A FIRE-RELATED LATE TRIASSIC VERTEBRATE FOSSIL ASSEMBLAGE FROM NORTH-CENTRAL NEW MEXICO

The Snyder quarry is a unique Late Triassic bonebed located near Abiquiu in north-central New Mexico. The quarry is stratigraphically high in the Petrified Forest Formation of the Chinle Group, and tetrapod biostratigraphy places it in the Revueltian land-vertebrate faunachron, which is mid-Norian (~210 - 215 Ma). This locality has yielded fossils of a wide variety of organisms, including phytosaurs, aetosaurs, theropod dinosaurs, reptiles, fish, bivalves, conchostracans, and decapods, as well as plant material.

A taphonomic analysis of the skeletal and plant material, as well as the sedimentology of the quarry indicates that this deposit is the result of a catastrophic mass mortality event. The sediments of the deposit contain rip-up clasts from the surrounding floodplain, a significant portion of the bone and wood is aligned, there is a high density of bones over a large area, and there is a moderate degree of hydraulic sorting of the skeletal material. These data are evidence for very rapid movement and deposition of the bonebed. There is no evidence of abrasion of the bones, and there is a significant amount of associated charcoal, which is buoyant, indicating that transport was minimal.

The skeletal material is associated, and in rare cases articulated, indicating that the animals were in a state of partial decay prior to transport and deposition. There is no evidence of weathering of the bones or of vertebrate scavenging, which is evidence for the rapid burial of the material. Also, an age profile constructed for the phytosaurs reveals a high percentage of subadult or young adult animals. Scanning electron microscopy and reflectance microscopy work on the charcoalized wood show that the internal structure of the cell walls has been homogenized, and the reflectance of the material is substantially higher than other forms of coal. This indicates that the wood was burned in a moderate temperature ground fire. Thus, the evidence from the Snyder quarry fossil assemblage best fits the scenario of a catastrophic Late Triassic wildfire.