Adam D. Marsh

Elasmobranch teeth are useful index fossils and allow correlation among different stratigraphic units (Ginter, 2022; He et al., 2017; Murry & Kirby, 2002; Williamson et al., 1993). Among extinct sh...

Unguals (“claws”) are important tools that allow terrestrial vertebrates to navigate their environments, from subduing prey to climbing, running, and digging, with the shape of the element being broadly correlated with their ecological niche. Middle Triassic‐Late Triassic drepanosauromorphs (Sauropsida: Diapsida) have been hypothesized to occupy various niches and functional modes from arboreality to fossoriality based on a range of evidence including claw anatomy. Hypertrophied manual unguals are present in many members of Drepanosauromorpha and vary in shape from the strongly curved, mediolaterally compressed, and dorsoventrally tall Drepanosaurus‐ like forms to the shallowly curved and mediolaterally wide unguals of Skybalonyx . Here, we detail and compare the external and internal features of western North American drepanosauromorph unguals. Using osteohistology and micro‐computed X‐ray tomography, we find that drepanosauromorph manual unguals possess a combination of characteristics that differentiate drepanosauromorph taxa. Using external and internal characteristics, we identify a new taxon of drepanosauromorph, Fabanychus monos gen. et sp. nov., from the upper Sonsela Member of the Upper Triassic Chinle Formation (~214 Ma) at Petrified Forest National Park in northeastern Arizona. More broadly, we find that drepanosauromorph manual unguals share a set of unique character states compared to other extant and extinct reptilian and mammalian unguals. Each drepanosauromorph ungual morphotype is diagnostic to a distinct taxon, revealing biostratigraphic patterns of the clade. The spatiotemporal and lithostratigraphic occurrence of drepanosauromorphs in equatorial Pangaea reveal a long record of the group, co‐occurring taxa, and possible genus‐level turnover in the middle Norian (~215 Ma).

Temporally constrained microvertebrate bone beds are powerful tools for understanding continent-scale biotic change. Such sites are rare globally in nonmarine settings during the 12 million years (Ma) preceding the end-Triassic extinction (ETE; ~201.5 Ma), obscuring patterns of faunal change across this interval. A vertebrate assemblage from Arizona, USA, provides unique insights into community composition and ecology prior to the ETE. PFV 393 is a macro- and microvertebrate bone bed preserved in a volcaniclastic fluvial channel-fill with a high-precision U-Pb zircon age of 209.187 ± 0.083 Ma. The fossil assemblage consists of three-dimensionally preserved, delicate, and small skeletal elements of known and new taxa that document a local paleocommunity including hybodontiformes, actinopterygians, actinistians, metoposaurids, salientians, synapsids, lepidosaurs, testudinatans, trilophosaurids, Vancleavea , doswelliids, Revueltosaurus , loricatans, phytosaurs, and pterosaurs. The new early-diverging pterosaur is one of the few Triassic pterosaurs found outside of Europe and the only one with a documented precise radioisotopic age. The testudinatan material shows the rapid dispersal of terrestrial stem-turtles across the Pangaean supercontinent in the Norian and refines temporal constraints on the origin of the turtle shell. The presence of vertebrate lineages endemic to the Triassic highlights their persistence in a mesic, fluvial paleocommunity through a prolonged phase of environmental change preceding the ETE. These lineages coexisted with frogs, lepidosaurs, turtles, and pterosaurs- all key elements of post–Triassic Mesozoic communities. The arrival of turtles and pterosaurs in west-central Pangaea therefore may have been driven by the northward drift of Laurentia from humid equatorial conditions into more arid subtropical latitudes.

Calyptosuchus wellesi is an aetosaur known from the upper Blue Mesa Member and lowermost Sonsela Member of the Chinle Formation in Arizona, and the Tecovas Formation of the Dockum Group of Texas. The taxon is considered biostratigraphically informative and aids in correlating Upper Triassic strata across the southwestern United States. Our anatomical understanding of Calyptosuchus was primarily based on the holotype specimen from Texas and referred specimens, including several disarticulated elements from the Placerias Quarry in northern Arizona. We provide a re-interpretation of the holotype specimen and describe two new specimens referrable to Calyptosuchus from northern Arizona. Together these specimens provide new information on the cranial anatomy, including dentition, and the positional and intraspecific variation within the carapace and vertebral column of the taxon. This new evidence suggests that the skull anatomy of Calyptosuchus is more similar to that of Neoaetosauroides and Stagonolepis than to other aetosaurs. The dentition of Calyptosuchus supports recent hypotheses suggesting that aetosaurs were more omnivorous/faunivorous. The associated carapace expands our anatomical understanding of characters that vary among trunk paramedian osteoderms. We identify variation in the vertebral column within the trunk region, including the prominence of the centrodiapophyseal lamina and anatomy of the neural spine. Additionally, we report the first documentation of co-ossified sacral vertebrae within Calyptosuchus, which is also the first observation of this state within the Aetosauria outside of the Desmatosuchini. This suggests that co-ossification of the sacral vertebrae may be more prevalent within the Aetosauria than previously recognized.http://zoobank.org/da3735b8-edf1-465d-820a-316174ed7bf9

The upper Blue Mesa Member of the Chinle Formation at Petrified Forest National Park, Arizona, U.S.A., contains a remarkable diversity of Triassic vertebrate fossils. The locality PFV 456 includes hundreds of bones of the long-necked archosauromorph clade Tanystropheidae. Using cervical vertebrae morphotypes as a metric for diversity, there are three distinct taxa of tanystropheids present at the locality, including aff. Tanystropheus and a novel taxon, described here and named as a new genus and species, Akidostropheus oligos, recognizable by its autapomorphic spike emanating dorsally from the neural spine that may be a defensive structure. This new taxon is also present at locality PFV 396 and the nearby possibly contemporaneous MNA 207/UCMP A269 (the Placerias and Downs quarries) near St. Johns, Arizona. This unexpected diversity of tanystropheid archosauromorphs indicates that semi-tropical nonmarine ecosystems may have provided opportunities for speciation and niche partitioning in Triassic vertebrates and provides valuable three-dimensional anatomical data for a group that is often characterized by two-dimensional preservation.

Fossils of embryonic and hatchling individuals can provide invaluable insight into the evolution of prenatal morphologies, heterochronies, and allometric trajectories within Archosauria but are exceptionally rare in the Triassic fossil record, obscuring a critical aspect of archosaurian biology during their evolutionary origins. Microvertebrate sampling at a single bonebed in the Upper Triassic Chinle Formation within Petrified Forest National Park has yielded diminutive archosauriform femora (PEFO 45274, PEFO 45199) with estimated and measured femoral lengths of ~31 mm and ~ 37 mm, respectively. These new specimens provide the unique opportunity to assess the preservation, body size, and growth dynamics of skeletally immature archosauriforms in North America and compare the growth dynamics of archosauromorphs within an evolutionary and ontogenetic context. We assign PEFO 45199 and PEFO 45274 to Phytosauria (Archosauriformes) based on their strongly sigmoidal shape in lateral view, the presence of proximal anterolateral and posteromedial tubera, the absence of an anteromedial tuber of the proximal end, a teardrop‐shaped proximal outline, and a fourth trochanter that is not confluent with the proximal head. Osteohistological analyses of PEFO 45274 reveal a cortex comprising low vascularity, parallel‐fibered bone composed of primary osteons that lacks a hatching line and any lines of arrested growth. We interpret PEFO 45274 as a slow‐growing, post‐hatching individual of less than 1 year of age. Surprisingly, osteohistology of some larger phytosaur femora implies faster growth rates in comparison to PEFO 45274 based on the occasional presence of woven bone and overall higher degrees of vascular density, suggesting the ontogenetic shift from rapid‐to‐slow growth rates might not occur simply or uniformly as expected in Phytosauria and that non‐archosaurian archosauriforms may exhibit size‐dependent histological characteristics. This study highlights the importance of including osteohistology from multiple body sizes to investigate non‐archosaurian archosauriform ancestral growth rates given the phylogenetic position of phytosaurs near the divergence of Archosauria.

Reptile feeding strategies encompass a wide variety of diets and accompanying diversity in methods for subduing prey. One such strategy, the use of venom for prey capture, is found in living reptile clades like helodermatid (beaded) lizards and some groups of snakes, and venom secreting glands are also present in some monitor lizards and iguanians. The fossil record of some of these groups shows strong evidence for venom use, and this feeding strategy also has been hypothesized for a variety of extinct reptiles (e.g., archosauromorphs, anguimorphs, and a sphenodontian). However, evidence of systems for venom delivery in extinct groups and its evolutionary origins has been scarce, especially when based on more than isolated teeth. Here, we describe a potentially venomous new reptile, Microzemiotes sonselaensis gen. et sp. nov., from a partial left dentary recovered from the Sonsela Member of the Chinle Formation (middle Norian, Upper Triassic) of northeastern Arizona, U.S.A. The three dentary teeth have apices that are distally reclined relative to their bases and the tip of the posteriormost tooth curves mesially. The teeth show subthecodont implantation and are interspaced by empty sockets that terminate above the Meckelian canal, which is dorsoventrally expanded posteriorly. Replacement tooth sockets are positioned distolingually to the active teeth as in varanid-like replacement. We identify this new specimen as a diapsid reptile based on its monocuspid teeth that lack carinae and serrations. A more exclusive phylogenetic position within Diapsida is not well supported and remains uncertain. Several features of this new taxon, such as the presence of an intramandibular septum, are shared with some anguimorph squamates; however, these likely evolved independently. The teeth of the new taxon are distinctively marked by external grooves that occur on the entire length of the crown on the labial and lingual sides, as seen in the teeth of living beaded lizards. If these grooves are functionally similar to those of beaded lizards, which use the grooves to deliver venom, this new taxon represents the oldest known reptile where venom-conducting teeth are preserved within a jaw. The teeth of the new species are anatomically distinct from and ~10x smaller than those of the only other known Late Triassic hypothesized venomous reptile, Uatchitodon, supporting venom use across multiple groups of different body size classes. This new species represents the third Late Triassic reptile species to possibly have used envenomation as a feeding (and/or defensive) strategy, adding to the small number of venomous reptiles known from the Mesozoic Era.

Maximum individual body size in pseudosuchian archosaurs is not well constrained in the fossil record, but it may be influenced by a variety of factors including basal metabolic rate, evolutionary relationships, and environmental conditions. Body size varies among the Aetosauria in which estimated total length ranges between 1 m (e.g., Coahomasuchus kahleorum) and 5 m (e.g., Desmatosuchus spurensis). A new, very large specimen of the aetosaurian Typothorax coccinarum from Petrified Forest National Park in northeastern Arizona is nearly twice the size of all other known specimens of Typothorax and is the largest aetosaur specimen currently known worldwide. The specimen lacks co‐ossified neurocentral sutures in the trunk vertebrae which may suggest that the individual had not yet reached skeletal maturity, yet smaller specimens of T. coccinarum exhibit partially or fully co‐ossified neurocentral sutures in the same region. If body size correlates with skeletal maturity in aetosaurs, this discrepancy warns that timing of neurocentral suture co‐ossification in aetosaurs may not be a reliable indicator of ontogenetic stage. Osteohistological observations of a trunk rib demonstrate that although PEFO 42506 shows a large body size, the specimen did not deposit an external fundamental system despite depositing as many as 19 growth lines, further indicating that it had not yet reached skeletal maturity. Thus, at least within Aetosauria, neurocentral suture co‐ossification and skeletal maturity may correlate, whereas body size can be incongruent in comparison. Furthermore, this specimen indicates that non‐desmatosuchin aetosaurs could exhibit large body sizes and suggests that some aetosaurs may have experienced indeterminate growth.

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 period^2–6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls^7–10, but the absence of pre-Jurassic period caecilian fossils^11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial^1,8,13,14. 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 dual jaw closure mechanism^15,16, reduced orbits¹⁷ and the tentacular organ¹⁸. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology¹⁹, in combination with vicariance patterns driven by plate tectonics²⁰. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.