OUTCROP-SCALE VARIATION IN DRILLING PREDATION IN THE LOWER WACCAMAW FORMATION OF SOUTHEASTERN NORTH CAROLINA

Drilling by predatory gastropods may vary at a range of scales, from across latitudes to within outcrops. We examined variation in drilling at outcrop scale for the lower Waccamaw Formation (late Pliocene) at the Register Quarry near Old Dock, Columbus Co., North Carolina. As part of a Research Experiences for Undergraduates study, samples were taken from two measured sections ~2m apart on opposite sides of a gully. Replicates were taken ~1m apart within each section from each of two shell beds (3 replicates for section 1 and 6 for section 2). Two additional samples were collected ~1m apart from a faunally distinct unit rich in the bivalve Laevicardium, 100 meters northwest of sections 1 and 2. Paleoecological analysis of the latter two samples was conducted as an Invertebrate Paleontology class research project at University of North Carolina Wilmington. Samples were sieved through a quarter-inch mesh and specimens picked, sorted and identified to genus level, and predatory drillholes were documented. We hypothesized that spatial variation in drilling frequency (DF) should reflect proximity of samples.

DF for bivalves was remarkably consistent among samples. Overall bivalve DF for sections 1 and 2 was 14.6%, with nearly all samples from those sections ranging from 11 – 19%. Only two samples differed significantly from the other section 1 and 2 samples; DF of one sample from the lower bed of section 2 was anomalously high (30%) and one from the upper bed of section 2 was anomalously low (8%). When replicates were combined within beds of sections 1 and 2, combined DF was 13 – 16% and no units differed significantly from one another. One of the two Laevicardium-rich samples had a DF of 17%, consistent with most other samples. The other Laevicardium-rich sample had a significantly lower DF (7%) than most other samples.

Gastropods showed much greater variation in DF than did bivalves, perhaps due to lower sample sizes (10 – 52 specimens vs. 100 – >600 for bivalves). Overall gastropod DF = 9% for sections 1 and 2, but DF ranged from 0 – 28%; one of the samples with unusually high bivalve DF also had significantly greater drilling on gastropods. The two Laevicardium-rich samples had DF of 4% and 8%, within the range of most samples. Outcrop-scale variation in drilling was thus relatively limited and was not necessarily linked to distance between samples.