LACK OF EVIDENCE FOR MOLLUSCAN GENUS-LEVEL DIVERSITY DECLINE ACROSS THE REPUTED MULTI-PHASED PLIO-PLEISTOCENE MASS EXTINCTION IN SOUTHEASTERN NORTH CAROLINA?

Western Atlantic molluscs are widely considered to have experienced a multi-phased Plio-Pleistocene mass extinction. To clarify timing and extent of the extinctions, a three-year study of Plio-Pleistocene mollusc diversity in the Carolinas has been conducted as part of a Research Experiences for Undergraduates program. Bulk samples have been studied from 16 sites; samples studied in 2010 include the Pliocene Duplin Fm (Robeson Farm near Tar Heel, NC) and the Pleistocene lower Waccamaw Fm at Acme, NC, Neils Eddy Landing on the Cape Fear River near Acme, and Holloman Pit in Whiteville, NC. Samples were sieved through ¼” mesh and specimens with beaks (bivalves) and apices (gastropods) were picked, sorted, identified to genus level and counted. The study included >1,500 gastropods and >16,000 bivalves. Genus-level richness was compared to our 2008 and 2009 REU data from other Duplin (>10,000 specimens) and Waccamaw (~19,000 specimens) sites using rarefaction; because a few samples showed high dominance, rarefactions were run both with and without the dominant genus in each sample.

When rarefied to 1440 specimens, expected genus-level richness for bivalves from the 2010 lower Waccamaw sites is high (Neils Eddy = 48, Holloman = 51.9). The 95% confidence intervals (CI) overlap with that of our most diverse sample, the Duplin on the Lumber River (Lumberton, NC; 56.7 genera). Bivalves at Acme are less rich (44.2 genera), but when rarefactions are run excluding the dominant genus the Acme sample becomes statistically equivalent to the Lumber River Duplin. Rarefied richness of the Duplin at Robeson Farm (40.5) is significantly less than the Lumber River Duplin (though greater than the Natural Well Duplin) but 95% CI overlap with those for Acme. In contrast, Robeson Farm gastropod richness (rarefied to 100 specimens) is not significantly different from that of previously studied Duplin sites (22-24 genera); however, the lower Waccamaw at Acme (with and without dominants) and Holloman also overlap in 95% CI with the Duplin samples. The data fail to support a significant decline in richness between the Duplin and lower Waccamaw in southeast North Carolina, though previously studied upper Waccamaw bivalves and gastropods from Shallotte, NC, are significantly less rich (~37 and 8 rarefied genera respectively) than the lower Waccamaw.