Coelophysis

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 and a , while the second was Coelurus bauri, named after German paleontologist Georg Baur, included the same regions of the skeleton. 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 , the , parts of the 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. == Description ==

Coelophysis is known from a number of complete fossil skeletons of the species C. bauri. This lightly built dinosaur measured up to long and was more than a meter tall at the hips. Gregory S. Paul (1988) estimated the weight of the gracile form at and the weight of the robust form at , Coelophysis was a bipedal, carnivorous, theropod dinosaur and a fast, agile runner. Coelophysis had narrow hips, arms adapted for grasping prey, and narrow feet. The pelvis and hindlimbs of C. bauri are also slight variations on the theropod body plan. It has the open acetabulum and straight ankle hinge that define Dinosauria. The leg ended in a three-toed foot (pes) with a raised dewclaw (hallux). The tail had an unusual structure within its interlocking prezygapophysis of its vertebrae, which formed a semi-rigid lattice, apparently to stop the tail from moving up and down. Coelophysis had a long and narrow head (approximately ), with large, forward-facing eyes that afforded it stereoscopic vision and, as a result, excellent depth perception. Rinehart et al. (2004) described the complete scleral ring found for a juvenile Coelophysis bauri (specimen NMMNH P-4200) and compared it to data on the scleral rings of reptiles (including birds), concluding that Coelophysis was a diurnal, visually oriented predator. The study found that the vision of Coelophysis was superior to most lizards' vision and ranked with that of modern birds of prey. The eyes of Coelophysis appear to be the closest to those of eagles and hawks, with a high power of accommodation. The data also suggested poor night vision, which would mean this dinosaur had a round pupil rather than a split pupil. Coelophysis had an elongated snout with large fenestrae that helped to reduce skull weight, while narrow struts of bones preserved the structural integrity of the skull. The neck had a pronounced sigmoid curve. The braincase is known in Coelophysis bauri, but little data could be derived because the skull was crushed. Unlike some other theropods, the cranial ornamentation of Coelophysis was not located at the top of its skull. Low, laterally raised bony ridges were present on the dorsolateral margin of the nasal and lacrimal bones in the skull, directly above the antorbital fenestra. Distinguishing anatomical features A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group. According to Ezcurra (2007) and Bristowe and Raath (2004), Coelophysis can be distinguished based on the absence of an offset rostral process of the maxilla, the quadrate being strongly caudally, a small external mandibular fenestra (which is 9–10% of the mandibular length), and the anteroposterior length of the ventral lacrimal process is greater than 30% of its height. Several paleontologists consider Coelophysis bauri to be the same dinosaur as Megapnosaurus rhodesiensis (formerly Syntarsus). However, this has been refuted by many paleontologists. Downs (2000) concluded that C. bauri differs from C. rhodesiensis in cervical length, proximal and distal leg proportions, and proximal caudal vertebral anatomy. Tykoski and Rowe (2004) concluded that C. bauri differs from M. rhodesiensis in that it lacks a pit at the base of the nasal process of the premaxilla. Bristowe and Raath (2004) concluded that C. bauri differs from M. rhodesiensis in having a longer maxillary tooth row. Barta et al. (2018) concluded that C. bauri differed from M. rhodesiensis in that it bears its 5th metacarpal. Griffin (2018) concluded that C. bauri differs from M. rhodesiensis in several differences in the musculature of the limbs. == Classification ==

When first described, Cope believed that Coelophysis was a member of the carnivorous dinosaur group Goniopoda, as a relative of Anchisaurus, Coelurus and at times Tanystropheus. == Paleobiology ==

Feeding , now interpreted as those of a crocodylomorph The teeth of Coelophysis were typical of predatory dinosaurs, as they were blade-like, recurved, sharp, jagged, and finely serrated on both the anterior and posterior edges. Its dentition shows that it was carnivorous, probably preying on the small, lizard-like animals that were discovered with it. It may also have hunted in packs to tackle larger prey. Coelophysis bauri has approximately 26 teeth on the maxillary bone of the upper jaw and 27 teeth on the dentary bone of the lower jaw. It has been suggested that C. bauri was a cannibal, based on supposed juvenile specimens found "within" the abdominal cavities of some Ghost Ranch specimens. Gay's position was lent support in a 2006 study by Nesbitt et al. In 2009, new evidence of cannibalism came to light when additional preparation of previously excavated matrix revealed regurgitate material in and around the mouth of Coelophysis specimen NMMNH P-44551. This material included tooth and jaw bone fragments that Rinehart et al. considered "morphologically identical" to a juvenile Coelophysis. Pack behavior 's Coelophysis block, originally American Museum of Natural History block XII collected by Colbert in 1948 There is a tendency to see this massive congregation of animals as evidence for huge packs of Coelophysis roaming the land. Growth and sexual dimorphism Rinehart (2009) assessed the ontogenic growth of this genus using data gathered from the length of its upper leg bone (femur) and concluded that Coelophysis juveniles grew rapidly, especially during the first year of life. The gracile form has a longer skull, a longer neck, shorter arms, and has sacral neural spines that are fused. The robust form has a shorter skull, a shorter neck, longer arms, and unfused sacral neural spines. Raath agreed that dimorphism in Coelophysis is evidenced by the size and structure of the arm. Rinehart et al. studied 15 individuals, and agreed that two morphs were present, even in juvenile specimens, and suggested that sexual dimorphism was present early in life, prior to sexual maturity. Rinehart concluded that the gracile form was female and the robust form was male based on differences in the sacral vertebrae of the gracile form, which allowed for greater flexibility for egg laying. However, more recent research has found that C. bauri and C. rhodesiensis had highly variable growth between individuals, with some specimens being larger in their immature phase than smaller adults were when completely mature. This indicates that the supposed presence of distinct morphs is simply the result of individual variation. This highly variable growth was likely ancestral to dinosaurs but later lost and may have given such early dinosaurs an evolutionary advantage in surviving harsh environmental challenges. Reproduction Through the compilation and analysis of a database of nearly three dozen reptiles (including birds) and comparison with existing data about the anatomy of Coelophysis, Rinehart et al. (2009) drew the following conclusions. It was estimated that average egg of Coelophysis was 31–33.5 millimeters across its minor diameter and that each female would lay between 24 and 26 eggs in each clutch. The evidence suggested that some parental care was necessary to nurture the relatively small hatchlings during the first year of life, where they would reach 1.5 meters in length by the end of their first growth stage. Coelophysis bauri invested as much energy in reproduction as other extinct reptiles of its approximate size. Paleopathology In a 2001 study conducted by Bruce Rothschild and other paleontologists, 14-foot bones referred to Coelophysis were examined for signs of stress fracture, but none were found. In C. rhodesiensis, healed fractures of the tibia and metatarsus have been observed, but are very rare. "[T]he supporting butresses of the second sacral rib" in one specimen of Syntarsus rhodesiensis showed signs of fluctuating asymmetry. Fluctuating asymmetry results from developmental disturbances and is more common in populations under stress and can therefore be informative about the quality of conditions a dinosaur lived under. == Ichnology ==

Edwin H. Colbert has suggested that the theropod footprints referred to the ichnogenus Grallator, located in the Connecticut River Valley across Connecticut and Massachusetts, may have been made by Coelophysis. The footprints are from the Late Triassic to Early Jurassic aged Newark Supergroup. They clearly show digits II, III, and IV, but not I or V. That condition is strange for footprints of their age. The digits I and V were presumed to be stubby and ineffective, not touching the ground when the dinosaur was walking or running. They have been thought to be from an unidentified, primitive saurischian similar to Coelophysis by David B. Weishampel and L. Young more recently. Skeletal remains resembling Coelophysis have also been found in the valley, supporting the idea that a species similar to Coelophysis is responsible for the footprints. == Paleoenvironment ==

Specimens of Coelophysis have been recovered from the Chinle Formation of New Mexico and Arizona, more famously at the Ghost Ranch (Whitaker) quarry in the Rock Point member but Thomas Holtz Jr. interpreted that it was during the Rhaetian stage from approximately 204 to 201.6 million years ago. C. rhodesiensis has been recovered in the Upper Elliot Formation in the Cape and Free State provinces of South Africa, as well as the Chitake River bonebed quarry at the Forest Sandstone Formation in Zimbabwe. Ghost Ranch was located close to the equator over 200 million years ago, and had a warm, monsoon-like climate with heavy seasonal precipitation. Hayden Quarry, a new excavation site at Ghost Ranch, New Mexico, has yielded a diverse collection of fossil material that included the first evidence of dinosaurs and less-advanced dinosauromorphs from the same time period. The discovery indicates that the two groups lived together during the early Triassic period 235 million years ago. Therrien and Fastovsky (2001) examined the paleoenvironment of Coelophysis and other early theropods from Petrified Forest National Park in Arizona and determined that this genus lived in an environment that consisted of floodplains marked by distinct dry and wet seasons. There was a great deal of competition during drier times when animals struggled for water in riverbeds that were drying up. In the upper sections of the Chinle Formation where Coelophysis is found, dinosaurs were rare. So far, only Chindesaurus and Daemonosaurus are known, the terrestrial fauna being dominated instead by other reptiles like the rhynchocephalian Whitakersaurus, the pseudosuchian Revueltosaurus, the aetosaurs Desmatosuchus, Typothorax and Heliocanthus, the crocodilomorph Hesperosuchus, the "rauisuchians" Shuvosaurus, Effigia, and Vivaron, along with other rare components like the dinosauriform Eucoelophysis and the amniote Kraterokheirodon. In the waterways there are the phytosaur Machaeroprosopus, the archosauromorph Vancleavea, the amphibians Apachesaurus and the fishes Reticulodus, Arganodus, and Lasalichthyes. In 2009, Rinehart et al. noted that in one case the Coelophysis specimens were "washed into a topographic low containing a small pond, where they probably drowned and were buried by a sheet flood event from a nearby river." The 30 specimens of C. rhodesiensis found together in Zimbabwe was also probably the result of a flash flood that swept away a large number of Coelophysis and buried them quickly and simultaneously as well. == Cultural significance ==

Coelophysis was the second dinosaur in space, following Maiasaura (STS-51-F). A Coelophysis skull from the Carnegie Museum of Natural History was aboard the Space Shuttle Endeavour mission STS-89 when it left the atmosphere on 22 January 1998. It was also taken onto the space station Mir before being returned to Earth. Since the discovery of Coelophysis fossils more than 100 years ago, it is one of the best-known dinosaurs in literature. It was designated as the official state fossil of New Mexico in 1981 and is now the logo of the New Mexico Museum of Natural History. ==Notes==