| Eucoelophysis | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria (?) |
| Clade: | †Ornithischia (?) |
| Family: | †Silesauridae |
| Clade: | †Sulcimentisauria |
| Genus: | †Eucoelophysis |
| Species: | †E. baldwini |
| Binomial name | |
| †Eucoelophysis baldwini | |
Eucoelophysis (meaning "truehollow form") is agenus ofsilesauriddinosauriform from theLate Triassic (Norian)periodChinle Formation ofNew Mexico. It was assumed to be acoelophysid upon description,[1] but a study byNesbittet al. found that it was actually a close relative ofSilesaurus,[2] which was independently supported by Ezcurra (2016), who found it to be thesister group toDinosauria, andSilesaurus as the next most basaltaxon.[3]
In the 1880s, David Baldwin collected vertebrates from theLate Triassic of north-centralNew Mexico for American paleontologistEdward Drinker Cope, who would then place these specimens in 1887 inCoelurus as the new speciesCoelurus bauri andCoelurus longicollis. Recognizing them as a separate genus fromCoelurus later in 1887, he movedC. bauri andC. longicollis toTanystropheus, and also named the new speciesTanystrophaeus willistoni. In 1889 he again revisited the genus classification, namingCoelophysis for the three species, all from the same area of New Mexico. Restudy of thestratigraphy 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 thePetrified 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 severalvertebrae, thescapulacoracoid, parts of thepelvis and most of the hindlimb.[1]
The original material that Cope namedCoelophysis was recognized as too incomplete to be distinguished, so in 1996 theInternational Commission on Zoological Nomenclature designated a complete skeleton and skull from a younger location in New Mexico as theholotype ofCoelophysis bauri, leavingC. longicollis,C. willistoni, and the specimens namedC. bauri by Cope as lacking a genus and species. This led to American paleontologistsRobert M. Sullivan andSpencer G. Lucas naming a new taxon for the NMMNH P-22298 specimen, to which they also referred a specimen previously included withinC. longicollis that Baldwin had collected. Sullivan and Lucas named this taxonEucoelophysis 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 originalCoelophysis, derived from theAncient Greek wordseu ("true"), andcoelo andphysi meaning "hollow" and "nature". There was also additional material found around Orphan Mesa that may belong toEucoelophysis, but that lacked overlap or diagnostic traits to make such a referral confident.[1]
When originally named Sullivan and Lucas placedEucoelophysis withinCeratosauria as a close relative ofCoelophysis,Syntarsus rhodesiensis, andSyntarsus kayentakatae.[1] American paleontologists Andrew Heckert and colleagues in 2000 referred further theropod material toEucoelophysis (asE. sp.) from the Snyder Quarry elsewhere in theChinle Formation, suggesting it may belong to a new species, which they reiterated in 2003.[4][5] These referrals and the identify ofEucoelophysis was reevaluated in 2006 by Argentine paleontologistMartín Ezcurra who identified that many of the features used by Sullivan and Lucas to support the identity ofEucoelophysis were instead shared amongst all early dinosaurs, withEucoelophysis instead appearing more similar to the non-dinosaurSilesaurus. Neither the Snyder Quarry specimens nor the material originally assigned toC. longicollis were found to share features withEucoelophysis by Ezcurra, who identified them instead as indeterminatecoelophysoids.[3] An articulated skeleton (TMP 1986.63.33) from theCoelophysis quarry previously considered a specimen ofCoelophysis was moved toEucoelophysis by American paleontologist Larry Rinehart and colleagues in 2009, but it was returned toCoelophysis in 2018 by American paleontologist Chris Griffin.[6][7]
Independent of Ezcurra, American paleontologistsSterling Nesbitt, Randall Irmis and William Parker reviewed the Late Triassic theropods of North America in 2007, includingEucoelophysis. They found thatEucoelophysis was not a theropod or even a dinosaur, instead related toSilesaurus andPseudolagosuchus. They also reconsidered what material belonged toEucoelophysis, as only the hindlimb was found articulated. The bonebed locality included other reptiles such asTypothorax, and the lack of clear features of the vertebrae, and differences between the pelvis ofEucoelophysis andSilesaurus, led Nesbitt and colleagues to only retain the hindlimb asEucoelophysis, with the rest of the material as uncertain identity.[2] Additional isolated material potentially referrable toEucoelophysis was identified from the nearby Hayden Quarry of the Chinle Formation by Irmis and colleagues in 2007, including adentary,ilium, and afemur. This material was not found together, but each individual bone shows similarities toSilesaurus and they are from the same age and region asEucoelophysis.[8] 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 ofEucoelophysis, the hindlimb material can be confidently referred and only a single silesaurid is presumed to have been present at the locality.[9][10]
