NATICID GASTROPOD DRILLING PREDATION ON MERCENARIA: ARE TEMPORAL PATTERNS IN THE NEOGENE OF THE SOUTHEASTERN UNITED STATES ENVIRONMENTALLY CONTROLLED?

Temporal patterns of naticid gastropod drilling predation have been used to test arms race hypotheses, including hypotheses of escalation and coevolution. However, the extent to which environmental variation may confound the temporal patterns is unclear. To examine the extent to which the fossil data are environmentally controlled, we compared Miocene – Pleistocene drilling on the bivalve Mercenaria with that from Recent samples from a range of latitudes on the U.S. east coast. Data for fossil Mercenaria were extracted from Kelley and Hansen’s naticid predation database for middle Atlantic Coastal Plain formations (all data from bulk samples). Drilling frequencies were high in the Miocene (55-65% for the middle Miocene of Maryland) and Pliocene Yorktown Formation (95%) and then dropped to 21% in the upper Pliocene Chowan River Formation and then to 7% in the Pleistocene. These patterns for Mercenaria are consistent with those for bivalves in general in the Kelley-Hansen database. Recent specimens were taken from bulk samples of mollusk assemblages collected from 13 localities from Maine to southern Florida, which were divided according to latitude into four molluscan provinces for analysis. Recent Mercenaria exhibited the highest drilling frequencies (59%) in the northernmost (Nova Scotian) province; drilling declined monotonically with decreasing latitude, reaching a low of 10% in the Gulf Province. These results indicate that, as temperature increases, drilling predation on Mercenaria decreases. Mercenaria shell thickness at a given length also increases with temperature. The results suggest that the temporal patterns are not simply controlled by climate; drilling on Recent Mercenaria increases with decreasing temperature, but during the period of cooling from the Pliocene to Pleistocene, a decline in drilling occurred. Drillhole location preference was also compared in the fossil and Recent samples. Fossil data indicate a drilling location preference for the central area of the shell, whereas Recent samples indicate a preferred location of the umbos or slightly anterior of the umbos. Prey size selectivity was also examined for the Recent samples; prey size and drillhole size (indicating predator size) were correlated, and naticids exhibited a preference for small- to medium-sized Mercenaria prey.