RELATIONSHIP OF DRILLING GASTROPOD PREDATION TO ESCALATION LEVELS OF MIOCENE AND PLIOCENE MOLLUSC FAUNAS OF THE U.S. ATLANTIC COASTAL PLAIN

Frequency of predation by shell-drilling gastropods varies in time and space, and the factors that influence such variation are complex. Students in an invertebrate paleontology class at University of North Carolina Wilmington investigated the relationship of drilling predation and escalation level (i.e., occurrence of antipredatory adaptations) of Miocene - Pliocene mollusc faunas from the mid-Atlantic Coastal Plain. Bulk samples from the Choptank Formation, Yorktown Formation (Sunken Meadow and Moore House members) and Chowan River Formation yielded 3775 bivalve and 785 gastropod specimens. Frequencies of successful drilling (DF) and failed drilling (prey effectiveness = PE) were tabulated for individual species and for each bivalve and gastropod assemblage as a whole. We compared drilling frequencies to the occurrence of characters that might affect drilling predation: maximum shell size and thickness, ornamentation, and (for bivalves only) tight valve closure.

Drilling frequencies were consistent with those reported previously from these formations; DF for the bivalve assemblage was high in the Choptank (0.35), generally lower in the Yorktown, and lowest in the Chowan River (0.11). As previously reported, DF for gastropod assemblages was typically less than for bivalves from the same unit. DF and PE were inversely related; at the assemblage level, declines in successful drilling were associated with increases in PE for both bivalves and gastropods. Results indicate a complex relationship between drilling and escalation level of molluscs. In bivalves, assemblage DF was inversely correlated with strength of ornamentation and tight valve closure, especially when the small sample from the Sunken Meadow Member of the Yorktown was excluded. These results are consistent with the hypothesis that such characters deterred drilling predation. However, contrary to predictions, assemblage values for DF were positively correlated with shell thickness relative to size in both bivalves and gastropods. Assemblage-level DF was also positively correlated with strength of ornamentation in gastropods, opposite expectations. However, individual species with failed drillholes tended to have escalated characteristics such as large maximum size, strong ornamentation, and/or thick shells relative to size.