Temporal patterns in the efficiency of naticid gastropod predators during the Cretaceous and Cenozoic of the United States Coastal Plain

The hypothesis of escalation predicts that highly armored (more escalated) prey will be selectively eliminated at mass extinctions. If recovery faunas were dominated by less escalated species, a relative decrease in the defensive abilities of prey should follow mass extinctions. We tested this hypothesis by examining two indices of the relative efficiency of drilling naticid gastropod predators and their molluscan prey: the percent of drill holes that were incomplete and the percent of holes occurring in multiply bored shells. Both represent unsuccessful drilling attempts, due to interruption of the drilling process by physical or biological factors, including escape tactics of the pray, excessive prey thickness, or predator inefficiency. If recovery faunas were more vulnerable to predation, the incidence of incomplete or multiple drill holes should decline following mass extinctions. The data do not support this hypothesis. Of a total of 16 comparisons across four extinction boundaries, two showed a significant decrease in failed drilling following mass extinctions, in accord with the hypothesis. Oligocene gastropods had fewer incomplete drill holes than did Eocene gastropods, and the frequency of multiply bored gastropod specimens was less in the Pleistocene than in the Pliocene. Two comparisons yielded a significant increase in failed drilling in the recovery fauna, contrary to the hypothesis; bivalve recovery faunas in the Oligocene and Miocene showed significantly greater proportions of multiply bored specimens than did their pre-extinction counterparts. No overall trend in unsuccessful drilling was apparent in the Cretaceous through Cenozoic data. Incomplete and multiple drilling were relatively rare in the Cretaceous, Paleocene, and most of the Eocene (0-12%). Failed drilling was significantly more common during the Oligocene (up to 20%), and returned to lower levels for most of the remainder of the Cenozoic (<10%). (C) 2001 Elsevier Science B,V, All rights reserved.