REPAIR SCAR FREQUENCY AS AN INDICATOR OF ECOLOGICAL CHANGE AND STABILITY THROUGH TWO EPISODES OF FAUNAL TURNOVER IN THE PLIO-PLEISTOCENE OF THE CAROLINA COASTAL PLAIN

Late Neogene mollusk assemblages in the western Atlantic underwent a change in trophic structure associated with at least two episodes of regional faunal turnover that accounted for the extinction of ~ 70% of mollusk species. A decrease in the relative abundance of suspension feeding mollusks in the Caribbean suggests that the turnover was caused by a drop in primary productivity due to changes in oceanographic circulation with the closure of the Central American Isthmus, but declining sea water temperatures may have also played a role at higher latitudes. An important component of this ecological reorganization was predator-prey interactions: predatory gastropods decreased in relative abundance and there was a reduction in predation intensity. In addition, change of feeding behavior and frequency of confamilial predation among some drilling gastropods indicate relaxation of competition after the Late Pliocene. It is not clear, however, if these changes characterize predator-prey interactions in general, including those involving the myriad of shell crushing predators (e.g. whelks, crabs, fishes), which often lack an abundant skeletal fossil record. We test the hypothesis that the effect of extinction on shell crushing predators produces a pattern of change and stability across two episodes of faunal turnover coinciding with the changes in drilling gastropod feeding strategy and intensity as well as assemblage trophic structure. Bulk samples were collected from six localities, two from each of three time intervals bracketing the turnover events: late Pliocene (Duplin Formation), early Pleistocene (lower Waccamaw Formation), and middle Pleistocene (upper Waccamaw Formation). Ongoing work uses the frequency of failed attacks by crushing predators, preserved as repair scars on the shells of intended prey, as a proxy for change in the host of ecological factors that regulate the outcome of predator-prey interactions. The number of repair scars per specimen is calculated for each of nine mollusk taxa (Glycymeris, Parvilucina, Caryocorbula, Mulinia, Cyclocardia, Plicatula, Anadara, Olivella, and Crepidula) selected to represent a variety of lifemodes. Repair frequency is standardized for abiotic factors and intrinsic prey characteristics that may alter the frequency of non-predatory shell breakage.