Rioarribasaurus

''Coelophysis'' ( traditionally; or , as heard more commonly in recent decades) is an extinct genus of coelophysid theropod dinosaur that lived approximately 228 to 201.3 million years ago during the latter part of the Triassic Period from the Carnian and Rhaetian faunal stages in what is now the southwestern United States. '' Megapnosaurus'' was once considered a species within this genus,Weishampel, David B; et al. (2004). "Dinosaur distribution (Early Jurassic, Africa)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 535–536. but this interpretation has been challenged since 2017 and the genus ''Megapnosaurus'' is now considered valid.

''Coelophysis'' was a small, slenderly-built, ground-dwelling, bipedal carnivore that could grow up to 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 given to 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

The type species of ''Coelophysis'' was originally named as a species of ''Coelurus''. Edward Drinker Cope first named ''Coelophysis'' in 1889 to name a new genus, outside of ''Coelurus'' and '' Tanystropheus,'' which ''C. bauri'' was previously classified in, for ''C. bauri'', ''C. willistoni'' and ''C. longicollis''. 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'' of the genus ''Coelurus''. Later in 1887 Cope reassigned the material to a yet another genus, ''Tanystropheus''. Two years later, Cope corrected his classification after realizing differences in the vertebrae, and named ''Coelophysis'', with ''C. bauri'' as the type species, which was named for Georg Baur, a comparative anatomist whose ideas were similar to Cope's. The name ''Coelophysis'' comes from the Greek words κοῖλος/koilos (meaning 'hollow') and φύσις/physis (meaning 'form'), together "hollow form", which is a reference to its hollow vertebrae. However, the first finds were too poorly preserved to give a complete picture of the 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 ''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.''"Syntarsus" rhodesiensis'' was first described by Raath (1969) and assigned to Podokesauridae.Raath (1969). "A new Coelurosaurian dinosaur from the Forest Sandstone of Rhodesia." Arnoldia Rhodesia. 4 (28): 1–25. The taxon "Podokesauridae", was abandoned because its type specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned the genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). In 2004 "Syntarsus" was found to be synonymous with ''Coelophysis'' by Tykoski and Rowe (2004). Ezcurra and Novas (2007) and Ezcurra (2007) also concluded that "Syntarsus" was synonymous with ''Coelophysis''.Tykoski, R. S., and Rowe, T., 2004, Ceratosauria, Chapter Three: In: The Dinosauria, Second Edition, edited by Weishampel, D.B., Dodson, P., and Osmolska, H., California University Press, p. 47-70. In a phylogenetic analysis by Ezcurra (2017), '' Megapnosaurus'' was recovered in a clade with ''Segisaurus'' and '' Camposaurus'', supporting the generic distinction of ''Megapnosaurus''. This was supported by Barta and colleagues in 2018, noting that ''Coelophysis'' still bears the vestigial 5th metacarpal, a feature absent in ''Megapnosaurus''.

Description

''Coelophysis'' is known from a number of complete fossil skeletons of the species ''C. bauri'': a lightly built dinosaur that measured up to in length and was more than a meter tall at the hips. Paul (1988) estimated the weight of the gracile form at , and the weight of the robust form at , but later presented a higher estimate of . ''Coelophysis'' was a bipedal, carnivorous, theropod dinosaur and 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 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 prey, and narrow feet.Tykoski, R.S. & Rowe, T. (2004). "Ceratosauria": In: 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.

''Coelophysis'' had a long 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 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 ''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 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.

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 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; 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 the following: Downs (2000) concluded that ''C. bauri'' differs from ''C. rhodesiensis'' in cervical length, proximal and distal hindlimb proportions and proximal caudal vertebral anatomy;Downs, A. (2000). "''Coelophysis bauri'' and ''Syntarsus rhodesiensis'' compared, with comments on the preparation and preservation of fossils from the Ghost Ranch Coelophysis quarry": In: 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; and several features in the musculature of the limbs according to Griffin (2018).Griffin, Christopher. (2018). Developmental patterns and variation among early theropods. Journal of Anatomy. 232. 604-640. 10.1111/joa.12775.

Classification

''Coelophysis'' is a distinct taxonomic unit (genus), composed of one species; '' C. bauri.'' Two additional originally described species, ''C. longicollis'' and ''C. willistoni'', are now considered dubious and undiagnostic. ''M. rhodesiensis'' was referred to ''Coelophysis'' for several years, but it is likely its own genus, and is known from the early Jurassic of southern Africa. A third possible species is ''Coelophysis kayentakatae'', previously referred to the genus ''Megapnosaurus,'' from the Kayenta Formation of the southwestern US. In recent phylogenetic analyses, ''"Syntarsus" kayentakatae'' has been shown to be distantly related to ''Coelophysis'' and ''Megapnosaurus'', suggesting that it belongs to its own genus.

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 specimens 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. This would make ''Rioarribasaurus'' a definite synonym of ''Coelophysis'', specifically a junior objective synonym. 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 taxa, some more recently discovered fossils were originally classified as new genera but may be species of ''Coelophysis''. For example, Prof. Mignon Talbot's 1911 discovery which she named ''Podokesaurus holyokensis'', has long been considered to be related to ''Coelophysis'', and some modern scientists consider ''Podokesaurus'' a synonym of ''Coelophysis''. Another specimen from the Portland Formation

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