New Mexico Geological Society Annual Spring Meeting — Abstracts

The Snyder quarry, in the Painted Desert Member of the Petrified Forest Formation in north-central New Mexico, is one of the richest and most di verse Upper Triassic bonebeds discovered in the past 50 years. This bonehed formed, at least in part, as a result of a catastrophic paleowildfire that we also implicate in the formation of the well-known Canjilon quarry and the recently discovered Hayden quarry.

The Snyder quarry yields a diverse array of vertebrates and a more depauperate invertebrate assemblage. Invertebrates include a new decapod crustacean, a conchostracan, and an allochthonous assemblage of the unionid bivalve "Antediplodon." Phytosaurs that we assign to Pseudopalatus buceros dominate the vertebrate assemblage (minimum number of individuals [MNI]=11). Aetosaurs present in the quarry include Typothorax coccinarum and Desmatosuchus chamaensis, both represented by diverse osteodcnns (scutes) and some other postcrania representing as many as five different individuals. A striking feature of the Snyder quarry is the relative abundance of theropod dinosaurs (MNI=4), most of which appear to be coeIophysoids congeneric with Eucoelophysis, previously known only from the type locality near Orphan Mesa to the east. Other vertebrates are considerably rarer, and include semionotid and redfie1diid fish, metoposaurid amphibians, a procolophonid reptile, a probable lepidosauromorph reptile and, possibly, a xenacanth shark. Recently we detetmined that rauisuchian archosaurs (Postosuchus-grade) are also a rare component of the fauna.

The tetrapod fauna of the Snyder quarry is a "typical" Revueltian assemblage, including the index taxa P. buceros and T. coccinarum. The assemblage is particularly important for many reasons, including (1) its exceptionally good preservation of many of the vertebrates, (2) it is the type locality of D. chamaensis, (3) it has the highest concentration of pre-Apachean (latest Triassic) ceratosaurian dinosaurs known, and (4) the extraordinarily rich concentration of phytosaur skulls and postcrania. D. chamaensis appears to be an anagenetic descendant of the more common D. haplocerus, known from older Chinle strata across the southwestern United States. Dinosaurs are exceedingly rare in the Chinle Group (except at the Ghost Ranch Coelophysis quarry), and the presence of multiple individuals of single species at the Snyder quarry is a significant discovery. Indeed, Eucoelophysis is one of the oldest known coelophysids and its presence marks the first appearanee of a body form that would be successful until the end of the Early Jurassic (with the extinction of Syntarsus). The abundance of phytosaurs at the Snyder quarry, while somewhat "typical" of Chinle localities, is still significant because the catastrophic nature of the assemblage has helped us document sexual dimorphism in the phytosaur Pseudopalatus (best shown at the nearby Canjilon quarry) and will facilitate studies of the ontogeny and variation of this genus.