DIVERSITY AND ECOLOGICAL STRUCTURE OF THE CANEPATCH FORMATION, UPPER PLEISTOCENE, HORRY COUNTY, SOUTH CAROLINA

Diversity and ecological structure of the Upper Pleistocene Canepatch Formation were analyzed by a graduate Paleoecology class at University of North Carolina Wilmington. Bulk samples were collected from the Mercenaria –rich layer (Bed 8) at the Windy Hill airstrip (Dubar’s WA 56) locality, Myrtle Beach, SC. Three samples (designated Canepatch 1, 2, and 4) were wet sieved with 5 mm mesh and bivalves with beaks and gastropods with apices were retained for study. Diversity indices and rarefaction curves were calculated and life modes were determined from the NMITA and Paleobiology Databases. We examined small-scale spatial variability of ecological structure within the Canepatch by comparing the three samples.

The samples in aggregate included 1559 bivalve specimens (26 genera) and 239 gastropods (12 genera). Canepatch 1 was significantly less rich than the other two samples based on rarefaction to 420 specimens (15.6 vs 18 and 19.7 bivalve genera). The gastropods of Canepatch 1 were also less diverse, though 95% confidence intervals overlap among all samples. All samples have very similar ecological structure. Dominance values ranged from 0.28 – 0.33 for bivalves and 0.22 - 0.32 for gastropods; equitability was 0.60 – 0.63 for bivalves and 0.70-0.79 for gastropods. Chione and Crepidula dominated the bivalve and gastropod faunas respectively, and rank order abundances were highly correlated among all samples (p<0.0002) for bivalves and between Canepatch 2 and 4 for gastropods (p < 0.001). Life modes were very similar among bivalve samples, especially Canepatch 1 and 2 (94-95% suspension feeders, 85% infaunal siphonate/semi-infaunal specimens; Canepatch 4 had 91% suspension feeders and infaunal siphonate/semi-infaunal specimens). Gastropod assemblages were all dominated by predatory carnivores (51-57%); Canepatch 1 and 2 had 43% suspension feeding individuals (Crepidula) whereas Canepatch 4 had 36% Crepidula, with some carnivorous browsers and herbivores. Drilling predation accounted for 12 – 19% of bivalve mortality, with Chione consistently drilled (9 – 22%). Mulinia, found only in Canepatch 4, had a 32% drilling frequency. Tellina was also drilled frequently (24% for 37 total specimens). Only one gastropod specimen in Canepatch 1 and 2 was drilled, but 13% of gastropod specimens in Canepatch 4 were drilled.