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To determine the quality of fossil preservation in the early Eocene Fossil Butte Member (FBM) of the Green River Formation in Fossil Basin, Wyoming, we excavated 1,133 fishes from the upper 30 cm of the 45.7-cm-thick FBM. Each fossil was evaluated for its relative skeletal articulation, ranging from near perfect articulation to almost complete disarticulation, and placed into one of four stages. About 70% of the fishes have near-perfect articulation of the skeleton. Ninety-seven percent of the specimens in the sample belong to five of 19 genera known from the FBM: †Knightia, †Diplomystus, †Cockerellites, †Mioplosus, and †Priscacara. Finally, 97% of the specimens belong to four of 15 families known to be present in the FBM: †Paraclupeidae, Clupeidae, Latidae, and Moronidae.
This chapter is a critical review of the problems presented by our current knowledge of the early fossil record of tetrapods. This record is marked by two major features. The first is a significant absence of preserved fossils between the Upper Devonian and the end of the Lower Carboniferous: Romer's Gap. The second is that the numerous lineages that appear at the end of the Lower Carboniferous are diverse and distinct from one another and do not present features that permit the confident assignment of relationships either to the Devonian taxa or among the several lineages themselves. Furthermore, convergence in anatomical characteristics is common. Phylogenetic systematics, also termed cladistics, currently plays a major role in the analysis of relationships and patterns of evolution among vertebrates. This mode of analysis does not consider the relative incompleteness of the fossil record, the relative frequency of convergence in the evolution of character changes that may occur independently in more than a single major lineage, development, or body function, none of which can be readily categorized with standard data matrices. These problems in the available data and its mode of analysis are discussed in the context of the early stages in the evolution of terrestrial vertebrates.
We report the discovery of an early tetrapod skull from the St. Louis Limestone of Missouri, USA. It was found among a collection of coelacanths in the Museum für Naturkunde in Berlin, Germany, part of a larger collection donated to that institution by Jaekel containing other fish fossils from the same locality. The exact locality remains uncertain, but sedimentological analysis suggests that the specimens derive from the lower or middle part of the Upper St. Louis Limestone. The lithology is consistent with a deeper water marine setting, suggesting that the tetrapod specimen is an erratic. The skull is a natural mold exposed in palatal view, showing good detail of the bones preserved. Phylogenetic analysis shows it to be most closely related to colosteids, though retaining some plesiomorphic characters. Stratigraphic correlation shows that the St. Louis Limestone is older than the Verdi and Waugh Members of Iowa, here assigned to the Ste. Genevieve Formation, from which other colosteid specimens and Whatcheeria were found. The new specimen is thus both the earliest post-Devonian tetrapod from North America, and also the oldest colosteid-like tetrapod known.
Temnospondyls—a major component of Permian and Carboniferous terrestrial ecosystems—display great diversity in skull shapes and proportions. To quantify and interpret this diversity, we conducted a geometric morphometric analysis using 45 landmarks on the dorsal skull surface of 90 species with well-represented cranial material. Results show a correlation between morphospace occupation and phylogenetic proximity of taxa for trees in which dvinosaurs and dissorophoids are sister groups and join an edopoid-–eryopoid–basal archegosauriform clade. Most large groups of Carboniferous and Permian temnospondyls occupy specific areas of morphospace. Nearest-neighbor analyses reveal significantly greater taxon clustering than expected under either uniform or Gaussian null models. Size correlates strongly with shape across the whole data set, highlighting the association of some features (short, broad snout; large orbits) with small size. A significant relationship between size and shape is not observed in clades such as branchiosaurids, dvinosaurs, and olsoniforms (the clade encompassing trematopids plus dissorophids). This suggests that evolutionary allometry patterns vary across temnospondyls. In the case of branchiosaurids, this pattern may be explained by the similar sizes and relatively conservative morphologies of the constituent species. Distance-based disparity measures indicate that edopoids, eryopoids, and basal archegosauriforms make the largest contributions to total disparity (reflecting the peripheral locations of some of the constituent taxa in morphospace), whereas amphibamids, dissorophids, and trematopids make the smallest contributions. Disparity correlates strongly with diversity within groups, suggesting that skull shape was not subject to character state exhaustion (decrease or cessation in the acquisition of novel morphological conditions during clade evolution). The Kasimovian, Roadian, Wordian, and Changhsingian are time intervals of high disparity despite their low diversity. We hypothesize that this pattern stems from the fact that times of high diversity are characterized by larger areas of morphospace, which results in mean shapes being located relatively close to the grand mean; in contrast, mean shapes for low-diversity stages are based on incomplete morphospace samples. Many of these patterns are similar to those observed in previous analyses of stereospondyls, suggesting that similar controls on skull shape may have operated throughout the history of temnospondyls.
The inner ear is a complex structure consisting of the vestibular and auditory systems. Across vertebrates, morphological variation in the inner ear provides a source of homologous features (characters) that may aid in resolving phylogenetic relationships. The morphology of the inner ear in extant frogs and salamanders is well known, and has been extensively studied from functional perspectives. However, the ability of its form and features to shed light on the broader question of lissamphibian origins and relationships has not been as thoroughly explored. Herein we review the morphology of the inner ear of the least well-known lissamphibian group, the caecilians, and present three-dimensional reconstructions of otic capsule endocasts and of soft-tissue labyrinths. We use these data to explore previous statements about the structure of the caecilian inner ear and its evolutionary significance. The postulate that the periotic canal has a posterior path is corroborated, and the periotic sacs of each ear are observed to extend into the brain cavity, where they are applied to a fluid-filled compartment that is located ventral to the brain. These features are shared with frogs and salamanders. Additionally, it is hypothesized that the regression of two endorgans in caecilians is correlated with the secondary loss of the two middle ear auditory pathways, the tympanum–stapes and opercularis hearing pathway, suggesting that the lissamphibian-type ear is present, but in a derived state in caecilians. Identification of osteological correlates of this lissamphibian-type ear permits the interpretation of the evolution of this distinct ear type in the context of the three competing hypotheses of lissamphibian phylogeny. The distribution of traits is shown to be most parsimoniously explained when optimized onto the phylogenetic pattern that incorporates a monophyletic temnospondyl-derived Lissamphibia. This interpretation is consistent with a single origin of a lissamphibian-type tympanic ear. Therefore, characters of the ear seemingly provide synapomorphies that unite lissamphibians with amphibamid temnospondyls, potentially improving the resolution of concepts about the affinities of frogs, salamanders, and caecilians and clarifying issues of tetrapod ear evolution.
The Lower Triassic amphibians Triadobatrachus massinoti and Czatkobatrachus polonicus are universally regarded as stem anurans. However, there is still uncertainty about whether or not they were capable of jumping like true anurans as their postcranial features have so far provided only equivocal evidence. Although previous work has concentrated on the anatomy of the hind limb, here we examine the anatomy of the forelimb, comparing stem and crown-group anurans to other amphibians. The forelimb and pectoral girdle of Triadobatrachus share several features with modern frogs, including a frog-like deltoid attachment of the scapula. Though the radius and ulna are unfused, the humerus is similar to those of modern anurans. The deltopectoral crest is slightly elongated and a lateral deflection of the ventral edge of the crest creates a concavity on the lateral face of the humerus. These features are uncommon in most tetrapod groups characterized by a sprawling stance but are typical of modern anurans, in which the anterior chest musculature is enlarged. Our findings indicate that the importance of the deltoid had increased relative to that of the pectoralis muscle in Triadobatrachus. Czatkobatrachus is somewhat less similar to modern jumping anurans in its deltopectoral crest and pectoral girdle. Most anurans extend the forelimbs forward during a jump and land on their forefeet, perhaps accounting for the enlargement of the deltoid and the orientation of the deltopectoral crest. This type of behavior during landing is not seen in Ascaphus and Leiopelma but several features of these two genera, such as the fusion of the radio-ulna, the forearm musculature, and the degree of medial rotation of the manus, indicate that their landing behavior may be derived rather than primitive. Overall, Triadobatrachus was certainly not as capable of long jumps as some modern anurans, yet its anatomy does suggest that jumping or hopping was part of its locomotor repertoire.
A new coelurosaurian theropod, Alnashetri cerropoliciensis, is reported here based on articulated hind limbs of a single individual discovered at the locality of La Buitrera (Candeleros Formation, Cenomanian–Turonian), Río Negro Province, Argentina. The new taxon differs from other coelurosaurs in the possession of a low ridge that separates the rostral tibial surface from the outer face of the lateral malleolus, and which extends proximally beyond the tip of the ascending process of the astragalus, and in the possession of ventral notches on the hemicondyles of the distal articulations on pedal phalanges III-1 and III-2. Alnashetri is easily distinguished from the dromaeosaurid Buitreraptor, the only other known small theropod from La Buitrera. Phylogenetic analysis supports alvarezsauroid affinities. The evidence supporting this relationship comes from the detailed anatomy of the ankle, however, and this concentration of character support within a single anatomical region may bias our results. If our proposed phylogenetic placement is accurate, Alnashetri antedates all other Argentinian alvarezsaurids and indicates that alvarezsaurids were present in the Neuquén Basin throughout the entire Late Cretaceous.
On some morphology-based phylogenies of extant snakes the capacity to ingest prey of a diameter larger than the snake's head optimizes as a derived condition of macrostomatan snakes such as boas and pythons. The evolution of macrostomatan jaw mechanics can be traced in the more basal scolecophidian and anilioid snakes, such as blind snakes, thread snakes, pipe snakes, and shield tails. Several recent morphology-based phylogenetic analyses of snake interrelationships including fossil snakes have placed fossil taxa of large body size and/or with a macrostomatan skull structure basal to either all extant snakes, or basal to the Alethinophidia (Anilioidea plus Macrostomata, excluding Scolecophidia). This has led to the characterization of scolecophidians and/or anilioids as “regressed macrostomatans”. These snakes would have lost their macrostomatan feeding capacities in adaptation to a fossorial or secretive mode of life, correlated in some forms such as scolecophidians and uropeltines with miniaturization. However, the characterization of scolecophidians and/or anilioids as regressed macrostomatans is not only a matter of character optimization on a phylogeny, but is also incompatible with morphological and physiological aspects of feeding mechanics in snakes.
Hyperelongate neural spines forming a prominent dorsal “sail” are known in eight genera distributed between two families of pelycosaurian-grade synapsids. Although the function(s) of the sail remain disputed, most researchers assume that resilient soft tissue stretched between the elongate neural spines, extending to the distal tips. Hypotheses to explain the purpose of the sail have included thermoregulation (Romer & Price, 1940; Bramwell & Fellgett, 1973; Haack, 1986; Tracy et al., 1986; Bennett, 1996; Florides et al., 1999) and sexual selection (Tomkins et al., 2010). In this paper, we analyze the natural pathologies found in the neural spines of a very large pelycosaur, Dimetrodon giganhomogenes, as a natural experiment: What would ensue in the event of sail breakage and what does that tell us about sail structure, development, maintenance, and the orientation of the sail?
A series of seven associated neural spines from fmnh UC 1134 demonstrate subtle though distinctly abnormal rugosities, a sign most often indicative of a well-healed hard callus of bone fracture. Microstructural examination revealed surprising facts: not only did the abnormal bone areas prove NOT to be fracture hard callus, but the abnormal tissue reflected underlying material failure resulting from slippage between adjacent lamellae of bone. Moreover, the characteristic cranial and caudal orientation of the deep longitudinal grooves contributing to the classic dimetrodont figure-8 spine cross section was rapidly reestablished in vivo by a combination of osteoclastic resorption and additional lamellar deposition of bone to regain the “correct” pre-injury orientation, underscoring the architectural importance of the dumbbell shape in resisting lateral bending. This bone disruption and repair occurred at least five seasons before death, which explains the well-healed external appearance of the lesions. The absence of vascular communicating canals casts doubt on the widely held hypothesis that these grooves contained blood vessels that supplied a thermoregulatory sail. Furthermore, the distal morphology of spines in more complete specimens, including the type fmnh UC 112 and omnh 01727, suggests that the dorsal margin of the sail was located well proximal to the tips of the elongate neural spines. The cross-sectional architecture of the spines suggests a further hypothesis: that the proximal portion of the sail may have also functioned as an energy storage device, facilitating fast locomotion in this top predator.
New craniodental material of the traversodontid Dadadon isaloi from Middle/Upper Triassic basal “Isalo II” beds of southwestern Madagascar is described. These specimens reveal several new autapomorphies of Dadadon, including paired foramina on the frontal near the anterior border of the postorbital and lower incisors with denticulated distal margins. The new material covers a broad size range, providing the first information on ontogeny in Dadadon. Larger (presumably older) specimens of Dadadon isaloi have more postcanine teeth, relatively longer, narrower snouts, and a higher degree of cranial ornamentation than smaller specimens. Postcanine replacement in Dadadon was similar to that of other traversodontids: new teeth erupted at the posterior end of the postcanine tooth row and moved forward. Using information from the new specimens, the position of Dadadon was tested in a new phylogenetic analysis of traversodontids. In the new analysis, Dadadon is strongly supported as a member of a clade also including the South American taxa Massetognathus and Santacruzodon, here named Massetognathinae subfam. nov. This clade is diagnosed by the presence of denticulated lower incisors, relatively small canines, three cusps in the labial margin of the upper postcanines, and low, flat skulls. Massetognathinae is the sister-group of Gomphodontosuchinae, which includes Gomphodontosuchus, Menadon, Protuberum, Exaeretodon, and Scalenodontoides. The Laurasian traversodontids (Arctotraversodon, Boreogomphodon, and Nanogomphodon) form a clade that is the sister-taxon of Massetognathinae Gomphodontosuchinae. Denticulated incisors evolved multiple times in traversodontid evolution (in massetognathines and Arctotraversodon), and thus this group represents another possibility (besides various archosauromorphs) to be considered when attempting to identify isolated Triassic teeth with denticulated carinae lacking cingula.
The question of the adaptive basis for the origin of mammalian endothermy remains unresolved despite a great deal of research effort. Controversy continues over which physiological adaptations were of greatest importance in starting ectothermic nonmammalian synapsids of the Late Paleozoic on the path that culminated in modern endothermic mammals. Models of the selective basis for the origin of endothermy fall into two main categories: “thermoregulation first” and “aerobic capacity first.” Studies of lizards show a dichotomy between a low-energy “sit-and-wait” (SW) foraging mode in Iguania and a more energy-intensive “widely foraging” (WF) mode in Autarchoglossa. It is proposed that in the transition from basal synapsids (“pelycosaurs”) to therapsids, a shift from the primitive SW mode to the WF mode put the ancestors of mammals on the path to increased aerobic capacity and the ability to sustain high levels of foraging activity. Selection for increased energy expenditure disproportionately increased the amount of food energy consumed, thus improving foraging efficiency. A shift from reliance on anaerobic muscle metabolism for short but rapid dashes to capture prey to a reliance on aerobic metabolism for active searching for prey necessitated improvements of the cardiovascular system and lungs for increased aerobic capacity and greater stamina. Over time, therapsids became locked into high food requirements, which selected for improvements in aerobic metabolism, locomotor and food-processing ability, and neurosensory/behavioral specializations. Evidence of a link between maximum activity metabolism and resting (basal) metabolism in anurans and rodents suggests that further increases in aerobic activity metabolism required an increased basal metabolic rate, which led to high body temperatures and, ultimately, homeothermy. Therapsids show adaptations for increased activity, greater food-getting and food-processing ability, and higher metabolic rates than basal synapsids (“pelycosaurs”). It is argued that the “foraging mode” model is preferable to the “parental care” model of Farmer and the “correlated progression” model of Kemp for understanding the origin of mammalian endothermy.