DIVERSITY OF MOLLUSCAN ASSEMBLAGES IN THE LOWER WACCAMAW FORMATION (PLEISTOCENE) AT SNAKE ISLAND PIT IN NORTH CAROLINA

Bulk samples of assemblages from the lower Waccamaw Formation at Snake Island Pit in Columbus County, southeastern North Carolina, were analyzed as part of ongoing research on the Plio-Pleistocene molluscan extinctions. Bags #6–#10 were processed by a paleontology class at Georgia State University (GSU) yielding ~5000 bivalve and ~800 gastropod specimens. These five samples were sieved using a ¼” mesh and only shells containing intact umbos (bivalves) and apices (gastropods) were retained. Taxa were sorted and identified to genus level and relative abundances calculated. Data on individuals for bivalves (specimens divided in half) and gastropods were used for comparing diversity across all samples at the 95% confidence level via Holland’s Analytic Rarefaction program. Taxa were ranked in order to recognize common genera. All results were compared with data from a smaller sample composed of 750+ specimens (85% bivalves, 15% gastropods) retrieved from the same locality as processed earlier by University of North Carolina Wilmington (UNCW) students.

The GSU samples combined contained at least 55 bivalve and 30 gastropod genera with ~3% of all specimens grouped as rare taxa still to be identified. These five bulk samples were represented by a minimum of 445 individuals with 48 genera and a maximum of 826 individuals with 65 genera. The UNCW sample contained 451 individuals and 50 genera (35 bivalve, 15 gastropod). The number of genera was compared across all six samples when richness was rarefied at 440 individuals. Confidence intervals at the 95% level overlapped for all samples except between the pair that exhibited the highest and lowest counts of individuals and genera.

Nearly 75% of the individuals in the GSU samples was made up of 10 genera comprising both bivalves (Anomia, Diplodonta, Glycymeris, Laevicardium, Lirophora, Plicatula) and gastropods (Crepidula, Conus, Olivella, Prunum). Over 80% of the individuals in the sample processed by UNCW students was represented by eight of the same genera as well as Oliva and Conradostrea instead of Olivella and Prunum. The dominance and evenness of these assemblages will be examined further upon the completion of rare taxa identification in the GSU samples.