Naticid Gastropod Prey Selectivity through Time and the Hypothesis of Escalation

Abstract

The hypothesis of escalation posits that biologic hazards such as predation have increased during the Phanerozoic. Previously, a survey of drilling frequencies in the Cretaceous and Paleogene of the North American Coastal Plain suggested an episodic pattern of escalation within the naticid gastropod predator-prey system. This study examines escalation from the perspective of naticid prey selectivity. If escalation occurred within the system, less selectivity of prey may be apparent in the Paleogene compared to younger assemblages. We test this hypothesis for four Eocene Coastal Plain assemblages. Contrary to predictions, intraspecific prey size selectivity was well developed for nine of eleven bivalve prey species. Drillhole size (indicating predator size) correlated significantly with prey size, especially for successful drillholes. Few incomplete or nonfunctional drillholes occurred, except within corbulid species. Interspecific prey selectivity was less developed than for Neogene or Recent assemblages. Naticid prey preferences predicted by cost-benefit analysis were consistent with actual drilling frequencies only for the Bashi Marl Member of the Hatchetigbee Formation (Alabama). In the Piney Point Formation of Virginia, all prey items were drilled at equivalent frequencies, despite their different cost-benefit rankings. Upper Lisbon (Alabama) and Moodys Branch (Mississippi and Louisiana) assemblages showed limited agreement with preferences predicted by cost-benefit analysis. Prey selectivity thus appears less developed in the Paleogene compared to the Neogene and Recent, in accordance with the hypothesis of escalation.