Sterling Nesbitt
Virginia Tech, Geosciences, Faculty Member
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Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan–batrachian molecular divergence1 suggests a major gap in the record... more
Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan–batrachian molecular divergence1 suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period2–6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls7–10, but the absence of pre-Jurassic period caecilian fossils11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial1,8,13,14. Here we report the geologically oldest stem caecilian—a crown lissamphibian from the Late Triassic epoch of Arizona, USA—extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the ...
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Non-archosaur archosauromorphs are a paraphyletic group of diapsid reptiles that were important members of global Middle and Late Triassic continental ecosystems. Included in this group are the azendohsaurids, a clade of allokotosaurians... more
Non-archosaur archosauromorphs are a paraphyletic group of diapsid reptiles that were important members of global Middle and Late Triassic continental ecosystems. Included in this group are the azendohsaurids, a clade of allokotosaurians (kuehneosaurids and Azendohsauridae + Trilophosauridae) that retain the plesiomorphic archosauromorph postcranial body plan but evolved disparate cranial features that converge on later dinosaurian anatomy, including sauropodomorph-like marginal dentition and ceratopsian-like postorbital horns. Here we describe a new malerisaurine azendohsaurid from two monodominant bonebeds in the Blue Mesa Member, Chinle Formation (Late Triassic, ca. 218–220 Ma); the first occurs at Petrified Forest National Park and preserves a minimum of eight individuals of varying sizes, and the second occurs near St. Johns, Arizona. Puercosuchus traverorum n. gen. n. sp. is a carnivorous malerisaurine that is closely related to Malerisaurus robinsonae from the Maleri Formatio...
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Archosauromorph reptiles underwent rapid lineage diversification, increases in morphological and body size disparity, and expansion into new adaptive landscapes. Several of the primary early archosauromorph clades (e.g. rhynchosaurs) are... more
Archosauromorph reptiles underwent rapid lineage diversification, increases in morphological and body size disparity, and expansion into new adaptive landscapes. Several of the primary early archosauromorph clades (e.g. rhynchosaurs) are easy to differentiate from others because of their characteristic body types, whereas the more lizard‐like and carnivorous forms with long necks (e.g. tanystropheids) were historically all relegated to the groups Protorosauria or Prolacertiformes. However, it is now clear that these groups are polyphyletic and that a lizard‐like, carnivorous form is plesiomorphic for Archosauromorpha, and multiple subclades started with that body plan. Among these early forms is Malerisaurus from the Upper Triassic of India (M. robinsonae) and the Upper Triassic of south‐western USA (M. langstoni). In this paper, we critically re‐evaluate the genus. We find both species of Malerisaurus as valid, and identify Malerisaurus as an early diverging, but late‐surviving, carnivorous member of Azendohsauridae within Allokotosauria. Our histological analysis and assessment of ontogenetic changes of limb bones of small and large individuals demonstrate that the skeletons of the small forms grew slowly and became more robust through ontogeny, and that the larger recovered bones are at or near the maximum size of the taxon. Malerisaurus and Malerisaurus‐like taxa were common members of the Otischalkian–Adamanian (late Carnian to mid‐Norian) faunal assemblages from Upper Triassic strata of the south western USA, but they are absent from the younger Revueltian holochronozone. Specimens from western North America show that Allokotosauria had a near‐Pangaean distribution for much of the Middle Triassic to Late Triassic.
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The radiation of archosauromorph reptiles in the Triassic Period produced an unprecedented collection of diverse and disparate forms with a mix of varied ecologies and body sizes. Some of these forms were completely unique to the... more
The radiation of archosauromorph reptiles in the Triassic Period produced an unprecedented collection of diverse and disparate forms with a mix of varied ecologies and body sizes. Some of these forms were completely unique to the Triassic, whereas others were converged on by later members of Archosauromorpha. One of the most striking examples of this is with Triopticus primus, the early dome‐headed form later mimicked by pachycephalosaurid dinosaurs. Here we fully describe the cranial anatomy of Triopticus primus, but also recognize a second dome‐headed form from a Upper Triassic deposit in present‐day India. The new taxon, Kranosaura kuttyi gen. et sp. nov., is likely the sister taxon of Triopticus primus based on the presence of a greatly expanded skull roof with a deep dorsal opening (possibly the pineal opening) through the dome, similar cranial sculpturing, and a skull table that is expanded more posterior than the posterior extent of the basioccipital. However, the dome of Kranosaura kuttyi gen. et sp. nov. extends anterodorsally, unlike that of any other archosauromorph. Histological sections and computed tomographic reconstructions through the skull of Kranosaura kuttyi gen. et sp. nov. further reveal the uniqueness of the dome of these early archosauromorphs. Moreover, our integrated analysis further demonstrates that there are many ways to create a dome in Amniota. The presence of ‘dome‐headed’ archosauromorphs at two localities on the western and eastern portions of Pangea suggests that these archosauromorphs were widespread and are likely part of more assemblages than currently recognized.
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Research Interests: Evolutionary Biology, Geology, Paleontology, Biology, Osteology, and 3 moreEcology, Skull, and Holotype
The relationship between dinosaurs and other reptiles is well established, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives... more
The relationship between dinosaurs and other reptiles is well established, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies or are known from poorly preserved or incomplete material. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after ...
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Following the end-Permian extinction, terrestrial vertebrate diversity recovered by the Middle Triassic, and that diversity was now dominated by reptiles. However, those reptilian clades, including archosaurs and their closest relatives,... more
Following the end-Permian extinction, terrestrial vertebrate diversity recovered by the Middle Triassic, and that diversity was now dominated by reptiles. However, those reptilian clades, including archosaurs and their closest relatives, are not commonly found until ~30 million years post-extinction in Late Triassic deposits despite time-calibrated phylogenetic analyses predicting an Early Triassic divergence for those clades. One of these groups from the Late Triassic, Phytosauria, is well known from a near-Pangean distribution, and this easily recognized clade bears an elongated rostrum with posteriorly retracted nares and numerous postcranial synapomorphies that are unique compared with all other contemporary reptiles. Here, we recognize the exquisitely preserved, nearly complete skeleton of Diandongosuchus fuyuanensis from the Middle Triassic of China as the oldest and basalmost phytosaur. The Middle Triassic age and lack of the characteristically-elongated rostrum fill a critic...
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Reptiles have a long history of transitioning from terrestrial to semi-aquatic or aquatic environments that stretches back at least 250 million years. Within Archosauria, both living crocodylians and birds have semi-aquatic members.... more
Reptiles have a long history of transitioning from terrestrial to semi-aquatic or aquatic environments that stretches back at least 250 million years. Within Archosauria, both living crocodylians and birds have semi-aquatic members. Closer to the root of Archosauria and within the closest relatives of the clade, there is a growing body of evidence that early members of those clades had a semi-aquatic lifestyle. However, the morphological adaptations to a semi-aquatic environment remain equivocal in most cases. Here, we introduce a new Middle Triassic (245-235 Ma) archosauriform, Litorosuchus somnii, gen. et sp. nov., based on a nearly complete skeleton from the Zhuganpo Member (Ladinian [241-235 Ma]) of the Falang Formation, Yunnan, China. Our phylogenetic analyses suggest that Litorosuchus is a stem archosaur closely related to the aberrant Vancleavea just outside of Archosauria. The well-preserved skeleton of L. somnii bears a number of morphological characters consistent with oth...
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Rauisuchids are large (2–6 m in length), carnivorous, and quadrupedal pseudosuchian archosaurs closely related to crocodylomorphs. Though geographically widespread, fossils of this clade are relatively rare in Late Triassic assemblages.... more
Rauisuchids are large (2–6 m in length), carnivorous, and quadrupedal pseudosuchian archosaurs closely related to crocodylomorphs. Though geographically widespread, fossils of this clade are relatively rare in Late Triassic assemblages. The middle Norian (∼212 Ma) Hayden Quarry of northern New Mexico, USA, in the Petrified Forest Member of the Chinle Formation, has yielded isolated postcranial elements and associated skull elements of a new species of rauisuchid.Vivaron haydenigen. et. sp. nov. is diagnosed by the presence of two posteriorly directed prongs at the posterior end of the maxilla for articulation with the jugal. The holotype maxilla and referred elements are similar to those of the rauisuchidPostosuchus kirkpatrickifrom the southwestern United States, butV. haydenishares several maxillary apomorphies (e.g., a distinct dropoff to the antorbital fossa that is not a ridge, a straight ventral margin, and a well defined dental groove) with the rauisuchidTeratosaurus suevicus...
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Research Interests: History, Zoology, Art, Biology, Conflation, and 4 morePhylogenetic Systematics, Misrepresentation, Auk, and Avialae
Research Interests: Geology and Paleontology
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Triassic predatory guild evolution reflects a period of ecological flux spurred by the catastrophic end-Permian mass extinction and terminating with the global ecological dominance of dinosaurs in the early Jurassic. In responding to this... more
Triassic predatory guild evolution reflects a period of ecological flux spurred by the catastrophic end-Permian mass extinction and terminating with the global ecological dominance of dinosaurs in the early Jurassic. In responding to this dynamic ecospace, terrestrial predator diversity attained new levels, prompting unique trophic webs with a seeming overabundance of carnivorous taxa and the evolution of entirely new predatory clades. Key among these was Crocodylomorpha, the largest living reptiles and only one of two archosaurian lineages that survive to the present day. In contrast to their existing role as top, semi-aquatic predators, the earliest crocodylomorphs were generally small-bodied, terrestrial faunivores, occupying subsidiary (meso) predator roles. Here we describe Carnufex carolinensis a new, unexpectedly large-bodied taxon with a slender and ornamented skull from the Carnian Pekin Formation (~231 Ma), representing one of the oldest and earliest diverging crocodylomor...
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Archosauria and their closest relatives, the non-archosaurian archosauriforms, diversified in the Early and Middle Triassic, soon after the end-Permian extinction. This diversification is poorly documented in most Lower and Middle... more
Archosauria and their closest relatives, the non-archosaurian archosauriforms, diversified in the Early and Middle Triassic, soon after the end-Permian extinction. This diversification is poorly documented in most Lower and Middle Triassic rock sequences because fossils of early groups of archosauriforms are relatively rare compared to those of other amniotes. The early Middle Triassic (? late Anisian) Manda beds of southwestern Tanzania form an exception, with early archosaur skeletons being relatively common and preserved as articulated or associated specimens. The Manda archosaur assemblage is exceptionally diverse for the Middle Triassic. However, to date, no non-archosaurian archosauriforms have been reported from these rocks. Here, we name a new taxon, Asperoris mnyama gen. et sp. nov., from the Manda beds and thoroughly describe the only known specimen. The specimen consists of a well-preserved partial skull including tooth-bearing elements (premaxilla, maxilla), the nasal, p...