PALEOECOLOGICAL RELATIONSHIPS WITHIN A MOLLUSCAN ASSEMBLAGE FROM THE PLEISTOCENE OF HORRY COUNTY, SOUTH CAROLINA

Students in an undergraduate invertebrate paleontology class at University of North Carolina Wilmington investigated paleoecological relationships at the classic DuBar WA 56 site at Windy Hill airstrip, Myrtle Beach, SC. Two bulk samples of an argillaceous fine sand, thought to represent the Waccamaw Formation, were sieved through 5 mm mesh. Whole mollusc specimens and fragments with beaks (bivalves) and apices (gastropods) were picked, sorted and identified to genus. Life modes (bivalve substrate relations, attachment, mobility, diet; gastropod diet) were assigned to genera using the NMITA molluscan life mode database. Complete and incomplete predatory drillholes were tallied.

The two samples included 300 gastropods and ~25,000 bivalves, including >23,000 Mulinia. Dominance of Mulinia suggests a somewhat different environment from other Waccamaw sites, perhaps an estuary. Due to the overwhelming dominance of this genus, bivalve life mode proportions were calculated with and without Mulinia. For the combined bivalve assemblages, 26% of genera were epifaunal; 3% of individuals (45% excluding Mulinia) were epifaunal. Actively mobile forms composed 74% of bivalve genera and 98% and 63% of bivalve individuals with and without Mulinia respectively. Suspension-feeding bivalves represented 76% of bivalve genera and 99% of bivalve individuals (93% without Mulinia). Among gastropods, predators comprised 60% of genera and 56% of individuals. Shell-drilling naticid gastropods (Neverita) ranked fourth in abundance among 20 gastropod genera. Drilling frequency (DF) was 15% and 9% on the bivalve and gastropod assemblages respectively. Of the 43 bivalve genera present, 21 were drilled, with most DFs ~14 – 21%. Neverita and Olivella were the most commonly drilled gastropods (DFs ~ 13%). Incomplete drillholes were rare (observed on only 4 genera). Comparison with 10 other Waccamaw sites in the Carolinas indicates a consistent proportion of suspension-feeding bivalves (~75% of genera and ~95% of individuals) but percent predatory gastropods ranges more widely among localities (38 – 79% of genera, 18 – 91% of individuals). A decrease in suspension-feeding bivalves and predatory gastropods signaled productivity decline in the Caribbean Plio-Pleistocene; results are less clear for the Carolinas.