Sterling Nesbitt

Virginia Tech (Virginia Polytechnic Institute and State University) | VT · Department of Geosciences

Placerias hesternus, a Late Triassic dicynodont, is one of the last megafaunal synapsids of the Mesozoic. The species has a tusk-like projection on its maxillary bone, known as the caniniform process. This process has been hypothesized to be sexually dimorphic since the 1950s, however this claim has not been thoroughly investigated quantitatively....

A vast array of pseudosuchian body plans evolved during the diversification of the group in the Triassic Period, but few can compare to the toothless, long‐necked, and bipedal shuvosaurids. Members of this clade possess theropod‐like character states mapped on top of more plesiomorphic pseudosuchian character states, complicating our understanding...

Proterosuchidae represents the oldest substantial diversification of Archosauromorpha and plays a key role in understanding the biotic recovery after the end-Permian mass extinction. Proterosuchidae was long treated as a wastebasket taxon, but recent revisions have reduced its taxonomic content to five valid species from the latest Permian of Russi...

Scleromochlus taylori is one of the most enigmatic members of the herpetofauna from the Lossiemouth Sandstone Formation (Upper Triassic) of Elgin (Moray, Scotland). For many years it was thought to be closely related to pterosaurs and dinosaurs, but the anatomy of this animal is difficult to interpret because of the notoriously poor preservation of...

The anatomy of the braincase and associated soft tissues of the lagerpetid Dromomeron gregorii (Archosauria: Avemetatarsalia) from the Late Triassic of the United States is here described. This corresponds to the first detailed description of cranial materials of Lagerpetidae, an enigmatic group of Late Triassic (c. 236–200 Million years ago) anima...

New specimens and the reassessment of many silesaurids have recently shed light on the origin and early evolution of dinosaurs and their close relatives. Yet, the group is relatively poorly represented in South America, an area that likely played an important role in dinosaurian origins. Since the discovery of Sacisaurus agudoensis from the Norian...

Dinosaurs and pterosaurs have remarkable diversity and disparity through most of the Mesozoic Era1–3. Soon after their origins, these reptiles diversified into a number of long-lived lineages, evolved unprecedented ecologies (for example, flying, large herbivorous forms) and spread across Pangaea4,5. Recent discoveries of dinosaur and pterosaur pre...

Understanding the evolution of the earliest avemetatarsalian (bird-line) archosaurs and inferring the morphology of the last common ancestor of Archosauria are hampered by a poor fossil record in critical temporal intervals. Here we describe an early-diverging avemetatarsalian from the ?Earliest Late Triassic (~235 Ma) ‘basal Isalo II’/Makay Format...

Documenting evidence of feeding behavior in extinct vertebrates is crucial to understanding trophic structure and stability of ecosystems over periods of Earth's history. Direct evidence of trophic interactions in the fossil record is rare, but proxies like dental microwear enable testable hypotheses of feeding behavior in extinct taxa. Here we pre...

Mass extinctions change the trajectory of evolution and restructure ecosystems. The largest mass extinction, the end-Permian, is a particularly interesting case due to the hypothesized delay in the recovery of global ecosystems, where total trophic level recovery is not thought to have occurred until 5-9 million years after the extinction event. Di...

The femora of diapsids have undergone morphological changes related to shifts in postural and locomotor modes, such as the transition from plesiomorphic amniote and diapsid taxa to the apomorphic conditions related to a more erect posture within Archosauriformes. One remarkable clade of Triassic diapsids is the chameleon-like Drepanosauromorpha. Th...

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 fin...

Background An ontogenetic niche shift in vertebrates is a common occurrence where ecology shifts with morphological changes throughout growth. How ecology shifts over a vertebrate’s lifetime is often reconstructed in extant species—by combining observational and skeletal data from growth series of the same species—because interactions between organ...

Significant evolutionary shifts in locomotor behaviour often involve comparatively subtle anatomical transitions. For dinosaurian and avian evolution, medial overhang of the proximal femur has been central to discussions. However, there is an apparent conflict with regard to the evolutionary origin of the dinosaurian femoral head, with neontologica...

Pterosaurs, the first vertebrates to evolve powered flight, were key components of Mesozoic terrestrial ecosystems from their sudden appearance in the Late Triassic until their demise at the end of the Cretaceous1–6. However, the origin and early evolution of pterosaurs are poorly understood owing to a substantial stratigraphic and morphological ga...

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 postcrania...

Exceptionally preserved fossils retain soft tissues and often the biomolecules that were present in an animal during its life. The majority of terrestrial vertebrate fossils are not traditionally considered exceptionally preserved, with fossils falling on a spectrum ranging from very well-preserved to poorly preserved when considering completeness,...

The vertebrate lineages that would shape Mesozoic and Cenozoic terrestrial ecosystems originated across Triassic Pangaea1–11. By the Late Triassic (Carnian stage, ~235 million years ago), cosmopolitan ‘disaster faunas’ (refs. 12–14) had given way to highly endemic assemblages12,13 on the supercontinent. Testing the tempo and mode of the establishme...

Living birds (Aves) have bodies substantially modified from the ancestral reptilian condition. The avian pelvis in particular experienced major changes during the transition from early archosaurs to living birds1,2. This stepwise transformation is well documented by an excellent fossil record2–4; however, the ontogenetic alterations that underly it...

The Manda Beds of southwest Tanzania have yielded key insights into the early evolutionary radiation of archosaurian reptiles. Many key archosaur specimens were collected from the Manda Beds in the 1930s and 1960s, but until recently, few of these had been formally published. Here, we describe an archosaur specimen collected in 1963 which has previ...

Placerias hesternus is a Late Triassic dicynodont, and one of the last megafaunal synapsids of the Mesozoic. The species has tusk-like projections of maxillary bone, known as caniniform processes, that have been speculated to be sexually dimorphic since the 1950s, but this claim has never been thoroughly investigated quantitatively. This study exam...

Red to red-orange spheres in the vascular canals of fossil bone thin sections have been repeatedly reported using light microscopy. Some of these have been interpreted as the fossilized remains of blood cells or, alternatively, pyrite framboids. Here, we assess claims of blood cell preservation within bones of the therizinosauroid theropod Beipiaos...

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 c...

Pseudosuchians, archosaurian reptiles more closely related to crocodylians than to birds, exhibited high morphological diversity during the Triassic with numerous examples of morphological convergence described between Triassic pseudosuchians and post‐Triassic dinosaurs. One example is the shuvosaurid Effigia okeeffeae which exhibits an “ostrich‐li...

Doswellia kaltenbachi is a long-snouted non-archosaur archosauriform known from the Late Triassic of the United States (Virginia and Texas). New material from the Chinle Formation of Arizona represents the first occurrence of D. cf. D. kaltenbachi from that formation and state. This occurrence is from the type section for the Adamanian estimated ho...

The evolutionary history of archosaurs and their closest relatives is characterized by a wide diversity of locomotor modes, which has even been suggested as a pivotal aspect underlying the evolutionary success of dinosaurs vs. pseudosuchians across the Triassic–Jurassic transition. This locomotor diversity (e.g., more sprawling/erect; crouched/upri...

Once known solely from dental material and thought to represent an early ornithischian dinosaur, the early‐diverging pseudosuchian Revueltosaurus callenderi is described from a minimum of 12 skeletons from a monodominant bonebed in the upper part of the Chinle Formation of Arizona. This material includes nearly the entire skeleton and possesses a c...

The Placerias/Downs’ Quarry complex in eastern Arizona, USA, is the most diverse Upper Triassic vertebrate locality known. We report a new short-faced archosauriform, Syntomiprosopus sucherorum gen. et sp. nov., represented by four incomplete mandibles, that expands that diversity with a morphology unique among Late Triassic archosauriforms. The mo...

Dinosaurs possess a form of tooth attachment wherein an unmineralized periodontal ligament suspends each tooth within a socket, similar to the condition in mammals and crocodylians. However, little information is known about tooth attachment and implantation in their close relatives, the silesaurids. We conducted a histological survey of several si...

Temnospondyl specimens collected recently in the Middle-?Late Triassic of the Ruhuhu (Tanzania) and Luangwa (Zambia) rift basins are described and figured. They are attributed to cf. Cherninia megarhina (Chernin & Cosgriff, 1975), Mastodonsauroidea indet., Stereospondyli indet., and cf. Stereospondyli, as well as intercentra of small adult individu...

Synopsis Allometry—patterns of relative change in body parts—is a staple for examining how clades exhibit scaling patterns representative of evolutionary constraint on phenotype, or quantifying patterns of ontogenetic growth within a species. Reconstructing allometries from ontogenetic series is one of the few methods available to reconstruct growt...

Living archosaurs (birds and crocodylians) have disparate locomotor strategies that evolved since their divergence ∼250 mya. Little is known about the early evolution of the sensory structures that are coupled with these changes, mostly due to limited sampling of early fossils on key stem lineages. In particular, the morphology of the semicircular...

Morphology forms the most fundamental level of data in vertebrate palaeontology because it is through interpretations of morphology that taxa are identified, creating the basis for broad evolutionary and palaeobiological hypotheses. Assessing maturity is one of the most basic aspects of morphological interpretation and provides the means to study t...

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 th...

Biomolecules preserved in fossils are expanding our understanding of the biology and evolution of ancient animals. Molecular taphonomy seeks to understand how these biomolecules are preserved and how they can be interpreted. So far, few studies on molecular preservation have considered burial context to understand its impact on preservation or the...

Pterosaurs were the first vertebrates to evolve powered flight¹ and comprised one of the main evolutionary radiations in terrestrial ecosystems of the Mesozoic era (approximately 252–66 million years ago), but their origin has remained an unresolved enigma in palaeontology since the nineteenth century2–4. These flying reptiles have been hypothesize...

The Lamy Quarry (= the Gunter bonebed) is known for its extensive accumulation of temnospondyl skulls and skeletons from the Norian (Upper Triassic) Garita Creek Formation, south of Lamy, New Mexico. Although the quarry is monodominant for metoposaurids, reptile fossils are also present. The reptile material, briefly described and identified to lea...

The Late Triassic fauna of the Lossiemouth Sandstone Formation (LSF) from the Elgin area, Scotland, has been pivotal in expanding our understanding of Triassic terrestrial tetrapods. Frustratingly, due to their odd preservation, interpretations of the Elgin Triassic specimens have relied on destructive moulding techniques, which only provide incomp...

The strata of the American Southwest, particularly the Chinle Formation and the Dockum Group, are critical to our understanding of faunal diversity and evolution in the Late Triassic. In recent decades, these strata have informed the evolution of close dinosaur relatives, which remain poorly sampled and enigmatic in their geographic distribution. H...

Significance Reptiles of the Mesozoic Era are known for their remarkable size: dinosaurs include the largest known land animals, and their relatives, the pterosaurs, include the largest creatures to ever fly. The origins of these groups are poorly understood, however. Here, we present a species ( Kongonaphon kely ) from the Triassic of Madagascar c...

The Triassic Period (252–201.5 Ma) records a great expansion of saurian diversity and disparity, particularly in skull morphology. Stem archosaurs exhibit substantial cranial disparity, especially by taxa either shortening or elongating the skull. This disparity is exemplified in the North American Late Triassic proterochampsians by the ‘short-face...

Allokotosauria, including Trilophosauridae and Azendohsauridae, is an extinct archosauromorph group that reached a near-Pangean distribution in the Middle Triassic to Late Triassic and evolved a broad range of cranial and dental morphologies. Within the Chinle Formation of western North America, allokotosaurs span the Norian-aged Blue Mesa Member (...

Ornithosuchidae is one of the most enigmatic clades of Triassic pseudosuchians. The group is composed by three carnivorous species that were excavated from Upper Triassic beds of Scotland and Argentina. We describe the first ornithosuchid from the Upper Triassic sediments of Brazil and explore its phylogenetic affinities and implications for the ev...

The sternum is a functionally important but poorly understood component of the pectoral girdle in both living and extinct dinosaurs. When ossified, the dinosaur sternum consists of paired plates situated just posterior to the clavicles and scapulocoracoid. In Avialae, the sternal plates are fused at the midline to form a single element, aiding in p...

Poposaurus gracilis is a bipedal pseudosuchian archosaur that has been poorly understood since the discovery of the holotype fragmentary partial postcranial skeleton in 1915. Poposaurus. gracilis is a member of Poposauroidea, an unusually morphologically divergent clade of pseudosuchians containing taxa that are bipedal, quadrupedal, toothed, edent...

The diagnosis of Dinosauria and interrelationships of the earliest dinosaurs relies on careful documentation of the anatomy of their closest relatives. These close relatives, or dinosaur “precursors,” are typically only documented by a handful of fossils from across Pangea and nearly all specimens are typically missing important regions (e.g., fore...

Following the Permo–Triassic mass extinction, Archosauriformes—the clade that includes crocodylians, birds, and their extinct relatives outside crown Archosauria—rapidly diversified into many distinct lineages, became distributed globally, and, by the Late Triassic, filled a wide array of resource zones. Current scenarios of archosauriform evolutio...

Chindesaurus bryansmalli is an early dinosaur of uncertain affinities from the Late Triassic Chinle Formation at Petrified Forest National Park, Arizona. Since its first description in 1995, the taxon has been considered a plateosaurid, a noneusaurischian saurischian, a herrerasaurid, and/or a non-neotheropod member of Theropoda. Chindesaurus bryan...

Late Cretaceous dinosaur assemblages of North America—characterized by gigantic tyrannosaurid predators, and large-bodied herbivorous ceratopsids and hadrosaurids—were highly successful from around 80 million years ago (Ma) until the end of the ‘Age of Dinosaurs’ 66 Ma. However, the origin of these iconic faunas remains poorly understood because of...

Mass extinctions change global ecosystems, and the end‐Triassic mass extinction was hypothesized to have precipitated the rise of dinosaur dominance, with dinosaurs filling resource zones of eliminated large‐bodied reptilian lineages. This replacement has been explicitly hypothesized for theropod dinosaurs, and the eastern North American theropod f...

Crown-group frogs (Anura) originated over 200 Ma according to molecular phylogenetic analyses, though only a few fossils from high latitudes chronicle the first approximately 60 Myr of frog evolution and distribution. We report fossils that represent both the first Late Triassic and the earliest equatorial record of Salientia, the group that includ...

Understanding growth patterns is central to properly interpreting paleobiological signals in tetrapods, but assessing skeletal maturity in some extinct clades may be difficult when growth patterns are poorly constrained by a lack of ontogenetic series. To overcome this difficulty in assessing the maturity of extinct archosaurian reptiles—crocodylia...

Complete version of this abstract online at: https://gsa.confex.com/gsa/2019AM/webprogram/Paper339845.html The Placerias/Downs quarry (PDQ) complex in east-central Arizona is the most diverse Late Triassic nonmarine vertebrate locality in the world, yielding dozens of taxa since the first excavations in the 1930s. The PDQ is low in the Chinle and...

Mass extinctions provide a biological “reset” often linked to subsequent biological radiations, generating a repeated macroevolutionary pattern. The End-Permian Mass Extinction, largest of the “Big Five,” decimated ecosystems and environments. Following the extinction, archosaurs (group including birds, crocodylians, and their extinct relatives) ra...

The evolution of the braincase and brain of early pseudosuchians through to the earliest crocodylomorphs is poorly understood given the paucity of specimens, lack of well-preserved material, and lack of consensus on the phylogenetic relationships of the major clades of Pseudosuchia. Here, we describe three differently sized braincases diagnosable a...

Dinosaurs and their close relatives grew to sizes larger than any other terrestrial animal in the history of life on Earth, and many enormous dinosaurs (e.g., Diplodocus , Spinosaurus , Tyrannosaurus ) have accessory intervertebral articulations that have been suggested to support these large body sizes. Some pseudosuchian archosaurs have been repo...

Figs S1–S4. Photographs of the manus and carpus of AMNH FARB 27435 Coelophysis bauri provide additional documentation of the morphology of this taxon.

Table S1. Manus measurements of selected Coelophysis bauri and Megapnosaurus rhodesiensis specimens.

A partial hind limb (femur and tibia) from the Triassic Lifua Member of the Manda Beds (Tanzania) is recognized as a new morphotype of kannemeyeriiform dicynodont. The femur of this specimen is the largest known dicynodont postcranial element from the Manda Beds and indicates an animal nearing the size of the Late Triassic Stahleckeria potens from...

During the recovery from the end-Permian extinctions in the Middle Triassic, small reptiles (<50 cm) are exceptionally rare, and this gap in the fossil record inhibits our understanding of small reptile ecology and evolution during such a critical transition in Earth history. Here, we fully describe two fragments of left and right hemimandibles of...

The diverse assemblage of extinct archosaur species known from the Manda Beds of Tanzania has provided key insights into the timing and tempo of the early part of the archosaur radiation during the Middle Triassic. Several archosaur specimens were collected from the Manda Beds in 1933 by F. R. Parrington, and three of these were subsequently descri...

We report new data on the climate, paleoenvironments, and burial history of tetrapod fossils in the Middle Triassic Lifua Member of the Manda Beds (Songea Group) of southern Tanzania. Two bone-bearing intervals have been identified, both hosted by rubified floodplain mudrocks deposited alongside rivers that flowed from the Ruhuhu rift scarps into a...

Stable carbon isotope analysis of coexisting soil calcite and organic matter sampled from modern, California soil profiles representing 18 different U.S. Department of Agriculture (USDA) official soil series yields 51 paired calcite–organic matter δ¹³C values (Δ¹³Ccc-om values). These paired values correspond to atmospheric pCO2 estimates ranging f...

Citation for this article: Sidor, C. A., and S. J. Nesbitt. 2018. Introduction to vertebrate and climatic evolution in the Triassic rift basins of Tanzania and Zambia; pp. 1–7 in C. A. Sidor and S. J. Nesbitt (eds.), Vertebrate and Climatic Evolution in the Triassic Rift Basins of Tanzania and Zambia. Society of Vertebrate Paleontology Memoir 17. J...

Bird-line archosaurs (= Avemetatarsalia, the clade containing birds, dinosaurs, pterosaurs, and their kin) originated in the Triassic Period. However, the earliest evolution of this group is poorly documented because fossils are extremely rare and consist mostly of postcrania. Here, we document the osteology of Teleocrater rhadinus, an early avemet...

The Permian-Triassic mass extinction (PTME) was one of the transformative events of the Phanerozoic, marked by extinction, post-Permian transformation of surviving ecosystems, and the formation of new communities. The South African Karoo Basin has served as the primary source of data on the terrestrial component of these events, but its global appl...

The origin of the avian hand, with its reduced and fused carpals and digits, from the five-fingered hands and complex wrists of early dinosaurs represents one of the major transformations of manus morphology among tetrapods. Much attention has been directed to the later part of this transition, from four- to three-fingered taxa. However, earlier an...

Osteoderms are common in most archosauriform lineages, including basal forms, such as doswelliids and proterochamp-sids. In this survey, osteoderms of the doswelliids Doswellia kaltenbachi and Vancleavea campi, and proterochampsid Chanaresuchus bonapartei are examined to infer their palaeobiology, such as histogenesis, age estimation at death, deve...

The Triassic Period saw the first appearance of numerous amniote lineages (e.g. Lepidosauria, Archosauria, Mammalia) that defined Mesozoic ecosystems following the end Permian Mass Extinction, as well as the first major morphological diversification of crown-group reptiles. Unfortunately, much of our understanding of this event comes from the recor...

Supplemental Appendix A: A. Modifications to characters and codings from the matrix of Nesbitt et al. (2015).; Appendices for Main Text PDF (A–C)

This corrects the article DOI: 10.1038/srep46028.

Hypotheses of feeding behaviors and community structure are testable with rare direct evidence of trophic interactions in the fossil record (e.g., bite marks). We present evidence of four predation, scavenging, and/or interspecific fighting events involving two large paracrocodylomorphs (= ‘rauisuchians’) from the Upper Triassic Chinle Formation (~...