REVUELTIAN (EARLY-MID NORIAN) MICROVERTEBRATES FROM THE UPPER TRIASSIC SNYDER QUARRY, PAINTED DESERT MEMBER, PETRIFIED FOREST FORMATION, NORTH-CENTRAL NEW MEXICO

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 demonstrate that the Snyder quarry is of Revueltian (early-mid Norian) age. Screenwashing the matrix of the primary bonebed at the Snyder quarry yields 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 surprisingly rare. New records include a tooth of the hybodontoid shark Lonchidion and numerous scales of a palaeoniscid fish tentatively assigned to aff. Turseodus. Not surprisingly, the microvertebrate assemblage differs somewhat from the known macrovertebrate assemblage, and includes many more fossils of bony fish. Indeed, 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 which are represented by fragments of dentigerous toothplates, fossils previously assigned to colobodontids. The microvertebrate tetrapod fauna represented by teeth includes metoposaurid amphibians, procolophonid anapsids, 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 vertebrae and other non-cranial elements greatly outnumber intact teeth. 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.