Temporal range: Late Triassic, 216.5–203.6Ma
|Mounted skeleton cast at the Cleveland Museum of Natural History|
Coelophysis (pron.: // or //), is one of the earliest known genera of dinosaur. It was a small, carnivorous biped that lived during the Late Triassic (Norian stage) of the southwestern United States, with scattered material representing similar animals to Coelophysis found worldwide in some Late Triassic and Early Jurassic formations.
The type species, C. bauri, was described by Edward Drinker Cope in 1889. The name Rioarribasaurus is synonymous with Coelophysis. Another dinosaur, Megapnosaurus, is also often considered synonymous with Coelophysis.
The name Coelophysis comes from the Greek words κοιλος/koilos (meaning 'hollow') and φυσις/physis (meaning 'form'), thus "hollow form" which is a reference to its hollow limb bones.
Coelophysis bauri is known from a number of complete fossil skeletons. C. bauri was a lightly built dinosaur which measured up to 3 metres (9.8 ft) in length and which was more than a meter tall at the hips. Paul (1988) estimated the weight of the gracile form at 15 kg (33 lb), and the weight of the robust form at 20 kg (44 lb). Coelophysis was a bipedal, carnivorous, theropod dinosaur that was a fast and agile runner. Despite being an early dinosaur, the evolution of the theropod body form had already advanced greatly from creatures like Herrerasaurus and Eoraptor. The torso of Coelophysis conforms to the basic theropod body shape, but the pectoral girdle displays some interesting special characteristics: C. bauri had a furcula (wishbone), the earliest known example in a dinosaur. Coelophysis also preserves the ancestral condition of possessing four digits on the hand (manus). It had only three functional digits, the fourth embedded in the flesh of the hand.
Coelophysis had narrow hips, forelimbs adapted for grasping, and narrow feet. Its neck and tail were long and slender. 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 the Dinosauria. The hindlimb ended in a three-toed foot (pes), with a raised 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. This may have let the tail act as a rudder or counterweight when the animal was maneuvering at high speeds.
Coelophysis had a long narrow head, with large, forward-facing eyes that afforded it stereoscopic vision and as a result excellent depth perception. Rinehart et al. (2004) described the complete sclerotic ring found for a juvenile Coelophysis bauri (specimen NMMNH P-4200), and compared it to data on the sclerotic rings of reptiles and birds and concluded that this Coelophysis was a diurnal, visually oriented predator." The Rinehart et al. study found that its vision 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 rather than a split pupil.
Coelophysis had an elongated snout with large fenestrae which 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.
According to Ezcurra (2007), Coelophysis can be distinguished based on the following features:
- the absence of an offset rostral process of the maxilla
- the quadrate is strongly caudally
- a small external mandibular fenestra, which is 9-10% of the mandibular length
According to Tykoski (1998), Coelophysis can be distinguished based on this feature:
- an anterior pedunclar foramina in present in the cervical vertebrae
According to Bristowe and Raath (2004), Coelophysis can be distinguished based on this feature:
- the anteroposterior length of the ventral lacrimal process is greater than 30% of its height.
Several paleontologists consider Coelophysis to be the same dinosaur as Megapnosaurus (formerly Syntarsus), however this has been refuted by the following:
- Downs (2000) concluded that Coelophysis differs from Megapnosaurus in cervical length, proximal and distal hindlimb proportions and proximal caudal vertebral anatomy
- Tykoski and Rowe (2004) concluded that Coelophysis differs from Megapnosaurus in that it lacks a pit at the base of the nasal process of the premaxilla
- Bristowe and Raath (2004) concluded that Coelophysis differs from Megapnosaurus in having a longer maxillary tooth row
Sexual Dimorphism 
Two "morphs" of Coelophysis have been found, a more gracile form (as in specimen AMNH 7223) and a slightly more robust form (as in specimens AMNH 7224, NMMNH P-42200). Skeletal proportions were different between these two forms. The gracile form has a longer skull, a longer neck, shorter forelimbs, and has sacral neural spines that are fused. The robust form has a shorter skull, a shorter neck, longer forelimbs, and the sacral neural spines are not fused. Raath agreed that dimorphism in Coelophysis is evidenced by the size and structure of the forelimb. Opinion among paleontologists is now that these were female and male variants (see: sexual dimorphism).< Rinehart et al. studied 15 individuals, and agreed that two morphs were present, even in juvenile specimens. This demonstrates 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. Further support for this position is provided by the study where analysis showed that each morph comprised 50% of the population, as would be expected in a 50/50 sex ratio.
History of discovery 
Edward Drinker Cope first named Coelophysis in 1889 during his competition to name species with Othniel Charles Marsh, known as the "Bone Wars". An amateur fossil collector working for Cope, David Baldwin, had found the first remains of the dinosaur in 1881 in the Chinle Formation in northwestern New Mexico. Early in 1887 Cope referred the specimens collected to two new species, C. bauri and C. longicollis and erected the genus Coelurus. Later in 1887 Cope reassigned the material to a yet another new genus Coelophysis, with C. bauri as the type species, which was named for Baur, another fossil collectors who supplied Cope. However, these first finds were too poorly preserved to give a complete picture of this new dinosaur. In 1947, a substantial 'graveyard' of Coelophysis fossils was found by George Whitaker, the assistant of Edwin H. Colbert, in New Mexico, at the Ghost Ranch, close to the original find. American Museum of Natural History paleontologist Edwin H. Colbert conducted a comprehensive study of all the fossils found up to that date and assigned them to Ceolophysis. It is from Colbert's work that we take most of our knowledge about Coelophysis. The Ghost Ranch specimens were so numerous, including many well-preserved and fully articulated specimens, that one of them has since become the diagnostic, or type specimen, for the entire genus, replacing the original, poorly preserved specimen (see Classification below). Since the Ghost Ranch specimens were discovered, more skeletons have been found in Arizona, New Mexico and an as-yet unconfirmed specimen from Utah, including both adults and juveniles, the deposits where Coelophysis has been discovered date from the late Carnian to the early Norian faunal stages of the Triassic Period.
Coelophysis is a distinct taxonomic unit (genus), composed of a single species, C. bauri. Two additional species were originally described in addition to C. bauri, C. longicollis, and C. willistoni; however, they are not diagnostic and are considered synonymous with C. bauri. C. rhodesiensis is probably part of this generic complex, and is known from the Jurassic of southern Africa (see below for more). In phylogenetic taxonomy, Coelophysis is treated as a clade within the Coelophysidae.
In the early 1990s, there was debate over the diagnostic characteristics of the first specimens collected, compared to the material excavated at the Ghost Ranch Coelophysis quarry. Some paleontologists were of the opinion that the original specimens were not diagnostic beyond themselves and, therefore, that the name C. bauri could not be applied to any additional specimens. They therefore applied a different name, Rioarribasaurus, to the Ghost Ranch quarry specimens.
Since the numerous well-preserved Ghost Ranch specimes were used as Coelophysis in most of the scientific literature, the use of Rioarribasaurus would have been very inconvenient for researchers, so a petition was given to have the type specimen of Coelophysis transferred from the poorly preserved original specimen to one of the well-preserved Ghost ranch specimens. In the end, the International Commission on Zoological Nomenclature (ICZN) voted to make one of the Ghost Ranch samples the actual type specimen for Coelophysis and dispose of the name Rioarribasaurus altogether (declaring it a nomen rejectum, or "rejected name"), thus resolving the confusion. The name Coelophysis therefore became a nomen conservandum ("conserved name").
In a situation affecting many dinosaur genera, many specimens were originally classified as new species but were in fact species of Coelophysis. For example, Prof. Mignon Talbot's 1911 discovery which she labeled Podokesaurus holyokensis, may be related to Coelophysis. Another specimen from the Portland Formation of the Hartford Basin, now at the Boston Museum of Science, has also been referred to Podokesaurus, and consequently, Coelophysis. However, both the type specimen of Podokesaurus and the other specimen are typically considered indeterminate theropods today. In addition, C. posthumus, named by Friedrich von Huene in 1908, also needs reclassification and is tentatively titled Halticosaurus longotarsus at the moment.
Related Genera 
Coelophysis shares the Coelophysidae taxon with Megapnosaurus, Camposaurus, and Podokesaurus. It is also closely related to Liliensternus, Procompsognathus, ?Pterospondylus, ?Segisaurus and ?Gojirasaurus. Yates (2005) analyzed Coelophysis and Megapnosaurus and concluded that the two genera are almost identical, and suggested that Megapnosaurus was possibly synonymous with Coelophysis. In 2004, Raath co-authored two papers in which he argued that Megapnosaurus (formerly Syntarsus) was a junior synonym of Coelophysis. Megapnosaurus is regarded by Paul (1988) and Downs (2000) as being congeneric with Coelophysis. Then in 1993, Paul suggested that Coelophysis should be placed in Megapnosaurus (then known as Syntarsus) to get around the above-mentioned taxonomic confusion. Downs (2000) examined Camposaurus and concluded that it is a junior synonym of Coelophysis, because of its similarity to some of the Coelophysis Ghost Ranch specimens.
Sullivan & Lucas (1999) referred one specimen from Cope's original material of Coelophysis (AMNH 2706) to what they thought was a newly discovered theropod, Eucoelophysis. However, subsequent studies have shown that Eucoelophysis was misidentified, and is actually a primitive, non-dinosaurian ornithodiran closely related to Silesaurus.
The teeth of Coelophysis were typical of predatory dinosaurs, blade-like, recurved, sharp and jagged with fine serrations 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 maxillary bone of the upper jaw and 27 teeth on the dentary bone of the lower jaw. Carpenter (2002) examined the bio-mechanics of theropods forelimbs and attempted to evaluate their usefulness in predation. He concluded that the forelimb of Coelophysis was flexible and had a good range of motion, but it's bone structure suggested that is was comparatively weak. The “weak” forelimbs and small teeth in this genus, suggested that Coelophysis preyed upon animals that were substantially smaller than itself. Rinehart et al. (2009) agreed that Coelophysis was a "hunter of small, fast-moving prey". Carpenter (2002) also identified three distinct models of theropod forelimb use and noted that Coelophysis was a “combination grasper-clutcher” as compared to other dinosaurs that were “clutchers” or “long armed graspers”
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. However, Rob Gay showed in 2002 that these specimens were misinterpreted. Several specimens of "juvenile coelophysids" were actually small crurotarsan reptiles such as Hesperosuchus. 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.
In 2010, Gay examined the bones of juveniles found within the chest (thoracic) cavity of AMNH 7224, and calculated that the total volume of these bones was 17 times greater than the maximum estimated stomach volume of the Coelophysis specimen. Gay observed that the total volume would be even greater when considering that there would have been flesh on these bones. This analysis also noted the absence of tooth marks on the bones as would be expected in defleshing, and the absence of expected pitting by stomach acids. Finally Gay demonstrated that the alleged cannibalized juvenile bones, were deposited stratigraphically below the larger animal that supposedly cannibalized them. Taken together these data suggested that the Coelophysis specimen AMNH 7224 was not a cannibal and that the bones of the juvenile and adult specimens were found in their final position as a result of "coincidental superposition of different sized individuals."
Pack behavior 
The discovery of over 1000 specimens of Coelophysis at the Whitaker quarry at Ghost Ranch, has suggested gregarious behavior to researchers like Schwartz and Gillette. There is a tendency to see this massive congregation of animals as evidence for huge packs of Coelophysis roaming the land. The television series Walking with Dinosaurs, for example, showed small flocks together (and did not cite the Ghost Ranch deposits as evidence). No direct evidence for flocking exists; the deposits only indicate that large numbers of Coelophysis, along with other Triassic animals, were buried together. Some of the evidence from the taphonomy of the site indicates that these animals may have been gathered together to feed or drink from a depleted water hole or to feed on a spawning run of fish, and then became buried in a catastrophic flash flood.
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 like birds (and not slowly like reptiles), especially during the first year of life. Coelophysis likely reached sexual maturity between the second and third year of life and reached its full size, just above 10 feet in length, by its eighth year. This study identified four distinct growth stages: 1-year, 2-year, 4-year, and 7+ year.
Through the compilation and analysis of a database of nearly three dozen birds and reptiles, 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 24-26 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.
Provenance and Occurrence 
Specimens of Coelophysis have been recovered at the Ghost Ranch (Whitaker) quarry of the Chinle Formation in New Mexico, the Petrified Forest Member of the Chinle Formation in Arizona and New Mexico, and the Bull Canyon Formation in Texas. In Ghost Ranch, Coelophysis was discovered in sediments deposited between the Rhaetian stage of the Triassic and the Hettangian stage of the Jurassic, approximately 208 to 199 million years ago. In the Petrified Forest Member of the Chinle Formation, this genus was discovered in strata that are from the Norian stage of the Late Triassic, approximately 228 to 208 million years ago. In Bull Canyon, Coelophysis was discovered in sediments deposited during the Norian stage of the Late Triassic, approximately 228 to 208 million years ago.
Fauna and Habitat 
Ghost Ranch was located close to the equator 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 during the Late Triassic 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. A contemporary of Coelophysis in Bull Canyon Formation was the phytosaur Rutiodon, several aetosaurs, an archosauriform Technosaurus, and the dinosaurs Chindesaurus, Revueltosaurus and Lucianosaurus.
The multitude of specimens deposited so closely together at Ghost Ranch was probably the result of a flash flood, which swept away a large number of Coelophysis and buried them quickly and simultaneously. In fact, it seems that such flooding was commonplace during this period of the Earth's history and, indeed, the Petrified Forest of nearby Arizona is the result of a preserved log jam of tree trunks that were caught in one such flood. Whitaker quarry at Ghost Ranch is considered a monotaxic Site because it features multiple individuals of a single taxon. The quality of preservation and the ontogenic range (age range) of the specimens helped make Coelophysis one of the best known of all genera. 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 buriedby a sheet flood event from a nearby river."
In popular culture 
Coelophysis were featured in the BBC television series Walking with Dinosaurs, as well as the Discovery Channel special When Dinosaurs Roamed America and the IMAX film Dinosaurs Alive!. The 1974 children's television series Land of the Lost also featured a Coelophysis, nicknamed "Spot".
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 January 22, 1998. It was also taken onto the space station Mir before being returned to Earth.
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