EVOLUTION OF PREDATION-RELATED CHARACTERS IN MIOCENE THROUGH PLEISTOCENE ATLANTIC COASTAL PLAIN CORBULIDS AND LUCINIDS

The hypothesis of escalation proposed by Geerat Vermeij predicts that the expression of traits that give organisms an advantage over their enemies should increase through time. The record of predator-prey interaction between drilling naticid gastropods and their molluscan prey is ideal for testing this hypothesis. The hypothesis of escalation would predict an increase in shell thickness of bivalve prey (as this factor exerts primary control over the cost of drilling infaunal, slow-moving bivalves) and a decrease in internal volume of the shell (as this represents the benefit gained by the predator). An increase in TH or decrease in IV would make a prey item less attractive to a selective predator.

Specimens of Caryocorbula and Parvilucina, ranging in age from mid-Miocene to Pleistocene of the Atlantic Coastal Plain, were measured for thickness (TH), internal volume (IV), and anterior-posterior length (L). TH and IV were regressed on L for each stratigraphic interval. TH and IV at a standard L (the mean of mean L from each stratigraphic interval) were calculated from the regressions for each stratigraphic interval. Caryocorbula displayed a net increase in TH at the standard L of ~19% and a net decrease in IV of ~32% over the time interval. Parvilucina demonstrated a net increase in TH of ~25% and a net increase in IV of ~14% over the time interval.

Previous work by Kelley on mid-Miocene Chesapeake Group bivalves found that characters related to predation (valve thickness and internal volume) evolved in a gradual mode and had higher rates of evolution than characters less closely related to predation. Significant changes in TH (8-157% for five different lineages) were related to predation intensity over a several million year interval. Internal volume (IV) showed no consistent temporal trends. The net increases in TH of Caryocorbula and Parvilucina in this study are consistent with Kelley's findings, though not as dramatic. The net decrease in IV of Caryocorbula was not consistent with Kelley's findings, although it is understandable in the context of the hypothesis of escalation. The net increase of IV in Parvilucina occurred through a series of fluctuations. Mass extinctions appeared to have no effect on trait evolution, with the exception of decrease in Caryocorbula IV across the Plio-Pleistocene boundary.