A new taxon of saurischian dinosaur from the Coelophysis Quarry of New Mexico, USA (Triassic: latest Norian or Rhaetian) highlights herrerasaurian diversity in the latest Triassic

Abstract The most complete record of the earliest dinosaur lineages is from the Carnian from the higher latitudes of Pangea (e.g. present‐day Brazil, Argentina), but dinosaurian assemblages from the upper stages of the Upper Triassic are better known from the low latitudes of Pangea (present day southwestern USA). How early carnivorous dinosaurian diversity matches or mismatches at various latitudes remains to be documented because of uncertainty around the spatio‐temporal ranges and phylogenetic relationships of early dinosaur lineages. We examine low‐latitude diversity patterns through the lens of the saurischian dinosaur Tawa hallae and close relatives, including a new species, Ptychotherates bucculentus gen. et sp. nov. The new taxon is known from an incomplete but well‐preserved skull (CM 31368) from the uppermost Triassic Coelophysis Quarry in northern New Mexico. The new taxon clearly shares synapomorphies with Tawa hallae , such as distinctive fossae on the quadrate and otoccipital and a dorsoventrally tall and laterally flat jugal. However, the new taxon is distinguishable from all other coeval ornithodirans by a combination of many character states, including the proportionally dorsoventrally deepest jugal known for any Triassic‐aged dinosaur. Higher‐palaeolatitude ecosystems across Pangea show a complete turnover of carnivorous dinosaurs by neotheropods in the Norian and Rhaetian, but the ‘ Chindesaurus – Tawa ’ clade (Morphoraptora clade nov.) coexisted with neotheropods possibly until the End‐Triassic Extinction Event. This suggests a low‐latitude ‘museum’ where early‐diverging lineages survived much longer than at higher latitudes, and that the End‐Triassic Extinction Event affected dinosaur diversity more than previously hypothesized.