BIODIVERSITY AND PALEOECOLOGY OF PLIO-PLEISTOCENE MARINE MOLLUSCS, CARPINTERIA, SANTA BARBARA COUNTY, CALIFORNIA

The Rincon Hill section near Carpinteria (California) contains a sedimentary succession that is highly fossiliferous, with diverse bivalves and gastropods, among other marine invertebrates. It has been correlated with the Santa Barbara Formation, which is distributed more widely to the west in Santa Barbara Basin. The multiple shell beds and intervening mudstones exposed in this section appear to comprise several distinct faunal associations. Bulk samples (5 gallon) were collected from 25 beds within the section, providing insights into the species diversity, paleoecology, and taphonomy of each of assemblages. More than 180 molluscan species are thus far recovered, including 59 bivalves and 120 gastropods. Of more than 18,000 specimens analyzed, Cyclocardia occidentalis (23%), Lirobittium sp. (14%), Turritella cooperi (6%), Alia carinata (12%), and Astyris tuberosa (11%) are most abundant. Despite uniform sample size, species richness varies considerably from bed to bed, with as many as 82 species (2900 specimens) and as few as 20 species (>500 specimens) retrieved from any given sample. This is in part related to the skeletal packing of individual beds (post-mortem concentration), but may also reflect variations in the richness and evenness of contemporaneous biological communities. A range of paleoenvironments are represented in these samples, ranging from intertidal rocky shore and shallow subtidal gravels through offshore silty bottom habitats. Sampling standardized species richness is higher in shallow subtidal assemblages than offshore assemblages, although not consistently, perhaps suggesting the influence of factors not related to water depth.

These sediments have been mapped as Santa Barbara Formation, implying a Middle Pleistocene age. Nevertheless, it has been argued that these sediments belong to a distinct, and as yet unnamed, unit of Pliocene age. Few fossils have been identified from these samples that are undoubtedly Pliocene in age, and many of the distinctive species that typify other late Pliocene units in California are yet to be recognized at Rincon Hill. It is plausible that environmental factors bias against their presence in this section, or that it was indeed deposited during the early or middle Pleistocene and can be correlated with the Santa Barbara Formation in its type area.