# Eucoelophysis

Eucoelophysis (meaning "true hollow form") is a genus of silesaurid dinosauriform from the Late Triassic (Norian) period Chinle Formation of New Mexico. It was assumed to be a coelophysid upon description, but a study by Nesbitt et al. found that it was actually a close relative of Silesaurus, which was independently supported by Ezcurra (2016), who found it to be the sister group to Dinosauria, and Silesaurus as the next most basal taxon.

## History and naming

In the 1880s, David Baldwin collected vertebrates from the Late Triassic of north-central New Mexico for American paleontologist Edward Drinker Cope, who would then place these specimens in 1887 in Coelurus as the new species Coelurus bauri and Coelurus longicollis. Recognizing them as a separate genus from Coelurus later in 1887, he moved C. bauri and C. longicollis to Tanystropheus, and also named the new species Tanystrophaeus willistoni. In 1889 he again revisited the genus classification, naming Coelophysis for the three species, all from the same area of New Mexico. Restudy of the stratigraphy of the region Baldwin collected allowed for the recognition of the type locality, which Baldwin referred to as Arroyo Seco, to be the area of Orphan Mesa in the Petrified Forest Formation. Another specimen was found in 1983 from the Orphan Mesa region, at a site names Cross Quarry after its discoverer Robert Cross, and it became known as the "Orphan Mesa theropod". The specimen, NMMNH P-22298, is an incomplete skeleton of an immature theropod, preserving several vertebrae, the scapulacoracoid, parts of the pelvis and most of the hindlimb.
The original material that Cope named Coelophysis was recognized as too incomplete to be distinguished, so in 1996 the International Commission on Zoological Nomenclature designated a complete skeleton and skull from a younger location in New Mexico as the holotype of Coelophysis bauri, leaving C. longicollis, C. willistoni, and the specimens named C. bauri by Cope as lacking a genus and species. This led to American paleontologists Robert M. Sullivan and Spencer G. Lucas naming a new taxon for the NMMNH P-22298 specimen, to which they also referred a specimen previously included within C. longicollis that Baldwin had collected. Sullivan and Lucas named this taxon Eucoelophysis baldwini in 1999, with the species name honoring David Baldwin, while the genus name was a reference to the status of the material as the original Coelophysis, derived from the Ancient Greek words eu ("true"), and coelo and physi meaning "hollow" and "nature". There was also additional material found around Orphan Mesa that may belong to Eucoelophysis, but that lacked overlap or diagnostic traits to make such a referral confident.
When originally named Sullivan and Lucas placed Eucoelophysis within Ceratosauria as a close relative of Coelophysis, Syntarsus rhodesiensis, and Syntarsus kayentakatae. American paleontologists Andrew Heckert and colleagues in 2000 referred further theropod material to Eucoelophysis (as E. sp.) from the Snyder Quarry elsewhere in the Chinle Formation, suggesting it may belong to a new species, which they reiterated in 2003. These referrals and the identify of Eucoelophysis was reevaluated in 2006 by Argentine paleontologist Martín Ezcurra who identified that many of the features used by Sullivan and Lucas to support the identity of Eucoelophysis were instead shared amongst all early dinosaurs, with Eucoelophysis instead appearing more similar to the non-dinosaur Silesaurus. Neither the Snyder Quarry specimens nor the material originally assigned to C. longicollis were found to share features with Eucoelophysis by Ezcurra, who identified them instead as indeterminate coelophysoids. An articulated skeleton (TMP 1986.63.33) from the Coelophysis quarry previously considered a specimen of Coelophysis was moved to Eucoelophysis by American paleontologist Larry Rinehart and colleagues in 2009, but it was returned to Coelophysis in 2018 by American paleontologist Chris Griffin.
Independent of Ezcurra, American paleontologists Sterling Nesbitt, Randall Irmis and William Parker reviewed the Late Triassic theropods of North America in 2007, including Eucoelophysis. They found that Eucoelophysis was not a theropod or even a dinosaur, instead related to Silesaurus and Pseudolagosuchus. They also reconsidered what material belonged to Eucoelophysis, as only the hindlimb was found articulated. The bonebed locality included other reptiles such as Typothorax, and the lack of clear features of the vertebrae, and differences between the pelvis of Eucoelophysis and Silesaurus, led Nesbitt and colleagues to only retain the hindlimb as Eucoelophysis, with the rest of the material as uncertain identity. Additional isolated material potentially referrable to Eucoelophysis was identified from the nearby Hayden Quarry of the Chinle Formation by Irmis and colleagues in 2007, including a dentary, ilium, and a femur. This material was not found together, but each individual bone shows similarities to Silesaurus and they are from the same age and region as Eucoelophysis. The referrals of the Hayden Quarry material was supported by American paleontologists Benjamin Breeden and colleagues in 2017 and Jeffrey Martz and Bryan Small in 2019, including additional skull material and bones of the skeleton. Though there is not clear overlap between the skull bones and the diagnostic elements of Eucoelophysis, the hindlimb material can be confidently referred and only a single silesaurid is presumed to have been present at the locality.