SPATIAL VARIATION IN ECOLOGICAL STRUCTURE OF A MOLLUSCAN ASSEMBLAGE FROM THE LOWER WACCAMAW FORMATION, SOUTHEASTERN NORTH CAROLINA

This study examined variation in ecological structure, including richness and trophic structure, within the late Pliocene lower Waccamaw Formation. Samples were collected at the Register Quarry near Old Dock, Columbus Co., North Carolina, as part of a Research Experiences for Undergraduates study. Two shell beds were sampled from each of two measured sections ~2m apart on opposite sides of a gully. Three replicates were taken ~1m apart from each bed of section 1; six similarly spaced replicates were taken for section 2. Two additional samples were collected 100m northwest of sections 1 and 2 from a unit marked by high abundance of the bivalve Laevicardium. Students in an Invertebrate Paleontology class at University of North Carolina Wilmington analyzed the paleoecology of the latter two samples. All samples were sieved through a quarter-inch mesh and specimens picked, sorted and identified to genus level; life modes were assigned to genera using the NMITA molluscan life modes database.

Paleoecological comparisons were made among the section 1 and 2 samples and with the Laevicardium-rich samples to determine how richness and trophic structure vary spatially at this locality. Samples ranged in size from 169 - 1002 bivalve specimens. Using Holland's Analytic Rarefaction program, we rarefied all bivalve samples to 160 specimens. Expected richness at this sample size ranged from 17.4-26.9 genera, and all samples overlapped in 95% confidence intervals, except for one unusually diverse sample from the lower bed of section 2. The Laevicardium-rich samples exhibited greater dominance but did not differ from other samples significantly in richness. Greater differences were seen in life mode proportions than richness. All samples were strongly dominated by suspension feeding bivalves (95% of section 1 and 2 samples overall; range = 75-99.5%), but the Laevicardium-rich samples had a lower percent of suspension feeders than did most other samples. The Laevicardium-rich samples also had more actively mobile and siphonate infaunal taxa and fewer epifaunal/cementing bivalves. Among gastropods, trophic composition was much more variable than for bivalves; for instance, predatory gastropods composed from 29-67% of gastropods, perhaps due to smaller sample sizes (25-94 specimens).