Predation in Molluscs: A Multi-Taxon Approach using Neontological and Paleontological Data.

Abstract

Predation is considered an important agent of natural selection, and studying predator-prey interactions in the fossil record allows evaluation of its evolutionary impact. Predator-prey systems involving drilling predators are especially relevant because fossil evidence of predation can be analyzed quantitatively. Frequencies of complete and incomplete drill holes in molluscs have been used extensively to evaluate success of predators relative to prey. Conventionally, drilling predation has been studied from a two-taxon perspective, the driller and its prey. I investigated the dynamics of such a system by experimentally measuring drilling and consumption rates that are essential for cost-benefit analysis. I have also investigated the effect of other, secondary, predators on drilling predation. In the presence of its own predator, (a shell-crushing crab), the success rate of the drilling predator (snail) decreased (frequencies of incomplete holes increased; drilling frequencies decreased). Extrapolating from the experimental results, I hypothesized that the presence of secondary predators should have the same effect in fossil assemblages. Using the frequency of repair marks as a proxy for secondary predators, I tested the hypothesis using Plio-Pleistocene molluscan assemblages. The results confirmed my predictions: frequency of complete drillholes varied inversely with frequency of crab predation marks and frequency of incomplete drillholes varied directly with crab predation frequency. Extending this approach further, I used a large synoptic database for the entire Cenozoic. As before, I found an inverse relationship between drilling and repair scar frequencies, consistent with my prediction. Drilling frequency could not be explained by the diversity change of drilling predatory gastropods, providing further support for a causal connection between activities of secondary, durophagous predators and drilling frequencies. Finally, I explored experimentally the hydrodynamic behavior of drilled and undrilled shells, assessed its potential impact on drill holes frequencies of shell assemblages, and provided guidelines for identifying such a bias.