phylogenetic signal (consistent with the results from the primary analysis) remains as evinced from the Adam's consensus of these trees (fig. 78). At present it appears that too little is known about the morphology of Pedopenna to reliably place its phylogenetic position.

Fig. 77

Reduced strict consensus cladogram of exploratory phylogenetic analysis including Pedopenna daohugouensis and Epidendrosaurus ningchengensis.

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Fig. 78

Adam's consensus cladogram of exploratory phylogenetic analysis including Pedopenna daohugouensis and Epidendrosaurus ningchengensis.

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Is Rahonavis a Dromaeosaurid?

When Rahonavis was first described this small Malagasy paravian was thought to represent a transitional basal avialan (Forster et al., 1998; Chiappe, 2002). Subsequent species-level analyses (Norell et al., 2001; Hwang et al., 2002; Makovicky et al., 2003; Novas and Pol, 2005) continued to recover Rahonavis as the sister taxon to Archaeopteryx lithographica and Confuciusornis sanctus. However, as discussed in multiple places above, nearly all the traits proposed by Forster et al. (1998) and Chiappe (2002) are now known to have wide distributions within maniraptorans.

The discovery of Buitreraptor gonzalezorum by Makovicky et al. (2005) and the characterization of Unenlagiinae, the South American clade of dromaeosaurids, expanded the character data used for testing the phylogenetic position of Rahonavis and provided the foundation for, and was first to propose, Rahonavis as a dromaeosaurid and not an avialan. Successive iterations of the dataset used by Makovicky et al. (2005) continue to yield dromaeosaurid status for Rahonavis, even with the inclusion of additional avialan taxa (Turner et al., 2007b). The remaining (and necessary) test for the affinities of Rahonavis rests on both the inclusion of basal avialans plus extensive character sampling of avialan synapomorphies and those that support the phylogenetic structure along the basal lineages of Avialae.

The dataset and the cladistic analysis conducted in our study provides this. Twenty-eight avialans and almost 200 morphological characters relating to avialan relationships were added. This substantial addition of data did not overturn the placement of Rahonavis with the Unenlagiinae clade of Dromaeosauridae. This relationship remains supported by at least six synapomorphies although support measures are somewhat weak. Constraining Rahonavis ostromi as a basal avialan requires seven additional steps beyond the most parsimonious topology, and constraining Rahonavis to a more derived placement within Avialae requires 11 additional steps (fig. 79). Taken together with the strong morphological support for the Unenlagiinae clade and strongly unparsimonious nature of an “avialan” Rahonavis, it has emerged that there is no reason to consider Rahonavis as a problematic taxon. Many of the features thought to be suggestive of avialan affinities are shared with its unenlagiine relatives and are most parsimoniously interpreted as convergent with derived avialans. Convergent evolution and mosaicism in character evolution among paravians is commonplace. So, to answer the question posed in this section—yes, Rahonavis is a dromaeosaurid.

Fig. 79

Reduced strict consensus cladogram of Paraves showing the number of extra steps required to place Rahonavis as basal within Avialae (7 steps longer), or to place it within Avialae in a position more derived than Archaeopteryx (11 extra steps).

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Are Neuquenraptor and Unenlagia the Same Taxon?

Although discussed above in evaluating the validity of the various dromaeosaurid species, the question whether Neuquenraptor and Unenlagia are the same taxon proves difficult to answer, let alone to answer definitively. We continue to feel that this is an important issue in dromaeosaurid evolution, although we will argue that it is not as critical as it once was. Because we lack the necessary fossils for each taxon to directly compare the morphology of overlapping elements (currently limited to femora and pedal unguals of digit II), we instead pose the related question—does Unenlagiinae monophyly hinge on Neuquenraptor and Unenlagia being the same taxon? In Makovicky et al.'s (2005) original analysis and the following analyses of Norell et al. (2006) and Turner et al. (2007a, 2007b), such a taxonomic conclusion was necessary for the monophyly of the Gondwanan dromaeosaurids. By including new information from Unenlagia paynemili, the contaxonomic status of Neuquenraptor and Unenlagia is no longer necessary for unenlagiine monophyly (fig. 80).

Fig. 80

Adam's consensus cladogram of Dromaeosauridae resulting from the exploratory analysis in which Unenlagia and Neuquenraptor are treated as separate terminal taxa.

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Scoring Unenlagia and Neuquenraptor as separate terminals results in a set of most parsimonious trees of the same length as the primary analysis. Neuquenraptor can occupy multiple placements across the base of Dromaeosauridae therefore collapsing the base of the clade in the strict consensus solution. However, Adam's consensus demonstrates that the monophyly and membership of Unenlagiinae is unchanged. Due to the paucity of material for Neuquenraptor argentinus we don't view this alternate analysis of dromaeosaurids as a strong indication of the phylogenetic position of Neuquenraptor or of the viability or validity of the contaxonomic status on these two taxa. Instead, this analysis should be viewed as exploratory and should only definitely illustrate that Unenlagiinae monophyly no longer rests on the assumed contaxonomic status of these two Gondwanan taxa.

The ultimate resolution of this question depends on either the discovery of a large amount of additional material referable to one or both of these taxa, or in coelurosaur systematists converging on a consensus.

Does Archaeopteryx have a Hyperextensible Second Digit?

Some authors have raised the possibility that Archaeopteryx possessed a hyperextensible second pedal digit homologous to the condition in deinonychosaurs (Gauthier, 1986; Sereno, 1997, 1999; Paul, 2002; Mayr et al., 2005, 2007). This claim is important to explore because of its relevance to character optimization at the base of Paraves. If Archaeopteryx indeed has a highly derived modified hyperextensible second digit this would indicate that it was a wider paravian synapomorphy and is not unique to Deinonychosauria as has traditionally been thought, and as recovered in the phylogenetic analysis discussed here.

The most recent incarnation of this argument was proposed by Mayr et al. (2005). These authors proposed that the foot of Archaeopteryx possesses “a hyperextensible second toe, as in Deinonychosauria…” (Mayr et al., 2005: 1485; also see Mayr et al., 2007). This claim was based on observation of the 10th described specimen of Archaeopteryx (WDC-CSG-100) as well as citations of Gauthier (1986), Elzanowski (2002), and Paul (2002). This observation, however, is problematic. Gauthier (1986a) does not say that the second digit of Archaeopteryx is hyperextensible, just that the distal articular surface of pedal phalanx II-1 was enlarged. Contrary to Mayr et al. (2005 and 2007), Elzanowski said that Archaeopteryx lacked an enlarged articular surface and stressed that the digit might in fact be hyperflexive.

It is our view that, at best, the distal articular surface is perhaps dorsoventrally enlarged (e.g., the WDC and Eichstätt specimens based on personal observations (A.H.T. and M.A.N.). However, unlike the condition present in deinonychosaurs, the distal articular surface of phalanx II-1 of Archaeopteryx does not extend proximally on the shaft (see Ostrom, 1969a) and lacks a deep midline trochlea. The depression that is present is rounded in Archaeopteryx and the sides are rounded, not convex as in Deinonychus antirrhopus (Ostrom, 1969a) and other dromaeosaurids like Velociraptor mongoliensis (IGM 100/985) and even the size-appropriate Mahakala omnogovae (IGM 100/1033) (Turner et al., 2011). Furthermore, the phalanx is not compressed along the long axis in Archaeopteryx lithographica as it is in deinonychosaurs. Archaeopteryx lithographica also lacks a number of other characters associated with the modified hyperextensible digit II in deinonychosaurs—even similarly sized animals like Microraptor. For example, digit II-2 lacks a proximal ventral heel and the distal end lacks the very large and very deeply grooved ginglymoid articular facet. The dorsal displacement and deepening of the ligament fossae on the lateral and medial surfaces of the phalanx present in deinonychosaurs are also lacking in Archaeopteryx lithographica.

It is apparent that it is an overstatement to say that the foot of Archaeopteryx possesses a hyperextensible second toe as in deinonychosaurs. Nonetheless, Mayr et al. (2005) added a new character state to character 170 from the TWiG dataset. This character state (177.2) is “penultimate phalanx of digit II modified for hyperextension but ungual not hypertrophied.” This additional character state was scored as present only in Archaeopteryx. Reformulation of this character is extremely problematic. First, it is phylogenetically uninformative because Mayr et al.'s state 2 is autapomorphic for Archaeopteryx. To provide phylogenetic structure, this character would have to be rearranged and ordered so as to treat this new character state as intermediate between a lack of hyperextension to fully hyperextensible with an enlarged ungual. However, this is not necessary because Archaeopteryx does not have any morphology indicative of hyperextension in the penultimate phalanx of digit II (i.e., phalanx II-2). Mayr et al.'s (2005) discussion was about a possible enlarged distal trochlear surface on phalanx II-1. This character state is entirely unnecessary, because no such character state exists within the sampled taxa.

An additional point worth noting is that the dataset coopted by Mayr et al. (2005) was not constructed nor intended to elucidate the interrelationships within Avialae (only two definitive avialans were included) and the character sampling was not intended to address the questions about which Mayr et al. (2005) were drawing conclusions. In fact, two more recent versions of TWiG datasets had been published by the time of the publication of Mayr et al. (2005). Furthermore, even if we could corroborate the observations of Mayr et al. (2005), or Mayr et al. (2007) for that matter, they would not necessarily alter paravian topology. Adding a new character for the presence of an enlarged distal end of phalanx II-1 to the Makovicky et al. (2005) matrix just renders it a paravian synapomorphy subsequently lost in all avialans more derived than Archaeopteryx (with some reversals within the crown clade and even in stem taxa like Patagopteryx).

Xiaotingia and the Position of Archaeopteryx

Recently Xu et al. (2011) reported the discovery of an interesting new paravian dinosaur from Late Jurassic of China, which according to their phylogenetic analysis significantly alters our understanding of avialan origins. Most significantly the authors claim to demonstrate for the first time that Archaeopteryx (and its sister taxon Wellnhoferia) is not a member of the clade Avialae and therefore not a basal bird. Through their discovery of a new clade of paravians comprised of Archaeopteryx, Xiaotingia, and Anchiornis, more closely related to the deinonychosaurs, these authors challenge the orthodox view of Archaeopteryx as the basalmost bird taxon. Using the data published by Xu et al. (2010) we reanalyzed the new dinosaur Xiaotingia within our larger phylogenetic dataset that samples paravian diversity more comprehensively and instead find Xiaotingia a basal troodontid, sister to the very similar Anchiornis, and Archaeopteryx lithographica still residing at the base of Avialae (fig. 58).

Review of the Xu et al. (2011) matrix reveals some character scorings that we dispute. Xu et al. (2011) atomize the presence of an enlarged and hyperextensible second pedal digit into several traits (chars. 201, 320–322). Character 322 describes the development of the flexor attachment heel on phalanx II-2 in deinonychosaurian taxa with the incipient condition of having a laterally displaced, small heel as state 2, while the possession of the more derived, enlarged heel is designated as state 1. Although Xu et al. (2011) describe and figure a ventrolateral flange or heel on pedal phalanx II-2 of Xiaotingia, they score it as absent in their matrix. Likewise, they code only the incipient condition as present in two unenlagiine taxa, whereas they consider a number of basal troodontids, such as Mei and Sinovenator, and basal dromaeosaurids, such as Microvenator, as having the more derived, enlarged heel below the proximal articulation. Based on our own study of the relevant specimens, we disagree with these scorings and instead code Xiaotingia, Mei, Sinovenator, Microraptor, Unenlagia, and Sinovenator as having state 2. Even without treating this character as an ordered transformation series, we recover a slightly different topology with Xiaotingia and Anchiornis as basal troodontids. Although we still discover Archaeopteryx as closer to deinonychosaurs than to Avialae, this result draws into doubt Xu et al.'s (2011) contention that their result was predicated entirely on the inclusion of Xiaotingia.

Another trait, whose scoring we find contentious, is their character 366 that describes the relative position of the postorbital process of the jugal along the length of that bone. Xu et al. (2011) score the process as close to the middle of the bone (state 0) in Epidexipteryx, which we agree with, but they go on to consider the basal avialans Jeholornis, Sapeornis, and Confuciusornis as well as the unenlagiine Buitreraptor as sharing this condition. Our study of these specimens, as well as their own figure 4, shows that the position of the postorbital process of the jugal is far closer to the caudal end of the bone in these paravian taxa, as it is in Archaeopteryx, Xiaotingia, and basal troodontids and dromaeosaurids. A reanalysis of the matrix combining changes in the scoring of this trait along with those in character 322 culminates in a strict consensus tree in which the position of Troodontidae, Avialae (without Archaeopteryx), Dromaeosauridae, and the Archaeopterygidae clade found by Xu et al. (2011) are fluid. The paravian node shows eight resolutions in the set of 231 MPTs (TL  =  1511), only one of which places Archaeopteryx as closer to a clade other than Avialae. A majority rule consensus reveals that 89% of resolutions favor the traditional alignment of Archaeopteryx as the most basal avialan, with Epidexipteryx and Epidendrosaurus grouping with oviraptorosaurs with an equal frequency (fig. 81). A possible relationship between these two enigmatic Middle Jurassic taxa and oviraptorosaurs has been previously proposed by Xu et al. (2009), although it was not tested at the time. As discussed above, an oviraptorosaur status for these two taxa is only one step longer than the most parsimonious trees from our matrix.

Fig. 81

Strict consensus cladogram of Coelurosauria based on the Xu et al. (2011) matrix having modified the scorings for their characters 322 and 366. Percentages indicate the frequencies of occurrence in the set of most parsimonious trees for the positions indicted by the arrows.

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Fig. 82

Character changes associated with avialan shoulder girdle evolution. A, right coracoid of Deinonychus antirrhopus (YPM 5236; top) and Sinovenator changii (IVPP 12583; bottom); B, Sapeornis chaoyangensis (IVPP V13396) in ventral view; C, Jeholornis prima (IVPP V13274) in ventral view; D, Jixiangornis orientalis (CAGS uncataloged) in dorsal view.

i0003-0090-371-1-1-f82.tif

Lee and Worthy (2011) recently reanalyzed the Xu et al. (2011) dataset using maximum likelihood and Bayesian optimality criteria. They recovered Archaeopteryx at the base of Avialae, and attributed the support for this position as coming from fewer, but less homoplastic characters than those recovered under maximum parsimony as supporting a position closer to Deinonychosauria. Other parts of their recovered topology conflict significantly with the parsimony results, however, a point Lee and Worthy (2011) did not comment on. Major discrepancies include paraphyly of Tyrannosauroidea and a more derived, but stratigraphically less congruent, position for Epidexipteryx and Epidendrosaurus among Avialae. This leaves some question as to whether the differences are due to opposing resolutions of conflicting characters resulting from different optimality criteria or uneven evolutionary rates, which in this case translate to uneven character sampling across the tree. We explored this question by analyzing the data with parsimony using implied weighting (Goloboff, 1993), which downweights characters with increasing homoplasy. The results (9 best-fit trees for weight k  =  3; fit score 164.72) placed Archaeopteryx within Avialae as more derived than Epidexipteryx and Epidendrosaurus, with Xiaotingia and Anchiornis posited as basal troodontids. Overall, the results conflict less with those from the unweighted parsimony analysis than those found by model-based analysis, although a couple of taxa with copious missing data (Hagryphus, Haplocheirus) exhibited anomalous affinities.

Taken altogether, these results show that it may be premature to declare Archaeopteryx a nonavian theropod. Much hinges on the exact position of the Middle Jurassic taxa Epidendrosaurus and, in particular, Epidexipteryx. These taxa share with oviraptorosaurs and basal avians a foreshortened rostrum, highly modified mandible, and reduced, unserrated dentition. A number of the relevant traits of the dentition and mandible have recently been identified as representing a homoplastic suite of characters that correlate with herbivorous habits (Zanno and Makovicky, 2011) and evolve independently in multiple maniraptoran lineages including oviraptorosaurs and avialans. Thus, it is possible that the phylogenetic result offered by Xu et al. (2011) is driven in part by adaptive ecological traits, a fact supported by the implied weights analysis and, to a less certain degree, by the model based analyses of Lee and Worthy (2011).

It is nevertheless also clear that the mosaic of character transformations surrounding avialan origins and evolution is far more complex than was appreciated just a few years ago. Taxa such as Xiaotingia, Epidexipteryx, and others play a significant role in elucidating these patterns, whether they lie directly on the avialan lineage or occupy position immediately adjacent to it.

Assembly of the Avian Shoulder Girdle

Our phylogenetic hypothesis for basal avialans within the broad context of other paravians, and the basal placement of Sapeornis among long-tailed avialans in particular, gives insight into the sequence of morphological changes that took place to restructure the avialan shoulder girdle. Like so many other features of the “bird” body plan (feathers, reproductive biology, etc.) the pattern that emerges is one of a sequential acquisition of modern “avian” features (fig. 81).

Plesiomorphically for paravians the coracoid is subquadrangular in shape (char. 136.2) and it articulated with the scapula along a simple flat sutural surface (char. 339.2). The glenoid facet in most paravians is located dorsal to or at the same level as the acrocoracoid process ( =  biceps tubercle) (char. 347.0) and the angle between scapula and coracoid is more than 90° (char. 351.0). Additionally, most paravians possess ossified sterna (char. 457.1), but the two plates usually do not coossify (char. 128.0). Most dromaeosaurids and troodontids exhibit this shoulder girdle architecture. Moreover, the basal avialans Archaeopteryx lithographica and Sapeornis chaoyangensis exhibit this same paravian-grade suite of morphological features with the exception that apparently in these two taxa the sternum never ossifies (the structure originally identified as a sternum in the Munich Archaeopteryx specimen is in fact a coracoid [Wellnhofer and Tischlinger, 2004]). Therefore, in overall construction the basal avialan shoulder girdle shows no morphological changes associated with increased stabilization.

Although derived in many aspects of the humerus, hand, and tail, Sapeornis shows little deviation from the plesiomorphic paravian condition in its shoulder girdle construction. It is not until the Jeholornis node that the shoulder girdle begins to take on more of a modern aspect. It is at this point in avialan evolution that the coracoids nascently become strutlike in morphology (char. 136.3). A number of other changes appear in concert with the elongation of the coracoid. The scapula and coracoid articulation takes on a “ball and socket” conformation with a pit-shaped scapular cotylus developed on the coracoid and coracoidal tubercle developed on the scapula (char. 339.0). The glenoid facet of the coracoid migrates ventral to the acrocoracoid process (char. 347.1) and the angle between coracoid and scapula at the glenoid is 90° or less (char. 351.1). It is also in Jeholornis prima that we first see ossified sternal plates (as in most other paravians) (char. 457.1), although they remain unfused to each other (char. 128.0).

Several of these features are correlated with increased rigidity of the shoulder girdle. The proximodistal elongation of the coracoid marks the beginning of the element's role as a compressive strut between the wing and the sternum like is seen in modern birds (Gray, 1968; Pennycuick, 1967). Similarly, the ossification of the sternal plates is an important corollary in the increased bracing system of the shoulder girdle (given that the forces directed through the coracoid are transmitted to the sternum). Also ossified sternae distribute compressive forces to the rib cage through the sternal ribs, so all the forces are not contained in the anterior part of the thoracic cavitly. The dorsal placement of the acrocoracoid process and the reduced angle between the scapula and coracoid served to redirect the acrocoracohumeral ligament and thereby provide incipient passive shoulder stabilization (Baier et al., 2007).

We do not see fusion of the ossified sternal plate on the midline (except in apomorphic taxa like some oviraptorosaurs and alvarezsaurs) (char. 128.1) until Jixiangornis. Fusion of the sternal plates into a single sternal element further strengthens the role the sternum played in bracing the shoulder girdle and absorbing the forces redirected to it through the coracoid (by restricting ventral midline flexion).

The pattern of trait evolution at the base of Avialae highlights the sequential nature of the acquisition of “avian” shoulder girdle features among basal birds. This pattern indicates that many of the features involved in shoulder stabilization and compressive force redirection had not yet evolved in the earliest avialans like Archaeopteryx and Sapeornis. Others characters involved in this system, like the furcula, have ancient origins within theropods (Nesbitt, et al., 2009). It wasn't until Jeholornis and Jixiangornis that a bracing system like that seen in modern flyers evolved in an incipient form. This is again more evidence that the powered flight that we see in modern avians is not comparable with the “volant” activity that perhaps existed in basal avialans.

CONCLUSIONS

The morphological gap between the paravian clades has blurred to the point that basal dromaeosaurids, troodontids, and avialans are nearly indistinguishable from one another, and in life these animals would appear extremely similar. However, important morphological divisions exist that allow us to understand the basic relationship between these three clades. This study was undertaken with the intention of reviewing and revising dromaeosaurid systematics and taxonomy, supplying an extensive morphological data matrix for paravian theropods, and discussing the phylogenetic relationships derived from that data matrix. Taxon sampling within Paraves is the most exhaustive to date, but the phylogenetic hypotheses discussed herein will certainly not be the last word on paravian or coelurosaurian relationships. Indeed, changes and additions to similar data matrices are already yielding interesting results within the various clades of Coelurosauria. Moreover, the potential for new discovery that will modify these results is a given.

Paraves is an extraordinary clade with an extant diversity of nearly 10,000 species. The early radiation of this clade is first picked up in the fossil record in the Middle Jurassic and Early Cretaceous, and witnessed a plethora of taxa of remarkably similar body types within the basal members of the constituent clades Avialae, Dromaeosauridae, and Troodontidae. Because of their importance for understanding the evolution of avian flight and the rapid but divergent body size trajectories exhibited by the paravian clades, dromaeosaurids, troodontids, and avialans remain some of the more important clades for paleontological study. Extinct paravians highlight the important role fossils play in understanding the complex biology of modern organisms.

Acknowledgments

Numerous people made this project possible and deserve thanks. We are grateful for conversations with Nate Smith, Sterling Nesbitt, Paul Olsen, Diego Pol, Jack Conrad, Amy Balanoff, Gabe Bever, Randy Irmis, Greg Erickson, Jonah Choiniere, Lindsay Zanno, John Callery, Sunny Hwang, Julia Clarke, Xu Xing, Dave Krause, Cathy Forster, Phil Currie, Mick Ellison, Jason Brougham, and Carl Mehling. The beautiful photographs in this work are the masterful products of Mick Ellison. The paper benefited from a detailed critique provided by L. Zanno and the reviews of two anonymous referees. Any errors or omissions are our own.

Research for this project was supported by the American Museum of Natural History Division of Paleontology, NSF grants DEB 9300700, DEB 0608003, and ATOL 0228693, the Roosevelt Fund, Stony Brook Research Foundation, and the Jurassic Foundation. For access to collections, we would like to thank B. Battail (MNHN), S. Chapman and A. Milner (BMNH), M. Munt and S. Hutt (MIWG), J. Le Loeuff (MDE), X. Xu and Z. Zhou (IVPP), R. Barsbold (IGM), P. Sereno (University of Chicago), W. Joyce (YPM), C. Forster and D. Krause (UA), J. Calvo and J. Porifiri (MUCP), R. Coria and R. Garrido (MCF), R. Garrido and S. Cocca (MOZ), F. Novas and A. Kramarz (MACN), J. Gardner (TMP), R. Scheetz (BYU), J. Bartlett (CEUM), and M. Getty and L. Zanno (UMNH).

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