Taphonomic analysis of a fire-related Upper Triassic vertebrate fossil assemblage from north-central New Mexico

Abstract

—The Snyder quarry is an Upper Triassic bonebed located in north-central New Mexico. The locality is stratigraphically high in the Petrified Forest Formation of the Chinle Group, and tetrapod biostratigraphy places it in the Revueltian land-vertebrate faunachron (mid-Norian in age: 210-215 Ma). This site has yielded the remains of a wide variety of organisms, ranging from terrestrial and aquatic vertebrates to aquatic invertebrates, as well as a substantial volume of charcoalified wood. A taphonomic analysis of both the biological material and the associated sediments indicates that the bonebed is the result of a catastrophic mass mortality event. The sediments of the bonebed 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 a moderate degree of hydraulic sorting of the skeletal material, indicating brief transport and rapid deposition of the bonebed. There is no evidence of abrasion on the bones, indicating that transport was minimal. The skeletal material is associated, with no evidence of substantial weathering or vertebrate scavenging, reflecting a rapid burial of partially dissociated carcasses. A survivorship curve constructed for the phytosaurs, the dominant taxonomic group, shows a thanatocoenosis that matches the biocoenosis for a reptile population. Scanning electron microscopy of the charcoalified wood reveals that the internal structure of the cell walls has been homogenized, and the reflectance of the charcoal is significantly higher than that of other forms of plant fossil preservation. These two pieces of data are evidence that the wood was burned in a moderate temperature (300-450°C) ground fire. Thus, both the sedimentological and biological data from the Snyder quarry best fit the scenario of a catastrophic, Late Triassic wildfire.