Coelophysis

Coelophysis was a small, slenderly built, ground-dwelling, bipedal carnivore that could grow up to 3 m (9.8 ft) long. It is one of the earliest known dinosaur genera. Scattered material representing similar animals has been found worldwide in some Late Triassic and Early Jurassic formations.

The type species C. bauri, originally placed in the genus Coelurus by Edward Drinker Cope in 1887, was described by the latter in 1889. The names Longosaurus and Rioarribasaurus are synonymous with Coelophysis. Coelophysis is one of the most specimen-rich dinosaur genera.

History of discovery

In April of 1887 American paleontologist Edward Drinker Cope first described vertebrate remains from the Triassic of New Mexico as new species of the genus Coelurus, including nearly all parts of the skeleton except the skull and teeth. While Coelurus had been considered by its describer Othniel Charles Marsh as an uncertain type of reptile, the new material described by Cope demonstrated that it was a dinosaur, and likely closely related to Megadactylus (now Anchisaurus) from the Triassic of Massachusetts. Cope chose to name two new species from his collection of New Mexican material, the first being Coelurus longicollis for multiple vertebrae and a femur, while the second was Coelurus bauri, named after German paleontologist Georg Baur, included the same regions of the skeleton. Following this description, Cope reclassified his New Mexican material in July 1887 from Coelurus to the German taxon Tanystrophaeus based on the similarities of the vertebrae, creating the new combinations Tanystrophaeus longicollis and Tanysteophaeus bauri. Cope described additional material of both T. longicollis and T. bauri to cover more of the anatomy of both species, and he also named the new species Tanystrophaeus willistoni for a partial pelvis even smaller than both other species, named after American paleontologist Samuel Wendell Williston. Following this, Cope came to the realization that the vertebrae of Tanystrophaeus he considered similar to the New Mexican species were not definitively caudal vertebrae, and as a result the species were in need of a new genus name. As a result, in 1889 Cope described the new genus Coelophysis, with three species C. longicollis, C. bauri, and C. willistoni, the name being a combination of the Ancient Greek words κοῖλος (koilos), "hollow", and φύσεις (fysis), "form".

The original material of Coelophysis described by Cope was all collected by David Baldwin in 1881 from three localities of Rio Arriba County, New Mexico, with the only surviving collection information being labels written by Baldwin associated with the specimens, indicating a provenance of 120 m (400 ft) below a gypsum layer at "Arroyo Seco" and "Gallina Canyon". The layer of gypsum identified by Baldwin is now known as part of the Middle Jurassic Todilto Formation that overlies the Chinle Formation within the region of Ghost Ranch that "Arroyo Seco" (Arroyo Canjilon or Canjilon Creek) flows past, so while no other information on the area Baldwin collected can be known for certain, he must have collected from within a 4.8 km (3 mi) region around Ghost Ranch where the river cuts through the Chinle Formation below the Todilto gypsum layer. Williston and Ermine Cowles Case believed they located the area of Baldwin's collecting in a 1912 visit to the region where they located fragments they assigned to Coelophysis on the surface.

The German paleontologist Friedrich von Huene revisited the material and taxonomy of Coelophysis in a 1906 review of Triassic dinosaurs, following the species identifications of Cope and illustrating the material for the first time, with the additional description of elements of the hand of C. longicollis and vertebrae of C. bauri that Cope had not described. Huene did not see justification in considering Coelophysis a close relative of Anchisaurus (a replacement name for Megadactylus), instead classifying the genus within Coeluridae. In a further 1915 review limited to the New Mexican collection of Cope, Huene separated the specimens into the three species differently from previously, assigning bones based on size with C. longicollis as the largest species and C. willistoni as the smallest and suggesting that more than three species may be present. No type species for Coelophysis was designated by Huene, nor were specimen numbers identified, complicating the assignment of material to the species. In 1986 American paleontologist Kevin Padian revisited the type material of Coelophysis and identified which material was referred to which species by Cope and Huene in their studies, and identified some material in the Cope collection that had yet to be described.

Ghost Ranch discoveries

During the summer of 1946, American paleontologist Edwin H. Colbert began searching for Triassic fossils in the Petrified Forest National Park of northern Arizona, under the employment of the American Museum of Natural History. This exploration continued the following year, where Colbert planned to stop at Ghost Ranch on the way to Petrified Forest, where many skulls of phytosaurs had been found. Colbert, along with George Simpson, Bill Fish, George Whitaker, and Tom Ierardi, were the field crew for the 1947 season, with Simpson and Fish focusing on Eocene mammals of northern New Mexico while Colbert, Whitaker, and Ierardi would continue to northern Arizona for Triassic reptiles. On 21 June, Colbert, Whitaker and Ierardi began exploring the sediments of the Chinle Formation in the area near Ghost Ranch, uncovering a complete phytosaur skull, and on 22 June Whitaker discovered some fragments including a claw that Colbert identified as Coelophysis, from a talus slope above an intermittent stream. Upon excavating bones on the surface and removing some of the talus, a fossil layer was found containing numerous Coelophysis bones. After several more days of excavations, it was understood that the fossil layer contained hundreds of skeletons of Coelophysis in a single concentration, and upon the return of Simpson and Fish to the locality the former stated that "this was the greatest find ever made in the Triassic of North America".

Excavations of the Ghost Ranch Coelophysis quarry were continued throughout 1947 and 1948 by the crew of the AMNH, which also included Carl Sorensen after arrival from New York City, assisted by the landowners Arthur and Phoebe Pack and farm hand Herman Hall. Seven large blocks were excavated, each containing numerous skeletons, numbered I to VI for identification of location when transported back to the AMNH. Around 105 smaller blocks were also collected, created by the channels excavated to separate the larger blocks. A further 6 blocks, VII to XIII, were excavated as the quarry expanded to several meters in diameter, with the blocks being distributed amongst the AMNH, Museum of Northern Arizona, New Mexico Museum of Natural History, and Yale Peabody Museum. All of the excavations at Ghost Ranch took place within the upper sandstone level of the Chinle Formation, believed to be Norian in age at the time and making the expanse of Coelophysis one of the longest deposits within the Chinle despite being one of the earliest known dinosaurs. Colbert believed that the original material of Coelophysis may have been collected from the surface in a similar method to Whitaker's discovery, as part of the same fossil layer that was excavated by the AMNH.

Upon completion of the AMNH excavations at Ghost Ranch at the end of 1948 the quarry was abandoned, with the hillside above slumping down to cover the dig site, and the land itself gifted to the Presbyterian Church. After 32 years in this state, American paleontologist David Berman of the Carnegie Museum of Natural History approached Reverend Jim Hall of Ghost Ranch to propose reopening of the dinosaur quarry with the intention of collecting a specimen for display in the museum. Hall agreed to reopen the quarry on the condition that Colbert and the recently opened New Mexico Museum of Natural History would both participate, with the MNA, YPM and University of Toronto additionally contributing to the efforts. The quarry was worked from 1981 to 1982, with 16 large blocks of 6 to 10 short tons (5.4 to 9.1 t) excavated, and all but one block being removed. The last block remained at the site until 1985 when it was moved by American paleontologist David Gillette to the new Ruth Hall Museum of Paleontology at Ghost Ranch. Blocks from both collections were distributed by the primary institutions through trades and sale to be distributed across all of North America, and eventually even other continents.

A monograph on Coelophysis was then published by Colbert in 1989, focusing primarily on the material collected by him at the AMNH but supplemented by material in various other institutions. Under the belief that the material of Baldwin was found near at the same locality as the dinosaur quarry, Colbert diagnosed Coelophysis using the anatomy of the complete skeletons he had excavated, and considered C. bauri the senior synonym of C. longicollis and C. willistoni, with the partial sacrum AMNH 2722 as the lectotype of the species. C. longicollis had been believed by American paleontologist Samuel P. Welles to be a different genus than Coelophysis, which he had named Longosaurus in 1984 after American paleontologist Robert A. Long. All of the Ghost Ranch material was thus described as part of a single species known from hundreds of individuals of various ages and sizes, with some additional variation in proportions of individuals at the same size that gave the suggestion of a "robust" and "gracile" morph. While Colbert considered Coelophysis to be diagnostic, this opinion was not shared in other reviews of early theropods, where Padian instead considered it a "metataxon" or otherwise lacking unique characteristics of its own and having anatomy primitive to Theropoda. Padian believed that as Coelophysis was founded in scrappy and incomplete material, it is difficult to justify the referrals of additional material to the taxon. He still considered Coelophysis a useful taxonomic entity, being characterized by the lack of more derived traits common to other theropods, and as a result a partial skeleton from Petrified Forest, and the theropods Avipes, Podokesaurus, Procompsognathus, and Trialestes could be considered referrable to Coelophysis.

American paleontologists Adrian P. Hunt and Spencer G. Lucas went further than Padian, declaring Coelophysis a nomen dubium as its type material could not be distinguished from other theropods. As a result, they determined that a new taxon was required for the very complete specimens from Ghost Ranch (what they referred to as the Whitaker Quarry), for which they named Rioarribasaurus colberti. The genus name referred to the Rio Arriba County where the fossils were discovered, while the species name was in honor of Colbert for his work excavating and describing the material. Hunt and Lucas also designated a lectotype for C. longicollis as the specimen chosen by Welles as its type was invalid, identifying that the cervical AMNH 2701 should be used to identify the species. A lectotype was also designated for C. willistoni, the partial ilium AMNH 2726. None of the material of the three species was believed by Hunt and Lucas to bear any unique characteristics that could support their synonymy to C. bauri or any other theropod. The complete skeleton AMNH 7224, mounted on display at the museum, was selected as the holotype of Rioarribasaurus, and all the Ghost Ranch specimens collected from 1947 to 1982 were referred to the taxon as they lacked differences beyond sexual or growth-related variation.

In a following study in 1996, Hunt and Lucas as well as American paleontologists Robert M. Sullivan and Andrew Heckert established that the locality where Baldwin collected Coelophysis was not the same deposit or level as the dinosaur quarry as Colbert had believed. The color and concretions attached to the bones Baldwin collected differs from those of the specimens named Rioarribasaurus, but was instead identical to material found at a separate cluster of sites along Arroyo Seco 120 m (400 ft) below the Todilto Formation. The Baldwin localities would therefore be from the region of Orphan Mesa 3 km (1.9 mi) southeast of the dinosaur quarry, and from older deposits equivalent to the Canjilon Quarry and aetosaur quarry where Pseudopalatus and Typothorax are known. According to the stratigraphy of Sullivan, Lucas and colleagues, Coelophysis would therefore be from the Revueltian-age Petrified Forest Formation, with Rioarribasaurus from the Apachean-age Rock Point Formation. Additional collected material from these relocated sites was used as justification for Coelophysis being a distinct taxon separated from Rioarribasaurus, showing differences in the pubis, femur, and metatarsals. The stratigraphic divisions of Lucas for the Chinle are not supported by other paleontologists, who instead place the Coelophysis quarry within a "siltstone member" of the Chinle Formation with a Rhaetian age, with the Baldwin sites as part of the Petrified Forest Member radiometrically dated to 211.9±0.7 million years ago in the late Norian.

Since the numerous well-preserved Ghost Ranch specimens were used as Coelophysis in most of the scientific literature as well as public mentions, the introduction of Rioarribasaurus would have caused much confusion. As such, Colbert and other paleontologists petitioned to the International Commission on Zoological Nomenclature to replace the lectotype of Coelophysis bauri with AMNH 7224 as a neotype, thereby making Coelophysis bauri and Rioarribasaurus colberti objective synonyms, giving Coelophysis a much better type specimen, and preserving the use of Coelophysis for the Ghost Ranch material as it had been since its discovery in 1947. They emphasised their belief that the Ghost Ranch quarry was from approximately the same level and place as the original collection, as well as that the lectotype sacrum of C. bauri belonged to the same taxon as AMNH 7224 as justification for the neotype. The petition was met with opposing comments by Hunt, Lucas, and Sullivan, as well as Welles, George Olshevsky, and Philip Huber, on the grounds of stratigraphic and possibly anatomical differences between the original material and more complete specimens, and that the synonymy of Rioarribasaurus and Coelophysis should be established through anatomy rather than by ruling, while it was supported by paleontologists including Hans-Dieter Sues, Thomas R. Holtz Jr., Farish Jenkins, Ralph Molnar, Elizabeth Nicholls, Louis L. Jacobs, who presented views of the common use of Coelophysis, the potential that the stratigraphic and anatomical differences suggested were invalid, and the belief that a replacement of the type by a better specimen helps with the continued use of taxa over time even when originally diagnostic material is later shown to be inadequate. American paleoartist Gregory S. Paul also published in 1993 the opinion that Coelophysis was indeterminate, but that Rioarribasaurus and Syntarsus belonged to the same genus, creating the combination Syntarsus colberti. In the end, the ICZN voted in 1996 to designate the complete specimen AMNH 7224 as the neotype of Coelophysis and disposed of the name Rioarribasaurus, declaring the latter a nomen rejectum, or "rejected name". The name Coelophysis became a nomen conservandum ("conserved name"), and thus the confusion and uncertainty surrounding the status of Coelophysis was resolved.

Formerly assigned material and taxa

In 1922 Case described the first supposed material of Coelophysis from outside New Mexico with the referral of a series of vertebrae he collected in 1921 from Dockum Formation of Crosby County, Texas. The specimen, UMMP No. 7507, was considered by Case as an indeterminate species of Coelophysis, with similarities in the elongation to both Coelophysis and Anchisaurus. Huene did not agree with the referral by case, naming the new coelurosaur genus Spinosuchus for the material. The greatest difference from Coelophysis was in the elongate neural spines, but he also referred a partial braincase from the same region that shows differences from the only other comparable braincase, that of Thecodontosaurus. Redescription of Spinosuchus in 2009 showed that it was separate from Coelophysis and not even a theropod, being a member of the early archosauriform group Trilophosauridae.

Following the rediscovery of a specimen containing the bone impressions of an early dinosaur by Colbert and Donald Baird in 1958, Coelophysis was first compared to Podokesaurus as potentially synonymous. BSNH No. 13656 was found in the in 1864 from possibly-Triassic sediments of Connecticut, probably the Portland Formation, and remained undescribed until the work of Colbert and Baird. Podokesaurus had been named in 1911 by American paleontologist Mignon Talbot for a partial skeleton from the Portland Formation with the only species P. holyokensis, the same deposits which bore the BSNH specimen. Comparisons between the BSNH specimen, Podokesaurus, and Coelophysis showed that they all differed minimally in the limb material in common, with Colbert and Baird assigning BSNH 13656 to an indeterminate Coelophysis species and suggesting that Podokesaurus may not be valid. This was followed up by Colbert in 1964 where he proposed that Podokesaurus should be considered a junior synonym of Coelophysis and created the new combination C. holyokensis, to which he also referred the BSNH specimen. In his 1989 monograph, Colbert no longer followed this synonymy, finding that the shared similarities were widespread throughout podokesaurids; though some later authors considered Podokesaurus a synonym, most now consider it as an indeterminate or potentially undiagnostic taxon.

In 1983 another partial skeleton of a theropod was found from the Orphan Mesa region where it is possible Baldwin collected the original Coelophysis material, at a site named Cross Quarry after its discoverer Robert Cross. The specimen, NMMNH P-22298 and known informally as the "Orphan Mesa theropod", was an incomplete skeleton of an immature theropod, preserving several vertebrae, the scapulacoracoid, parts of the pelvis and most of the hindlimb. Following the designation of AMNH 7224 as the neotype of Coelophysis bauri, C. longicollis, C. willistoni, and the specimens named C. bauri by Cope lacked a genus and species, so the NMMNH P-22298 as well as material previously assigned to C. willistoni (AMNH 2706) were named Eucoelophysis by Sullivan and Lucas in 1999. 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. The identification of Eucoelophysis as a silesaurid, distant from Coelophysis and the Baldwin material, was upheld by independent work by American paleontologists Sterling Nesbitt, Randall Irmis and William Parker in 2007.

While the Zimbabwean genus Syntarsus was first described by South African paleontologist Michael A. Raath as a separate genus of podokesaurid, it was compared very favourably to Coelophysis by both Raath and Colbert, with differences in bone fusion and the pelvis separating the two taxa. The similarity between the two genera was extended upon by American paleontologist Gregory S. Paul in 1984, who considered Syntarsus a species of Coelophysis, C. rhodesiensis, as well as finding Halticosaurus to be a probable synonym. Syntarsus and Coelophysis were distinguished by Padian in 1986 despite his referral of many other Triassic theropods lacking unique features to Coelophysis, but Paul maintained the synonymy of the two genera in 1988 and 1993, including assigning Rioarribasaurus colberti to Syntarsus as S. colberti under the belief that Coelophysis was undiagnostic but the Ghost Ranch specimens were not. American paleontologist Timothy B. Rowe kept Syntarsus and Coelophysis separate in his 1989 description of the new species Syntarsus kayentakatae, but in 2000 American paleontologist Alex Downs reiterated the opinion of synonymy between Coelophysis and Syntarsus, crediting the poor description of some features of Coelophysis by Colbert to previous misinterpretations. Downs also found that the theropod Camposaurus could not be distinguished from Coelophysis, and considered it undiagnostic, but it was reidentified as a distinct coelophysid by Ezcurra and American paleontologist Stephen Brusatte in 2011.

Further complications with Syntarsus occurred when it was recognized that the genus name had been preoccupied by a colydiine beetle named in 1869, which prompted American and Polish entomologists Michael Ivie, Adam Ślipiński, and Piotr Węgrzynowicz to provide the new replacement name Megapnosaurus for the theropod in 2001, with M. rhodesiensis becoming its type. Many paleontologists did not like the naming of Megapnosaurus, partially because taxonomists are generally expected to allow original authors of a name to correct any mistakes in their work. Raath was aware of the homonymy between the dinosaur Syntarsus and beetle Syntarsus, but the group who published Megapnosaurus have claimed that they believed Raath was deceased and unable to correct his mistake, so they proceeded accordingly. American paleontologist Mickey Mortimer pointed out that "Paleontologists might have reacted more positively if the replacement name (Megapnosaurus) hadn't been facetious, translating to "big dead lizard". In 2024, it has been petitioned to the ICZN that Syntarsus be retained for the name of the theropod. In the interim, Bristowe and Raath (2004) first proposed the synonymy of both species of Syntarsus with Coelophysis, though they were uncertain whether S. kayentakatae belonged to this genus, referring it to as ?C. kayentatakatae. Multiple authors in the 2000s provisionally considered S. rhodesiensis to be a species of Coelophysis, while S. kayentakatae has been identified as belonging to a separate genus and in need of a new name. Phylogenetic analyses of Coelophysidae have found that S. rhodesiensis is not the sister taxon to Coelophysis, so the name Megapnosaurus has been used in those cases.