PALEOECOLOGY OF A MOLLUSCAN ASSEMBLAGE FROM THE WACCAMAW FORMATION, HORRY COUNTY, SOUTH CAROLINA

Students in an undergraduate invertebrate paleontology class at University of North Carolina Wilmington investigated ecological relationships, including predation by drilling gastropods, for the Waccamaw Formation at a site on the IntraCoastal Waterway near North Myrtle Beach, Horry Co., SC. The samples were collected as part of a Research Experiences for Undergraduates program. Three beds are exposed at this site; the UNCW students examined three bulk samples from the lowermost bed, a blue-gray mollusc-rich sand. Samples were sieved through a quarter-inch mesh and whole mollusc specimens and fragments with beaks (bivalves) and apices (gastropods) were picked, sorted and identified to genus level. Life modes (substrate relation, attachment, mobility and diet for bivalves; diet for gastropods) were assigned to genera using the NMITA molluscan life mode database. Complete and incomplete predatory drillholes were also tallied.

Sample sizes ranged from ~ 500-700 bivalves (~1800 total specimens) but only 19-44 gastropods (~100 total specimens). For the combined samples, 23% of bivalve individuals were epifaunal; 41% were siphonate infaunal. Of bivalve individuals, 61% were actively mobile and 63% were unattached. Individual samples are very similar in trophic composition. Suspension-feeding bivalves represented 72-75% of bivalve genera in the three samples and 91-95% of bivalve individuals. Predatory gastropods composed 16-18% of individuals and 36-44% of genera for the three samples. Although shell-drilling naticid gastropods were rare, drilling frequencies (DF) on bivalve prey for the three samples ranged from 17 – 24% (DF=19% for the combined bivalve samples). The most frequently drilled abundant bivalve prey included Astarte, Glycymeris, and Gouldia. Incomplete drillholes, likely indicating failed attacks, were most common on thick-shelled bivalves such as Chione and Lirophora; 9% of all attempted drillholes were incomplete for the total bivalve fauna. DF for the gastropod fauna was 13%, and 14% of attempted drillholes were incomplete. In general, the proportion of suspension-feeding bivalves/molluscs is similar to that for the lower Waccamaw Formation in Columbus Co., NC, but predatory gastropod individuals are significantly less common at this locality.