Article: Taxonomic and phylogenetic reassessment of the early neotheropod dinosaur Camposaurus arizonensis from the Late Triassic of North America

Camposaurus arizonensis, a small theropod dinosaur from the early–middle Norian of Arizona (USA), is widely considered the oldest known neotheropod. However, despite its importance, Camposaurus is the subject of taxonomic and phylogenetic uncertainty and is often considered a nomen dubium, largely because of a fragmentary holotype. We here reassess the holotype of Camposaurus and identify two autapomorphies: the posterior edge of the tibial articular surface for the fibula offset as a sharp and prominent ridge and the absence of an anteriorly expanded medial condyle of the astragalus. We therefore consider Camposaurus to be a valid and diagnostic taxon of basal theropod dinosaur. For the first time, we include Camposaurus in a phylogenetic analysis, which confirms its neotheropod placement and recovers it as a close relative of Coelophysis rhodesiensis within Coelophysoidea sensu stricto. The position of Camposaurus as the oldest neotheropod provides an important calibration point, but necessitates long ghost lineages, indicating that our knowledge of the early evolutionary history of theropod dinosaurs is still patchy. Furthermore, our phylogenetic analysis recovers a polytomy at the base of Neotheropoda, as most parsimonious trees disagree in recovering a monophyletic or paraphyletic ‘traditional’ Coelophysoidea. This suggests that basal theropod phylogeny remains in a state of flux, and the monophyly of ‘traditional’ Coelophysoidea remains an open question.