NATICID GASTROPOD PREDATION IN THE PALEOGENE OF THE GULF AND ATLANTIC COASTAL PLAINS: TESTING HYPOTHESES IN EVOLUTIONARY PALEOECOLOGY

The Paleogene of the U.S. Gulf and Atlantic Coastal Plains has provided a molluscan database (40,000 specimens) from which we have derived a variety of testable hypotheses in evolutionary paleoecology. In particular, the Paleogene naticid gastropod predator-prey system has served as a model for studying evolutionary dynamics, including the process of escalation. Based on the Cretaceous and Paleogene fossil record of drilling naticids and their bivalve and gastropod prey, we developed the following hypotheses: 1) escalation of naticid predation occurred in cycles punctuated by mass extinctions, which initiated surges in drilling followed by eventual stabilization and decline in drilling; 2) the increases in drilling were caused by preferential extinction of highly escalated (armored) prey; 3) overall effectiveness of prey defenses increased through time; 4) selectivity of naticid predators increased through time; 5) naticid cannibalism increased through time; 6) drilling frequencies increase with latitude. Hypotheses generated for the Paleogene of the U.S. Coastal Plain can be tested by examining the record of naticid predation from other regions and times. Expansion of the database (now 150,000 specimens) to the Coastal Plain Neogene supports the occurrence of drilling cycles, although the suspected cause (elimination of highly escalated prey by mass extinctions) has not been corroborated. Overall prey effectiveness increased through the Paleogene, but declined in the Neogene. Although naticid predators were more selective of prey in the Miocene than in the Paleogene, Plio-Pleistocene prey selectivity was weak (i.e., comparable to the Eocene). Cannibalism increased from the Cretaceous to the Paleogene and again into the Neogene. In general, these results support a complex pattern of escalation in the naticid predator-prey system. The hypothesis that drilling increases with latitude is supported by drilling frequencies from the Cretaceous of North Dakota, but is not corroborated by results from the Eocene of Antarctica. Our hypotheses can be tested further with other predator-prey systems, including muricid and cassid gastropod predators and their prey.