The microvertebrate fauna of the Upper Triassic (Revueltian) Snyder Quarry, north-central New Mexico

Abstract

—The Snyder quarry is a well-documented assemblage of Late Triassic invertebrates and vertebrates from the Painted Desert Member of the Upper Triassic Petrified Forest Formation in the Chama Basin, north-central New Mexico. The presence of Revueltian index taxa, including the aetosaurs Typothorax coccinarum and Desmatosuchus chamaensis and the phytosaur Pseudopalatus buceros, demonstrate that the Snyder quarry is of Revueltian (early-mid Norian) age. Screenwashing matrix from the primary bonebed at the Snyder quarry yielded a moderately diverse assemblage of microvertebrates, some of which were not represented in the macrovertebrate fauna. Microvertebrate fossils from the Snyder quarry are mostly scales and bone fragments—complete teeth are unusually rare. New records include a tooth of the hybodontoid shark Lonchidion and numerous scales of a palaeoniscid fish tentatively assigned to aff. Turseodus. The microvertebrate assemblage differs somewhat from the known macrovertebrate assemblage, and includes many more osteichthyan fossils. Osteichthyans dominate the microvertebrate fauna, and include semionotids, redfieldiids, palaeoniscoids, and indeterminate sarcopterygians. Osteichthyans are largely represented by scales, with the exception of the indeterminate sarcopterygians and actinopterygians, represented by fragments of dentigerous toothplates, fossils previously assigned to “colobodontids.” The microvertebrate tetrapod fauna represented by teeth includes metoposaurid amphibians, juvenile (?) phytosaurs (?), probable dinosaurs, aetosaurs and other diverse, unidentified archosauromorphs. Many of the vertebrae appear to pertain to small archosauromorphs. The microvertebrate assemblage is unusual in that recovered vertebrae and other non-cranial elements greatly outnumber intact teeth, which normally dominate Chinle microvertebrate assemblages. We interpret this as additional support for the hypothesis of a catastrophic origin for the Snyder quarry vertebrate assemblage, as more typical Chinle Group microvertebrate assemblages are attritional deposits in which teeth greatly outnumber vertebrae.