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

Undergraduate Invertebrate Paleontology students at University of North Carolina Wilmington investigated paleoecological relationships at DuBar’s WA 56 site, Myrtle Beach, SC. Two bulk samples of argillaceous fine sand, thought to represent the Pleistocene Waccamaw Fm, were sieved with 5 mm mesh. Whole mollusc specimens and fragments with beaks (bivalves) and apices (gastropods) were picked, sorted and identified to genus. Diversity was analyzed using Holland’s Analytic Rarefaction and PAST and compared to that at other Waccamaw sites. Life modes were assigned using the NMITA database. Complete and incomplete predatory drillholes were tallied.

The two samples included 300 gastropods and ~25,000 bivalves (20 gastropod and 43 bivalve genera). Dominance was low for gastropods (0.18) with Olivella, Crepidula and Prunum most prevalent. Bivalve assemblages were dominated overwhelmingly by (~23,000) small Mulinia specimens; dominance was ~0.88 and evenness ~0.04. Bivalve richness rarefied to 640 specimens was significantly less (15.4 genera) than at all other Waccamaw sites (~30 – 43 genera). Richness of samples rarefied without Mulinia overlaps (95% confidence interval) with 8 of 10 Waccamaw localities. Gastropod rarefied richness overlaps (95% CI) with all but one locality. Richness most closely matches that of upper Waccamaw localities on the Intracoastal Waterway at North Myrtle Beach, SC. If this unit is upper Waccamaw, results from all 11 localities indicate average rarefied richness dropped significantly from the lower to upper Waccamaw for bivalves (40 vs 34 genera without dominants) but not for gastropods (14 genera).

Bivalve life mode proportions were calculated with and without the dominant Mulinia; 26% of genera and 3% of individuals (45% excluding Mulinia) were epifaunal. Actively mobile forms composed 74% of bivalve genera and 98% and 63% of bivalve individuals with and without Mulinia respectively. Consistent with the other sites, suspension feeders represented 76% of bivalve genera and 99% of bivalve individuals (93% without Mulinia). Predators comprised 60% of gastropod genera and 56% of individuals; proportions varied widely among sites. Drilling frequency was 15% and 9% on the bivalve and gastropod assemblages respectively. Incomplete drillholes were rare (observed on only 4 genera).