Rise of present-day tetrapods in the paleotropics of Late Triassic equatorial Pangaea: new insights from microvertebrate data

The Triassic Period (~252–201.5 Ma) saw a transformative radiation and reorganization of continental tetrapod diversity following the end-Permian Extinction, including an assemblage of diverse forms that do not survive the end-Triassic (herein termed the 'endemic Triassic fauna', =ETF), as well as the earliest fossil representatives of all major modern tetrapod groups (herein termed the 'Living [Triassic to Recent] Fauna', =LTF; i.e. Salientia, Caudata, Gymnophiona, Mammaliaformes, Squamata, Rhynchocephalia, Testudinata, Crocodylomorpha, and Dinosauria). With few exceptions, only the LTF assemblage survives the end-Triassic Extinction (~201.5 Ma), highlighting the Late Triassic (~227–201.5 Ma) record as essential for understanding this pivotal transition and the evolutionary and ecological origins of post-Triassic non-marine tetrapod faunas, including those of present day. Micro-microvertebrate bonebeds are arguably the best proxy for tracking continental vertebrate biodiversity, however gaps in their Late Triassic record obscure patterns and drivers of evolutionary, ecological, and environmental change during the rise of LTF communities. In my dissertation, I use new data collected from Upper Triassic microvertebrate bonebeds from North America, and particularly the Thunderstorm Ridge site (PFV 456) in Petrified Forest National Park, Arizona, U.S.A, to fill gaps in the evolutionary record of specific groups (e.g., lissamphibians and lepidosaurs), as well as the vertebrate paleocommunity record of Triassic equatorial Pangaea. My first chapter describes and analyzes an assemblage of gymnophionomorph (stem caecilian) bones from PFV 456 which represent the oldest-known caecilian fossils globally. As the oldest caecilian fossils, they provide new support for the dissorophoid temnospondyl affinities of caecilians and other living amphibians, evidence of a step-wise acquisition of caecilian anatomies associated with fossoriality, and evidence of an ancient pattern of equa