A long-necked tanystropheid from the Middle Triassic Moenkopi Formation (Anisian) provides insights into the ecology and biogeography of tanystropheids

Archosauromorphs are a diverse and successful group of reptiles that radiated into a series of groups around the time of the end-Permian extinction. One of these groups of archosauromorphs, tanystropheids, consists of diverse forms, and some of the largest members of the group possessed extremely elongated cervical vertebrae (greater than five times longer than tall), resulting in a hyperelongate neck. These derived tanystropheids have been found in Tethyan marine deposits of Pangaea. Four partial cervical vertebrae from a hyperelongate-necked tanystropheid from the Middle Triassic Moenkopi Formation of Arizona and New Mexico are described in this paper. These cervical vertebrae are assigned to Tanystropheidae, specifically the clade that includes the hyperelongate-necked Tanystropheus based on character states, which include an elongate centrum (length to height ratio of 6.2), the presence of epipophyses, and an elongate axial centrum. The Moenkopi tanystropheid elements were found in lower latitude fluvial sequences without any marine influence, corresponding to western Pangaea, whereas Tanystropheus-like tanystropheids are typically associated with marginal marine environments in middle to high latitudes of eastern Pangaea. These fossils suggest that hyperelongate-necked, Tanystropheus-like tanystropheids were perhaps behaviorally bound to general semi-aquatic environments, both marine and freshwater, due to their unique morphology. These fossils also greatly extend the biogeographic range of large tanystropheids and increase the anatomical diversity of tanystropheids known from North America demonstrating that the clade persisted in a wide variety of environments throughout the Triassic Period.