Coelophysis

Coelophysis (meaning "hollow form" in reference to its hollow bones) is one of the earliest known genera of dinosaur. It was a small, carnivorous biped that lived during the Late Triassic (Carnian and Norian stages) 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.

Description
Coelophysis bauri is the earliest dinosaur known from a number of complete fossil skeletons. C. bauri was a lightly built dinosaur which measured up to 3 m in length and which was more than a meter tall at the hips. The name Coelophysis means "hollow form" or "hollow process", so named because of its hollow limb bones.

Coelophysis was very slim and it probably would have been a fast 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.

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.

Coelophysis had large eyes and good eyesight. Its neck and head were long. The tail was also long, and 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, and its sharp, curved, jagged teeth show that it ate meat - probably the small, lizard-like animals that were found with it. It may also have hunted in packs to tackle larger prey. 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.

Paleobiology
The teeth were typical of predatory dinosaurs, blade-like and recurved with fine serrations on both anterior and posterior edges.

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. 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.

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, and it appears that in some cases bigger individuals were crushed on top of smaller ones), 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.

Edwin H. Colbert has suggested that Connecticut Valley theropod footprints referred to the ichnogenus Grallator may have been made by Coelophysis.

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) 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.

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.

Coelophysis is also the state fossil of New Mexico.