PATTERNS OF DRILLING PREDATION IN MOLLUSCS ACROSS THE PLIOCENE-PLEISTOCENE BOUNDARY IN SOUTHERN CALIFORNIA

During the Neogene period, a series of marine basins and embayments covered what is today the continental borderland and coastal plain of Southern California. Sedimentary layers from these basins contain a rich fossil fauna, including thousands of molluscan species, and record ecological transitions through the mid-Pliocene warming event and the shift into the cooler Pleistocene. In this study, I examined evidence of predatory drilling in bivalves and gastropods from shallow marine habitats using specimens from Pliocene and Pleistocene (5.0 to 0.0117 Ma) sedimentary formations ranging geographically from present day Santa Maria to the California-Mexico border. Evidence of predatory shell drilling by muricid and naticid gastropods is widespread amidst these fossil assemblages. Because drill holes are direct evidence of predator behavior, study of the incidence and style of drilling gives clues to past trophic interactions. Previous studies, most of which focus on the Atlantic and Gulf regions of N.A., have suggested that changes in predator-prey interactions follow faunal turnover and examined latitudinal differences in predation. In my analysis of 12 genera from 19 localities in Southern California Plio-Pleistocene basins, the prevalence of predatory drilling does not appear to be tied to sedimentary unit or latitude. The drilling frequency does, however, appear to be taxon-specific, with presence of drilling more common in some genera than others, and the presence of incomplete borings present in only some of the genera studied. Results of this study also suggest no prey size selectivity within ecosystems, the exception being one prey genus in which drilling was absent in the largest shells. Drill site stereotypy is present in most of the taxa studied. Despite ecological turnover and climate change that marks the Plio-Pleistocene, drilling predator-shelled prey relationships appear stable.