TEMPORAL PATTERNS IN EDGE- AND WALL-DRILLING PREDATION ON OLIGOCENE THROUGH PLEISTOCENE BIVALVES OF THE MIDDLE ATLANTIC COASTAL PLAIN

Shell-drilling predatory gastropods most frequently drill through the shell walls of bivalve prey, although muricid snails in competitive environments place holes along the shell margin where the valve is thinner and can be penetrated more rapidly. Such edge drilling is risky because the bivalve may close its valves, amputating the muricid’s proboscis. Some previous studies attributed edge drilling by naticid gastropods to similar circumstances; however, prey handling techniques of naticids minimize amputation risk. We hypothesized that, if naticid drilling follows the same patterns of increased edge drilling in competitive environments, edge drilling frequency should not correlate with overall drilling frequency but should peak in intervals of competitive environments (e.g., the Pliocene Yorktown Fm). To determine whether temporal patterns in edge drilling differ from those for wall drilling, we extracted edge-drilling data from the Kelley-Hansen naticid gastropod predation database to compare with data for wall drilling. We calculated the frequency of edge drillholes relative to all drillholes in bivalves (EF) and of edge-drilled valves relative to all valves (E/N). Patterns were compared to those for drilling frequency (DF = frequency of individuals with ≥1 complete drillhole) and failed drilling (PE = prey effectiveness = proportion of drillholes that were incomplete; MULT = proportion of drillholes in multiply bored valves). Data represent 17 stratigraphic units in temporal succession, from the Oligocene Belgrade Fm to the Pleistocene Flanner Beach Fm.

The two edge-drilling metrics were highly and significantly correlated (r = 0.9017). Both were positively correlated with DF; the correlation of DF and E/N is statistically significant (p = 0.009). EF ranged from 0 (Morgarts Beach Mbr of the Yorktown Fm; Neuse and Flanner Beach Fms) to 0.09 (Miocene Eastover Fm). EF in the Pliocene Yorktown Fm was significantly less than in the early Pleistocene, in contrast to a previous study of muricid drilling of Chione in the Florida Plio-Pleistocene. EF was also positively correlated with failed drilling; r = 0.424 (p = 0.045) for PE vs EF. Edge drilling appears consistent with general patterns of drilling predation by naticids in the Oligocene – Pleistocene of the middle Atlantic Coastal Plain.