A LOW-DIVERSITY MOLLUSCAN ASSEMBLAGE FROM THE PLEISTOCENE OF HORRY CO., SOUTH CAROLINA

Diversity of a mollusc assemblage thought to represent the Pleistocene Waccamaw Formation was analyzed by an undergraduate Invertebrate Paleontology class at University of North Carolina Wilmington. Teams of students analyzed two bulk samples from the Windy Hill airstrip (DuBar’s WA 56) locality, Myrtle Beach, SC. Samples were sieved through a 5 mm mesh and bivalves with beaks and gastropods with apices were picked, sorted and identified to genus. Diversity was analyzed using Holland’s Analytic Rarefaction (http://www.uga.edu/~strata/software/) and PAST (http://folk.uio.no/ohammer/past/). Diversity was then compared to that at ten other Waccamaw sites in the Carolinas.

The WA 56 samples included 300 gastropod and ~25,000 bivalve specimens. The two samples did not differ in richness when rarefied to the same sample size (110 gastropod and 9900 bivalve specimens). The combined samples contained 20 gastropod and 43 bivalve genera. Dominance was low for gastropods (0.18); Olivella, Crepidula and Prunum were the dominant gastropods in both samples. The bivalve assemblages were dominated overwhelmingly by small Mulinia specimens; dominance was ~0.88 and evenness ~0.04 for each sample as well as the combined samples. Based on rarefaction to a sample size of 640 (size of smallest Waccamaw sample = Snake Island Pit), bivalve richness (15.4 genera) was significantly less than at all other Waccamaw sites (~30 – 43 genera). This low diversity is due to the dominance of Mulinia; when samples are rarefied without the dominant genus, WA 56 overlaps (95% confidence intervals) in richness with all Waccamaw localities except the lower Waccamaw Holloman and Neils Eddy sites. Richness most closely matches that of upper Waccamaw localities on the Intracoastal Waterway at North Myrtle Beach, SC. Richness of the gastropod assemblage rarefied to 110 specimens overlaps (95% CI) with all other localities except the lower Waccamaw on the Waccamaw River, SC. An extinction event has been proposed between the upper and lower Waccamaw; if this unit is upper Waccamaw, results from all 11 localities indicate average rarefied richness dropped significantly between the lower and upper Waccamaw for bivalves (40 vs 34 genera without dominants) but not for gastropods (14 genera).