ECOLOGICAL STRUCTURE OF A MOLLUSCAN ASSEMBLAGE FROM THE UPPER WACCAMAW FORMATION, SHALLOTTE, NORTH CAROLINA, AND THE PLEISTOCENE EXTINCTION

We conducted a paleoecological analysis of a mollusc assemblage from the upper Waccamaw Fm near Shallotte, Brunswick Co., NC. The samples were collected for a sedimentological study by Mabry (2001) from the Todd marl pit, a locality no longer accessible.

Seven bulk samples (~2 liters each) from three different stratigraphic sections within the pit were examined. Samples were sieved through ¼” mesh and whole mollusc specimens and fragments with beaks (bivalves) and apices (gastropods) were picked, sorted and identified to genus level. Life modes (substrate relation, attachment, mobility and diet for bivalves; gastropod diet) were assigned using the NMITA molluscan life mode database.

Samples varied lithologically (fine to medium sand with varying amounts of carbonate and mud, some with oxidation or induration), and also in abundance and quality of preservation of fossils. Sample sizes ranged from 54-938 bivalves (1459 total specimens) but only 0-79 gastropods (110 total specimens). In total, 38 bivalve genera and 8 gastropod genera are present. Rarefaction was used to compare richness of bivalve samples; at a sample size of 50 specimens, richness ranged from ~10 – 14 genera, except for samples from the northernmost section. These two Mulinia-dominated samples had the highest abundance (130 and 938 bivalve specimens) but lowest richness (~7 and 4 genera) and highest dominance (0.71 and 0.85, compared to all other samples in which dominance was 0.19 – 0.43).

For the combined samples, 8% of bivalve individuals were epifaunal and 7% were attached. Suspension feeders represented 93-100% of bivalve individuals (99% for the combined samples) and 73-100% of bivalve genera (76% for the combined samples). Gastropod individuals included 62% suspension feeders and 35% predators (those two groups each comprised 38% of gastropod genera).

A pulse of extinction separates the lower and upper Waccamaw. As expected, richness of both bivalves and gastropods was significantly less than for the lower Waccamaw near Old Dock, NC. Unexpectedly, bivalve richness was greater than in the correlative upper James City Fm at James City, NC; gastropod richness was lower. A drop in productivity is not supported as a cause of the extinction; % suspension feeders remained high from lower to upper Waccamaw, although % predatory gastropod genera declined.