PALEOECOLOGICAL RELATIONSHIPS WITHIN PLIOCENE BIVALVE ASSEMBLAGES FROM THE CAROLINAS AND VIRGINIA

Students in an undergraduate invertebrate paleontology class at University of North Carolina Wilmington investigated paleoecological relationships for four Pliocene bivalve assemblages. We bulk sampled the Duplin Fm at Tearcoat Branch (Sumter Co., SC) and Kirby Pond (Timmonsville, SC). We also examined coeval samples from the Yorktown Fm of Virginia: the Morgarts Beach Mbr from Fort Boykin and the Moore House Mbr at Deep Creek. Samples were sieved through 5 mm mesh and whole specimens and fragments with beaks were picked, sorted and identified to genus. Life modes (substrate relations, attachment, mobility, and diet) were assigned to genera using the NMITA molluscan life mode database. Complete and incomplete predatory drillholes were tallied for ~whole valves.

The most abundant genus in the South Carolina and Morgarts Beach samples was Mulinia, whereas Cyclocardia, Astarte, and Glycymeris were most abundant in the Moore House, perhaps due to environmental differences. Only 12% of Moore House specimens (25% of genera) were infaunal siphonates, but the other samples were dominated by infaunal siphonates (84 – 99% of specimens; 72 -98% without Mulinia; 50 – 69% of genera). Actively mobile forms composed 85 – 99% of specimens for all samples (74 – 96% excluding Mulinia) except the Moore House, which included 64% actively mobile specimens (50% of genera). All samples were dominated by suspension feeders (64 – 99% of specimens); omission of Mulinia reduced suspension feeders to 31% and 11% in the Tearcoat and Morgarts Beach, respectively, due to abundant chemosymbiotic lucinids. A decrease in suspension feeders signaled productivity decline in the Caribbean Plio-Pleistocene; results are unclear for the mid Atlantic coastal plain. The proportion of suspension-feeding bivalves (~75% of genera, ~95% of specimens) was consistent from the Pliocene Duplin to the Pleistocene Waccamaw elsewhere in the Carolinas. Morgarts Beach values are significantly less and Moore House values greater than for the previously studied Plio-Pleistocene samples.

Drilling frequency (DF) was ~16% for both South Carolina samples; DF for the Moore House was 19% and for Morgarts Beach was 29%, significantly greater than for any other sample due to heavy drilling on Parvilucina. Incomplete drillholes constituted 2 – 11% of drilling attempts.