A TAPHONOMIC SURVEY OF THE BONEBEDS OF THE UPPER TRIASSIC CHINLE GROUP, SOUTHWESTERN UNITED STATES

Upper Triassic Chinle Group sediments in the American Southwest were deposited over approximately 30 million years (Carnian to Rhaetian). Numerous vertebrate fossils have been recovered from these sediments, including those of dinosaurs, phytosaurs, aetosaurs, metoposaurs, dicynodonts, and fish, among others, and there are several well-known bonebeds (mass death assemblages). In Otischalkian (late Carnian) sediments, four Trilophosaurus quarries in West Texas have been discovered. The following quarries have been discovered in Adamanian-age (late Carnian) sediments: Lamy amphibian quarry (New Mexico), Placerias quarry (Arizona), Dying Grounds and Dinosaur Ridge (Petrified Forest National Park, Arizona), Ward's bonebed (Arizona), the Krzyzanowski bonebed (Arizona), and Rotten Hill (West Texas). In Revueltian (Norian) sediments, numerous well-known quarries have been excavated. The Snyder, Canjilon and Hayden quarries are phytosaur-dominated bonebeds near Ghost Ranch (New Mexico). All are stratigraphically equivalent and probably represent the same event. Other Revueltian bonebeds include the Billingsley bonebed (Arizona), Post quarry, Borden Trilophosaurus quarry (West Texas), Revuelto Creek and several Barranca Creek assemblages (New Mexico), and Dinosaur Hill, RAP Hill, Zuni Well Mound and Lacey Point (Petrified Forest National Park, Arizona). In sediments of Apachean (Late Norian-Rhaetian) age, the Coelophysis quarry, Gregory's Apache Canyon quarry 2, and two phytosaur-dominated bonebeds have been discovered in New Mexico. Of these quarries, at least six of them are catastrophic mass death assemblages: two Otischalkian Trilophosaurus quarries, Placerias quarry, Snyder/Canjilon/Hayden quarries, a Barranca Creek locality and the Coelophysis quarry. Most of the other assemblages are attritional fossil accumulations that formed over longer spans of time. While this analysis focuses on macrovertebrate assemblages, many of these macrovertebrate assemblages yield microvertebrate material (e.g., Dying Grounds), and there are also numerous microvertebrate localities that are not associated with macrovertebrate material. These isolated microvertebrate assemblages in the Chinle have different taphonomic modes than the larger vertebrate assemblages.