Coelophysis
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[Photo] Coelophysis Animatronics model, Natural History Museum, London displaying supposed canibalistic behaviour. Photo created by Ballista http://en.wikipedia.org/wiki/User:Ballista

One of the earliest known dinosaurs, Coelophysis (see-low-FYS-iss) meaning "hollow form" in reference to its hollow bones (Greek κοιλο??/koilos meaning 'hollow' and φυσι??/physis meaning 'form') is a small, carnivorous biped from North America. It first appeared in the Mid Triassic Period, around 228 million years ago.

Description
Coelophysis bauri is the earliest dinosaur known from a number of complete fossil skeletons. C. bauri was a lightly built dinosaur, between two to three meters in length, and less than a meter tall at the hips. The name Coelophysis means "hollow form" or "hollow process", so named because of its hollow limb bones.

Despite being an early dinosaur, the evolution of the theropod body form had already advanced greatly from creatures like Herrerasaurus and Eoraptor. 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 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.

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 of Coelophysis 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.

Paleobiology
Coelophysis was probably opportunistic, catching live prey and scavenging. The teeth were typical of predatory dinosaurs, blade-like and recurved with fine serrations on both anterior and posterior edges. They were rooted in the jaws in sockets, and were continually replaced throughout the animal's life.

Since our knowledge of Coelophysis comes mainly from the specimens excavated at Ghost Ranch, there is a tendency to see this massive congregation of animals as evidence for huge packs of Coelophysis roaming the land (as seen in the television series Walking with Dinosaurs). There is no evidence for this. What the deposit does tell us is 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, then became buried in a catastrophic flash flood.

It has been suggested that C. bauri was a cannibal, based on 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), and there is no longer any evidence to support cannibalistic behavior in Coelophysis. Gay's study was corroborated in 2006 in a subsequent study by Nesbitt et al. There may be other evidence coming to light that may show stomach contents from some of these specimens, which might bring greater resolution to the subject.

Two forms of Coelophysis have been found, a more gracile form and a slightly more robust form. Opinion among paleontologists is now that these were female and male variants (see: sexual dimorphism).

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, David Baldwin, had found the first remains of the dinosaur in 1881. The type species, C. bauri was named for Baur, one of the many 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 in New Mexico, at the Ghost Ranch, close to the original find. So many fossils together were 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 such flooding was commonplace during this period of the Earth's history and, indeed, the Petrified Forest of nearby Arizona is caused by a preserved log jam of tree trunks that were caught in one such flood. Edwin H. Colbert made a comprehensive study of all the fossils found up to that date, and it is from him that we take most of our information about Coelophysis. The Ghost Ranch specimens were so numerous, including many well-preserved 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.

Classification
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").

Sullivan & Lucas (1999) suggested that the original type specimen was referable to what they thought was a newly discovered coelophysid, Eucoelophysis baldwini. However, subsequent studies have shown that Eucoelophysis was misidentified, and is actually a primitive, non-dinosaurian ornithodiran closely related to Silesaurus. Therefore, the original type specimen of Coelophysis cannot be considered a specimen of Eucoelophysis.

In addition to all of this, there is a competing controversy with another coelophysoid, Megapnosaurus, which many regard to be congeneric with Coelophysis. To make matters more confusing, Paul suggested that Coelophysis should be placed in Megapnosaurus (then known as Syntarsus) to get around the above-mentioned taxonomic confusion.

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 (or is) Coelophysis. In addition, C. posthumus, named by Friedrich von Huene in 1908, also needs reclassification and is tentatively titled Halticosaurus longotarsus at the moment.

Trivia
Coelophysis was the second dinosaur in space. Although Maiasaura had been taken into space three years earlier, 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.

Coelophysis is also the state fossil of New Mexico.

Coelophysis were featured in the BBC television series Walking with Dinosaurs, and in When Dinosaurs Roamed America, in which they are depicted hunting insects. The 1974 children's television series Land of the Lost also featured a Coelophysis, nicknamed "Spot".

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