About 100 species, mostly pelecypods and gastropods, with smaller numbers of scaphopods, brachiopods, echinoderms, barnacles, bryozoans and vertebrates occur in the Castaic, many of them farther south than previously reported in the upper Miocene. Many of the Castaic taxa also are found in the Recent Panamanian Molluscan Province, indicating that the Castaic Formation was deposited near the northern limit of the late Miocene equivalent of that province.