A revision of coelophysoid theropod specimens from Petrified Forest National Park, Arizona (USA), reveals a new species from the Upper Triassic Chinle Formation

Abstract

The vertebrate fossil record of the non-marine Upper Triassic Chinle Formation (southwest United States) is one of the richest worldwide for the Norian-Rhaetian stages. This unit includes one of the oldest neotheropods (Camposaurus arizonensis, Sonsela Member, ~219.5-218.4 Ma) and a mass accumulation of the best-known Triassic neotheropod (Coelophysis bauri, 'siltstone member', ~209-205 Ma). Here we revise three neotheropod associated partial postcrania from the middle portion of the Petrified Forest Member of the Chinle Formation, temporally intermediate between the latter occurrences (~212-210 Ma), including one found in 1982 (PEFO 21373/UCMP 129618), referred to Coelophysis bauri in 1986, but subsequently reinterpreted as Coelophysis sp. or a possible new taxon (the 'Padian theropod'). The other two specimens (PEFO 33981, PEFO 33983), found in 2004, were preliminarily referred to Coelophysis sp. or Coelophysidae. PEFO 21373/UCMP 129618 and PEFO 33983 possess a unique combination of character states that allows assigning both to a single, new species of neotheropod. The third specimen has a congruent morphology, but lacks preservation of key features, preventing an unambiguous referral to the new taxon. A phylogenetic analysis found the new taxon as one of the earliest branching coelophysoids, outside Coelophysidae (Coelophysis + Megapnosaurus +"Syntarsus"). When the three specimens are analysed as independent terminals, PEFO 33981 and (PEFO 21373/UCMP 129618 + PEFO 33983) are recovered as a paraphyletic group outside Coelophysidae. The new taxon provides key information for the optimization of character states at the base of Coelophysoidea and increases the richness of Triassic neotheropods.