PREDATOR-PREY RELATIONSHIPS IN A MIDDLE EOCENE MOLLUSCAN FAUNA FROM ORANGEBURG, SOUTH CAROLINA

Bivalve and gastropod drilling frequencies and site selectivity were analyzed in a bulk sample of the middle Eocene McBean Formation collected from an outcrop located five miles north of Orangeburg, South Carolina. Based on previous work on latitudinal drilling gradients, we hypothesized that drilling frequencies should be intermediate between those documented in the Gulf Coast and those recorded from Virginia. The quartz-rich siltstone has abundant, internal and external molds and casts, of which 606 were collected and examined. The dominantly infaunal, suspension feeding, fauna includes bivalves, gastropods, scaphopods and bryozoans, listed in order of abundance. Of these fossils, 139 are considered whole specimens, identified either by a bivalve umbo, gastropod apex or encompassing more than one half of the specimen. The rest were partials, or fragments, and in many cases could only be identified to the class level. Two hundred forty-six specimens were sorted into 26 genera, of which 85% were bivalves and 8% were gastropods. We recorded 125 complete pelecypod valves. The most abundant bivalves were Venericardia, Calorhadia, and Pteropsella lapidosa. Gastropods included Eopleurotoma, various turritellids and the only predatory gastropod, Natica semilunata. Only five whole specimens, all bivalves, had drill holes, giving a drilling frequency of 8%. Drilled bivalves included the genera Tellina and Leda. This result is closer to Gulf Coastal Plain drilling frequencies than to Atlantic Coastal states. This result does not fit easily into the trend of decreasing drilling frequencies with decreasing latitude since this value is less than drilling frequencies recorded for the stratigraphically equivalent Lisbon Formation of Alabama, which ranged from 9.2% to 30.1%; however, this may be due to the sample size, poor preservation, and the low diversity of this assemblage. Prey drill-hole site selectivity was examined using a 9-sector grid for bivalve shells. Three of the five drilled specimens had drill holes located in sector 2 near the umbo with the other two in sectors 5, centrally located, and 7 which is anteroventral. Additional studies of southeastern Atlantic coastal plain molluscan samples are needed to clarify latitudinal trends in predator-prey relationships.