Systematic paleontology of marine gastropods from the Upper Cretaceous Chatsworth Formation, Simi Hills, Southern California

Fossiliferous deposits of the Upper Cretaceous Chatsworth Formation are exposed in the Simi Hills of Los Angeles and Ventura counties, southern California, and have yielded a rich and diverse molluscan fauna. At least 36 species of gastropods, representing 18 families are present in collections from the Chatsworth Formation stored at the Invertebrate Paleontology Department of the Natural History Museum of Los Angeles County. The gastropod family showing greatest diversity in these collections is Perissityidae, which is represented by five species. Specimens of the trochid Atira ornatissima (Gabb, 1864) and ringiculid Biplica obliqua (Gabb, 1984) show the greatest abundance. The gastropods are generally indicative of warm-temperate waters. Tropical Tethyan species, such as nerineids and actaeonellids are lacking, although some of the Chatsworth gastropods probably tolerated Tethyan waters. A rudist bivalve indicates at least locally warmer waters. Deposits in the lower part of the Chatsworth Formation, in Bell and Dayton Canyons, contain the most mollusks, and are middle Campanian in age. Based on previous work by others analyzing depositional facies and their associated fossils, these strata were deposited on the continental slope at bathyal depths as part of an extensive submarine fan complex. The gastropods were transported downslope via turbidity currents and underwater debris flows. The Chatsworth fan was probably part of a larger offshore shelf and slope complex that includes deposits presently exposed in the Santa Monica and Santa Ana mountains in southern California.