341 New Mexico Geological Society, 56 th Field Conference Guidebook, Geology of the Chama Basin, 2005, p. 341-354. TAPHONOMIC ANALYSIS OF A FIRE-RELATED UPPER TRIASSIC VERTEBRATE FOSSIL ASSEMBLAGE FROM NORTH-CENTRAL NEW MEXICO KATE E. ZEIGLER 1 , ANDREW B. HECKERT 2 AND SPENCER G. LUCAS 2 1 Dept. of Earth & Planetary Science, University of New Mexico, Albuquerque, NM 87131 2 New Mexico Museum of Natural History, 1801 Mountain Rd. NW, Albuquerque, NM 87104 ABSTRACT.—The Snyder quarry is an Upper Triassic bonebed located in north-central New Mexico. The locality is strati- graphically 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 evi- dence 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. INTRODUCTION The Snyder quarry is an exceptionally productive and diverse Upper Triassic bonebed located in north-central New Mexico, in the Chama River basin, near the town of Abiquiu (Fig. 1) (Heckert et al., 1999a, b). The locality was discovered in 1998 by an amateur fossil hunter, Mark Snyder, and was extensively excavated from then through the summer of 2001 by crews from the New Mexico Museum of Natural History (NMMNH). The Snyder quarry has produced a rich array of fossil remains. Archosaurs dominate the bonebed, and include coelophysoid dinosaurs, two genera of aet- osaurs, rauisuchians, and phytosaurs. Rarer taxa include a pos- sible procolophonid, metoposaurid amphibians, semionotid fish, a decapod, a conchostracan, and unionid bivalves. There is also a significant quantity of charcoalified wood, which is an unusual occurrence in nonmarine Upper Triassic strata. The geologic and biological data retrieved from excavations of the Snyder quarry provide evidence of a catastrophic event. The presence of the charcoal, when combined with the other data, indicates that a wildfire or series of wildfires occurred. Abbreviations: NMMNH = New Mexico Museum of Natural History, Albuquerque; UCMP = University of California Museum of Paleontology, Berkeley. STRATIGRAPHY AND AGE The Snyder quarry is stratigraphically high in the Painted Desert Member of the Petrified Forest Formation of the Upper Triassic Chinle Group (Fig. 1). The designation of Chinle Group is used here following Lucas (1993) and Lucas et al. (1997), wherein the Chinle Formation was raised to group status in order to unify the lithostratigraphic nomenclature of nonmarine Upper Triassic strata in the western United States. The quarry is approxi- mately 60 m below the base of the Middle Jurassic Entrada Sand- stone. This horizon is approximately stratigraphically equivalent to the famous Canjilon phytosaur quarry, 4 km to the east, which was excavated by Charles Camp for the UCMP in 1930 and 1933. Based on the presence of the phytosaur Pseudopalatus and the aetosaur Typothorax coccinarum, the Snyder quarry is late Revueltian (mid-Norian) in age (Lucas, 1998, 1999) and thus is approximately 210-215 million years old. LITHOSTRATIGRAPHY The main bone-bearing horizon of the Snyder quarry (Level 1) is less than 30 cm thick and consists of a greenish-gray, intra- formational, mud-clast conglomerate that becomes finer-grained upward and grades into a greenish silty mudstone (Fig. 2). The conglomerate is dominantly a sandy mudstone that is yellowish gray (5Y 7/2), with approximately 20% clasts. This conglomerate has a sharp, erosional contact with the underlying unit and has locally scoured the lower unit away completely. A second conglomerate (Level 2) lies approximately 60 cm above the mudstone. This conglomerate is not as well defined as the underlying main horizon, and interfingers with silty mud- stones. This conglomerate also fines upward into a pebbly to silty mudstone that is locally laminated and in places shows evidence of soft-sediment deformation in the form of disrupted laminae and pockets of pebbles. Overlying the laminated mudstones is a third conglomerate (Level 3) that is almost one meter thick, with