PRESERVATION AND FAUNAL CHANGE IN THE BARSTOW FORMATION, SOUTHERN CALIFORNIA: IMPLICATIONS FOR TURNOVER AT THE HEMINGFORDIAN-BARSTOVIAN BOUNDARY

The Barstow Formation preserves rich middle Miocene mammalian assemblages that form the basis of the Barstovian North American Mammal Age. In the Barstow Formation, the Hemingfordian-Barstovian boundary is characterized by an increase in large-mammal (>1 kg) diversity and the lowest stratigraphic occurrences of several taxa. This increase in diversity coincides with a change in facies, depositional environments, and an increase in the number of fossil localities. We tested the effects of changing facies and fossil productivity on the clustering of first occurrences at the Hemingfordian-Barstovian boundary. Six facies associations representing the dominant environments at the time of deposition changed through time in relation to the tectonic and climatic history of the basin. Over time, environments transitioned from playa lakes and alluvial fans to floodplains, representing significant changes in habitats and preservation potential. We tested whether diversity is higher in specific facies associations and how their stratigraphic distribution influences observed patterns of turnover for large-mammal species.

We compiled species-abundance and occurrence data from 148 localities in order to document changes in faunal composition and turnover through the formation. We calculated evenness, observed turnover, and inferred turnover from 80% confidence intervals on the stratigraphic ranges of 54 large-mammal species. Species body size and facies were important determinants of preservation potential and contributed to observed patterns in richness and evenness among facies associations, as well as species ranges through the formation. Observed lowest and highest occurrences cluster at specific stratigraphic intervals that correspond with facies transitions, including high observed turnover at the Hemingfordian-Barstovian boundary. In contrast, patterns of inferred turnover based on confidence intervals are stratigraphically dispersed. Thus, the high turnover marking the Hemingfordian-Barstovian boundary in the Barstow Formation is a product of favorable preservation, and the increase in diversity is more likely due to a change in depositional environments than to an influx of immigrant taxa.