MOLLUSCAN DIVERSITY AND TROPHIC STRUCTURE IN TIME AND SPACE: DYNAMICS OF THE REPUTED PLIO-PLEISTOCENE EXTINCTION IN THE CAROLINAS

Plio-Pleistocene western Atlantic molluscs are generally considered to have experienced a multi-phased mass extinction. Over the past three years, our Research Experiences for Undergraduates program has examined diversity and trophic structure of Plio-Pleistocene mollusc communities to clarify extinction dynamics in the Carolinas. We bulk sampled 16 sites: from the Neuse River northward, an unnamed Pliocene unit (Yorktown Fm?) at Fountain, NC, the Chowan River Fm at its type locality, the lower James City Fm at Lee Creek, and upper James City near its type locality; in southeast NC and adjacent SC we sampled four Duplin Fm, five lower Waccamaw Fm, and three upper Waccamaw Fm sites. Samples were sieved through a ¼” mesh and specimens were picked, sorted, identified to genus, and counted (totaling ~5600 gastropod and >50000 bivalve specimens). Genus-level richness was compared among units using rarefaction (data reported here are rarefied to 1440 bivalve and 100 gastropod specimens); life modes were taken from the NMITA database.

The reputed mass extinction apparently had relatively little impact on richness and trophic structure prior to the upper Waccamaw and correlative upper James City; some geographic differences occur. Samples from the Neuse River northward are less diverse than the southern Duplin and Waccamaw sites. The Pliocene Duplin on the Lumber River is our most diverse sample (57 bivalve, 24 gastropod genera) but 95% confidence intervals (CI) overlap with several Pleistocene lower Waccamaw sites; less diverse Duplin and lower Waccamaw samples also overlap. Upper Waccamaw bivalve assemblages are significantly less rich (37 genera); the least rich gastropod sample (8 genera at Todd Pit) is also upper Waccamaw. Chowan River and lower James City (Pleistocene) bivalve samples are significantly less rich (36 genera) than Duplin and lower Waccamaw samples and overlap in 95% CI with the upper Waccamaw. The upper James City is the least rich bivalve sample (26 genera) and one of the least rich gastropod samples (16 genera) but all northern gastropod samples overlap in 95% CI. Fewer geographic differences occur in trophic structure. Suspension feeders consistently make up ~75% of bivalve genera across all formations. However, fewer predatory gastropods occur in the northerly samples and in the upper Waccamaw.