DRILLING PREDATION IN MOLLUSCAN ASSEMBLAGES OF THE LOWER WACCAMAW FORMATION (PLEISTOCENE) AT SNAKE ISLAND PIT IN NORTH CAROLINA

Bulk samples of the lower Waccamaw Formation were collected from Snake Island Pit in southeastern North Carolina as part of ongoing research into the Plio-Pleistocene molluscan extinctions. Five of these samples (Bags #6–#10) were analyzed by a paleontology class at Georgia State University (GSU) yielding ~5800 specimens. Students sieved samples using a ¼” mesh and picked bivalves with umbos (~5,000) and gastropods with apices (~800) to be sorted. Specimens were identified to genus level with complete and incomplete drillholes documented. Nearly whole specimens (>85% of the shell intact) were counted separately from fragmented specimens for calculating drilling frequency (=DF) and prey effectiveness (=PE). The results were compared with that from another bulk sample (750+ specimens) collected from the same locality that was analyzed previously by students at the University of North Carolina Wilmington (UNCW).

DFs for bivalves (# of complete drillholes divided by half of the whole specimens) ranged between 16% and 19% except for one sample at 25%. Much lower DFs were found for gastropods varying between 1% and 3% across the five samples. Total DFs were 19% for bivalves and 2% for gastropods with all samples combined. In each of the samples, fewer than 10 incomplete drillholes were documented for bivalves; no incomplete drillholes were recorded for gastropods. PE for bivalves based on all samples combined was 6.5%. UNCW DFs were 26% (bivalves) and 9% (gastropods) with only one incomplete drillhole noted.

Taxa drilled in these samples and their DFs are represented by Anadara (2%), Arcinella (24%), Astarte (35%), Caryocorbula (11%), Chama (4%), Conradostrea (13%), Cyclocardia (14%), Diplodonta (42%), Glycymeris (35%), Gouldia (23%), Laevicardium (6%), Lirophora (35%), and Plicatula (2%). Data from the UNCW sample revealed commonly drilled genera comprised Arcinella (53%), Diplodonta (43%), Glycymeris (40%), Laevicardium (26%), Lirophora (41%), and Plicatula (12%). Gastropods drilled in the GSU samples included Conus (2%), Crepidula (1%), and Olivella (5%). Both Conus (5%) and Crepidula (10%) were drilled in the UNCW sample. Fourteen other genera in the GSU samples and four other genera in the UNCW sample were drilled, but DFs were not calculated due to insufficient specimens (<10 individuals per genus).