SPATIAL VARIATION IN PREDATORY AND NONPREDATORY BIOEROSION IN SELECTED BIVALVES FROM THE PLIO-PLEISTOCENE OF THE U.S. COASTAL PLAIN

Interpreting temporal changes in predatory and nonpredatory bioerosion requires knowledge of spatial variation in such patterns. We examined spatial variation in the occurrence of predatory drillholes and nonpredatory bioerosion at several scales within the Plio-Pleistocene of the U.S. Coastal Plain. The study included species of arcoid, chionine, and carditid bivalves, all from bulk samples. Occurrence of bioerosion was compared for: 1) two different samples from a single locality of the Caloosahatchee Fm (La Belle, FL; samples from the American Museum of Natural History); 2) a single species from three localities of the Waccamaw Fm in North Carolina; 3) several species from the correlative Waccamaw and James City formations of North Carolina; 4) several taxa from approximately contemporaneous formations from two different latitudes (North Carolina vs Florida).

Two different samples of Chione elevata (1753 and 2121 specimens) from the Caloosahatchee at La Belle both had drilling frequencies (DF) of 10%, though bioerosion differed slightly but significantly between the samples (37% vs 40% of specimens were bioeroded). Anadara aequicostata from three localities of the Waccamaw yielded DF of 0, 17% and 18%, and the percent of bioeroded specimens was 8%, 27%, and 12%. With the exception of bioerosion at the Shallotte locality (27%), none of the differences between localities for either DF or bioerosion is statistically significant. Likewise, DF and percent of specimens that were bioeroded were consistent between the Waccamaw and James City for individual species. Greater variation in degree of bioerosion occurs within taxa at different latitudes. In all cases, drilling frequencies were greater in North Carolina than in Florida: 39% vs 24% for Lirophora latilirata (difference not significant); 9% vs 4% for Carditamera; 13% vs 4% for arcoids. The results support a previously documented pattern of greater drilling in the Carolina Province than in the Gulf Province. In contrast, percent of specimens with nonpredatory bioerosion was significantly greater in Florida than in North Carolina for Carditamera (47% vs 4%) and arcoids (54% vs 15%), though Lirophora latilirata from the two latitudes had equivalent amounts of bioerosion (3 – 4%). Higher percentages of bioerosion in Florida may represent greater productivity.