SPATIAL VARIATION IN DRILLING PREDATION WITHIN A BIOGEOGRAPHIC TRANSITION IN THE PLEISTOCENE LOWER WACCAMAW FORMATION OF THE CAROLINAS

The lower Waccamaw Formation (lower Pleistocene) in southeastern North Carolina and northeastern South Carolina represents a biogeographic transition zone. South Carolina faunas share more molluscan taxa with Florida and the Caribbean than occurs farther north. We examined spatial variation in drilling predation from six lower Waccamaw localities in this transition zone for the bivalve assemblage and three common species, Astarte concentrica, Cyclocardia granulata, and Lirophora latilirata. Understanding how dynamics change in ecological transition zones is key to understanding broad-scale geographic patterns, such as latitudinal gradients in drilling predation. We hypothesized that faunal changes, particularly new combinations of predators and prey, within the transition zone should yield differences in drilling frequencies and predator preferences. We expected the Waccamaw River (SC) locality, which has a more southern aspect to the fauna, to differ most from the other, more northerly, localities.

Based on bulk samples, drilling frequency (DF) in SC was significantly greater (19%) than at all NC localities (11-14%) except Snake Island (27%) for all bivalves. Only the SC locality differed significantly in DF for Lirophora (8% vs 35-44%). DF for Cyclocardia ranged from 19-24% with no significant differences among localities. For Astarte, DF was 24-30% for all localities except Neils Eddy Landing, NC (42%), and the SC locality did not differ significantly from any other locality. Thus assemblage and species DF patterns differ, possibly due to influence of other prey in the assemblage, and no clear pattern occurs in geographic variation. The extent to which the drilling predator fauna (primarily naticid gastropods) varies in composition geographically is unclear due to low abundance of naticids in bulk samples (<3% of gastropods present), but Neverita is the most common naticid at most localities. Our results show that drilling predation in the “core” of a transition zone does not vary in a predictable way. Future work on the edges of transition zones with major faunal boundaries may be more informative; preliminary data from the correlative lower James City Formation at Lee Creek, at the northern edge of the transition zone, show much lower DFs for all bivalves (4%), Astarte (6%), and Cyclocardia (3%).