PREDATION RATES IN A MODERN CARBONATE SETTING, SAN SALVADOR ISLAND, BAHAMAS

Predation rates in modern carbonate settings, as measured by drilling frequencies in mollusc assemblages, can provide an important framework for paleontological analyses. An analysis of 1,947 shells from two beaches of San Salvador Island, Bahamas, shows that statistically significant differences in drilling frequencies exist between the beaches located within ~15 km of each other. At Sandy Point Beach (southwest San Salvador), 978 shells were analyzed and approximately 6% of these shells were drilled. At Haitian Boat Beach (northwest San Salvador), an assemblage of 969 shells was examined and 16% of these shells contained drill holes. Based on beveling present in most of the drill holes, the dominant predator belongs to the genus Polinices; members of this gastropod genus are present, although never abundant, in these assemblages.

At Sandy Point Beach, the most abundant genera are Americardia, Glycymeris, and Divericella, accounting for nearly ~65% of all the shells counted. Although each genus contained bored shells, high drilling frequencies were also found in less abundant groups, such as Chione, Lima, and some genera of gastropods. At Haitian Boat Beach, the most abundant genera are Americardia, Chione, and Tellina, which account for 66% of the shells in the Haitian Boat assemblage. These show drilling frequencies of 14%, 25% and 15%, respectively, but higher drilling frequencies occur in Divaricella and Cerithium (26% and 29%, respectively). These results suggest that shells with the highest drilling frequencies are not necessarily the most abundant.

In addition to different rates of predation at the two beaches, taphonomic differences also exist. Shells collected at Haitian Boat Beach tend to be thinner and more abraded than those at Sandy Point Beach. Sandy Point Beach shells are robust and colorful, although many shells are partially filled with lithified fine-grained carbonate sediment. Future work will focus on integrating the environmental, taphonomic, and ecological differences between Haitian Boat and Sandy Point beaches to elucidate what factors control drilling frequencies in modern carbonate settings.