TAPHONOMY OF MAMMAL FOSSILS IN THE BARSTOW FORMATION (MIDDLE MIOCENE), SOUTHERN CALIFORNIA, IN RELATION TO FACIES AND PALEOENVIRONMENTS

The Barstow Formation of southeastern California preserves a rich middle Miocene mammalian fossil record that forms the basis of the Barstovian North American Land Mammal Age. Three members of the Barstow Formation—the Owl Conglomerate, Middle, and Upper members—have been distinguished on the basis of broad lithological differences. Major facies associations within these members represent the dominant environments at the time of deposition, which changed over time from early-forming playa- and alluvial-fan-dominated environments to forested floodplain environments, to wooded grassland and spring-fed wetlands. Both the number of vertebrate fossil localities and mammal diversity increase upsection with changing paleoenvironments.

To determine how taphonomic histories changed in relation to changing environments, we examined skeletal material in museum collections from 70 vertebrate localities for taphonomic indicators (e.g., skeletal-element composition, weathering stage, abrasion, tooth marks) and measured detailed stratigraphic sections at 64 representative vertebrate localities from major facies associations. The depositional settings of the 64 fossil localities examined in the field include channel and bar deposits (12.5%), proximal-channel settings (26.5%), distal-channel settings (21.8%), well-drained floodplain settings (12.5%), and poorly drained floodplain settings (26.5%). For 47 of these localities with both field- and collections-based observations, we interpreted the taphonomic pathways through which bones accumulated. The dominant taphonomic pathway is the accumulation of bones at long-term mortality sites in channel fills and around waterholes (80.8%), with fewer concentrations forming exclusively from fluvial processes in channel lags (4.3%) and crevasse splays (4.3%) or through carnivore or scavenger activity (10.6%). Taphonomic pathways are more diverse in facies of the Upper Member than in facies of the Owl Conglomerate or Middle members. Facies associations that preserve a greater range of depositional settings preserve more vertebrate taxa and more assemblages with complex taphonomic pathways.