PREDATOR/PREY RELATIONSHIPS IN THE FOSSIL RECORD: PREDATION BY NATICID GASTROPODS ON MOLLUSCAN PREY IN THE MIOCENE CALVERT FORMATION, MARYLAND

Naticid drilling predation was examined in several thousand fossil molluscan specimens (N=2,233) collected from the Calvert Formation of the Chesapeake Group, near St. Leonard, Maryland. The sample set consists of eight of the most common genera from the site: six pelecypods (Marvacrassatella, Dallarca, Corbula, Chesapecten, Cardium, and Isocardium) and two gastropods (Turritella and Natica). Thanks to the erosive effects of Hurricane Isabel, which swept through the region in the Fall of 2003, these specimens comprise an unusually large and fresh sample from this locality. Of this sample set, 473 specimens (21.2%) show evidence of naticid predation in the form of distinctive, circular and beveled holes penetrating the shell; rates range from 1.3% in Chesapecten to 33.9% in Marvacrassatella. Kitchell and others (1981) proposed a model in which predators preferentially select prey based on such factors as prey size and shell thickness; this would permit the predator to minimize energy expended in drilling while maximizing energy intake. In a larger sense, the theories of coevolution and escalation respectively state that predator and prey systems evolve together and that predation pressure increases during times in which other ecological pressures on prey are minimal. When compared with other studies of varying temporal and spatial scales, these data provide a basis to evaluate the related hypotheses of coevolution and escalation in the context of a single middle Miocene molluscan community.