PREY SELECTIVITY BY NATICID GASTROPOD PREDATORS WITHIN THE WACCAMAW FORMATION (PLIO-PLEISTOCENE) OF NORTH CAROLINA

Prey selectivity of drilling naticid gastropod predators from the Plio-Pleistocene Waccamaw Formation was studied by student research teams in the context of an invertebrate paleontology course at the University of North Carolina at Wilmington. Bulk samples were collected near Old Dock, North Carolina, wet sieved through ¼ inch mesh, and all whole pelecypod valves and gastropods were picked and identified. Each sample yielded ~ 200 individuals. Teams tabulated data on frequency of complete and incomplete drillholes from their sample and, based on preliminary observations, developed hypotheses concerning selective behavior of the naticid predators. Team I hypothesized that prey preference was influenced by the degree of ornamentation on the bivalve shell. Each bivalve species in the assemblage was coded for degree of ornamentation (1=smooth, 2=fine ribs, 3=coarse ribs, 4=pronounced spines) and drilling frequencies were calculated. Results supported the hypothesis; bivalve species with coarse ribs or spines were drilled less frequently than less ornamented species. Mobile and/or epifaunal species, which typically do not come into contact with infaunal naticids, or which have active defenses, provided the exceptions to these results. Team II hypothesized that naticids should attack medium-sized prey that represent a favorable cost-benefit ratio to the predator but that are within a size range that can be subjugated successfully. The hypothesis was tested by dividing species into three size categories and tallying drilling for each category. The data supported this hypothesis, although small shells with unfavorable cost-benefit ratios were also drilled frequently. Team III investigated whether selectivity of drillhole position had changed through time by tallying drillhole position with respect to a nine-sector grid for the Waccamaw sample and a modern sample from a similar environment in northern Florida. Predator preference shifted from central placement of the drillhole for the Waccamaw Formation to a position on the umbo in the Recent sample. The shift was accompanied by a decrease in the percent of incomplete drillholes and may suggest an increase in predator capabilities since the Waccamaw. Alternatively, two different species of drillers, with different preferences, may have produced the holes.