TECTONIC INFLUENCES ON SPECIES RICHNESS OF MAMMALS IN THE MIDDLE MIOCENE DOVE SPRING FORMATION, CALIFORNIA

Periods of tectonic activity often coincide with marked changes in the species richness of fossil mammals. Basin extension and uplift of the surrounding area generate new habitats and changes corridors for the dispersal of mammal species. We investigated the link between faunas and an increase in basin area through a study of changes in mammalian species richness from the Middle to Late Miocene (12.5 – 8.0 Ma) Dove Spring Formation of the El Paso Basin in the northwestern Mojave region of California.

Two stages of tectonic activity occurred during deposition. Left-shear fault movement south of the basin led to counterclockwise rotation at approximately 10.0 Ma, and east-west extension contributed to subsidence beginning near 9.5 Ma. Increasing accommodation space accumulated sediments from sources south of the basin until the activation of a new source in the form of the Sierra Nevada near 8 Ma. We examined changes in the sediment accumulation rate and the frequency of fossil localities to assess the tectonic influence on preservation potential. These features display an inverse relationship in the El Paso Basin. The frequency of localities is greatest in the middle of the section, reaching a peak at 9.5 Ma after a long interval of declining sediment accumulation rate and coarsening lithology. A subsequent sharp decline in the frequency of localities coincides with a rapid but brief increase in sedimentation rate. The environmental conditions leading to this relationship were further examined in the field through stratigraphic analysis of depositional facies.

Using a dataset compiled from fossils at the Natural History Museum of Los Angeles, we determined changes in mammalian species richness over time. We reconstructed stratigraphic ranges for 31 large-mammal taxa (>1 kg) and determined temporal ranges supported by 50% confidence intervals. The species richness of most mammalian families peaks at 10.0 Ma and is positively correlated with the number of fossil localities per time interval. A notable exception to this pattern is the disappearance of grazing horses (Anchitheriinae and Equinae) from the basin by 9.0 Ma, prior to any other group, which may indicate a shift in habitat or a dearth of grazing resources. We will use soil organic matter and plant biomarkers to examine this potential change in vegetation in a future study.