Db_abstract

The oldest Coelophysoid (Dinosauria:Theropoda) skull from the Upper Triassic Chinle Group, north-central New Mexico, USA

Reports a Coelophysis-like skull from the Snyder quarry near Ghost Ranch.

Item Details

Abstract

A recently discovered fossil locality, NMMNH locality L-3825, the Snyder Quarry, has produced the oldest known skull material of a coelophysoid theropod. The Snyder Quarry is in the upper Petrified Forest Formation near Ghost Ranch in north-central New Mexico. The occurrence of the aetosaur Typothorax, an index taxon of the Revueltian (early-mid Norian) land vertebrate faunachron, suggests an age of approximately 215 Ma for the Snyder Quarry, as much as 10 Myr older than the famous Coelophysis quarry at Ghost Ranch.


The partial skull consists of a right premaxilla, maxilla, and lachrymal. Associated jaws include three partial dentaries, surangulars, and angulars, a left splenial, and a portion of the hyoid apparatus. Postcrania identified thus far include vertebrae from the cervical, dorsal, sacral, and caudal series, a partial scapulocoracoid and ilium, and complete or nearly complete left ischium, both femora, a left tibia, left fibula, and numerous metapodials. The presence of two right dentaries and proximal left tibiae demonstrates that at least two individuals are present.


Coelophysoid theropods were the dominant 1-3-m-sized terrestrial predators from the latest Triassic through at least the Sinemurian, an interval of approximately 30 Myr. The Snyder Quarry theropod represents the oldest known skull of a coelophysoid. Notably, the skull is already highly derived, with gracile proportions, extensive fenestrae, and a slightly heterodont dentition, and is thus similar to the younger coelophysoid taxa Coelophysis and Syntarsus. This indicates that the coelophysoid bauplan was established early and remained relatively unmodified across the Triassic-Jurassic boundary.


Particularly interesting is the association of the theropod with a diverse vertebrate and invertebrate fauna. Thus far, this fauna includes unionid bivalves, a decapod crustacean, another arthropod, semionotid fish, indeterminate metoposaurid amphibians, the phytosaurs Pseudopalatus andersoni and Pseudopalatus sp., the aetosaurs Desmatosuchus haplocerus and Typothorax coccinarum, a possible cynodont, and other, fragmentary tetrapods. Unidentified carbonized leaf and wood fragments are also common. Thus, the quarry offers a nearly unparalleled opportunity to study the terrestrial ecosystem during Revueltian time.