LACK OF TAPHONOMIC BIAS IN THE RECORD OF DRILLING PREDATION FOR PLEISTOCENE BIVALVES FROM NORTH CAROLINA AND THE GASTROPOD CREPIDULA FROM BEACH ASSEMBLAGES ALONG THE U.S. EAST COAST

Predation by shell-drilling gastropods is related to prey traits (e.g., shell size, thickness) and environmental conditions, which may also affect the likelihood of preservation of drilled shells in death assemblages and the fossil record. Distinguishing effects of taphonomy from other factors affecting drilling remains challenging. Although compression experiments showed drilled shells are more prone to breakage, such taphonomic bias may not be common under natural conditions. Even less clear is how such bias, if it exists, might affect latitudinal patterns in drilling frequency (DF).

We tested for taphonomic bias against drilled shells for six bivalve genera from the North Carolina Pleistocene and for the gastropod Crepidula from death assemblages on beaches from Maine to Florida. If bias exists against drilled shells, they should not survive to accumulate taphonomic damage. To test for bias, we compared taphonomic condition of drilled and undrilled shells. DF was not significantly correlated with shell condition (indicated by breakage and bioerosion) among Waccamaw Fm Anadara, Arcinella, Chama, Glycymeris, Lirophora, and Plicatula, supporting lack of bias. In a more detailed analysis of Waccamaw Glycymeris, each specimen was given a taphonomic grade (TG) of 1 (pristine) to 4 (poor preservation) and grades compared for drilled and undrilled shells. DF did not vary significantly in pairwise χ2 comparison of TG, except that DF for grade 2 was greater than for grade 3. Average TG was slightly greater for undrilled (2.49) than drilled (2.28) shells; the difference is significant (Mann Whitney U test).

Average TG for Crepidula was also greater for undrilled than drilled shells (2.01 vs 1.89) but the difference was not significant, nor did DF differ among TG in pairwise comparisons. Within a latitudinal context, neither DF nor the incidence of failed drilling was correlated with average TG across 24 localities. However, TG was related to Crepidula size and thickness: shells with higher TG tended to be larger and thicker. Although Crepidula at higher latitudes tended to be larger and relatively thinner, based on rank correlations of locality averages with latitude, no significant latitudinal trend in TG occurred. Results suggest taphonomic bias is not a significant contributor to latitudinal patterns in drilling.