Interpretation

Patterns of drilling frequencies indicate significant changes in the predator-prey system through time, as predicted by Vermeij's hypothesis of escalation and supporting an important role for ecology in evolution. The pattern is more complex, however, than implied by Vermeij's initial data that suggested limited Cretaceous drilling and escalation to modern levels by the Eocene. Vermeij (1987) predicted that warming, transgression, and high primary productivity should foster escalation and that mass extinctions involving a drop in productivity or cooling should interrupt it. Our data support a significant role for mass extinctions in escalation. We have proposed (Kelley and Hansen 1996a) that escalation in the naticid predator-prey system occurred in cycles controlled by mass extinctions. Following Vermeij (1987), we have suggested that highly escalated species with antipredatory adaptations would be lost selectively at mass extinctions (particularly those associated with cooling or a decline in productivity) due to the higher energy requirements for maintaining those adaptations. Loss of such highly escalated prey allows drilling frequencies to rise abruptly after mass extinctions; as prey escalate their defenses, drilling frequencies subsequently stabilize or decline until the next mass extinction. This model seems to fit the changes in drilling frequencies for the K/T and E/O

extinctions; work in progress (Hansen, Graham, and Kelley 1996) indicates consistency between the model and drilling frequencies across the Middle Miocene extinction. We are in the process of testing the proposed mechanism for episodic escalation by examining the nature of pre-extinction and postextinction faunas in greater detail, particularly with respect to the occurrence of antipredatory traits (Hansen and Kelley 1995b; Melland, Kelley, and Hansen 1996). Prey effectiveness or multiple drilling frequency, or both, were low in the Paleocene Brightseat Formation but high in the Oligocene Red Bluff Formation (tables 8.1 and 8.2). Those results suggest that highly escalated prey were more likely to have been eliminated at the K/T than at the E/O boundary.

What evolutionary mechanisms were involved in the apparent escalation observed in our comprehensive survey of naticid predation? Was coevolution involved in the changes in the predator-prey system indicated by the patterns in drilling frequencies? Was adaptation of predator and prey reciprocal? The eventual decline in drilling frequencies after the postextinction highs suggests that adaptation by the prey outpaced that by the predator. Data on incomplete and multiple drillholes from the Cretaceous and Paleogene also suggest a lack of reciprocal adaptation. Superimposed on the pattern of episodic escalation of drilling frequencies is an increase in the frequency of failed predation attempts, especially in the later Eocene and Oligocene. This relative increase in prey defensive abilities suggests that, during the Paleogene, adaptation of predator and prey was not reciprocal, or at least not balanced. Escalation appears to have occurred, but the coevolutionary component is not significant.

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