Taphonomic bias in analyses of drilling predation; effects of gastropod drill holes on bivalve shell strength

The proportion of bivalve shells with drill holes in fossil assemblages is commonly used as a measure of the intensity of predation by drilling gastropods. Previous studies have assumed that drilled and undrilled bivalve shells have equal preservation potentials. We tested this assumption by measuring the mechanical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis. Under compressive loads, drilled valves are significantly weaker than undrilled valves, a difference we attribute to local stress concentrations produced by the presence of the drill hole. Our results suggest that drilled valves may break preferentially and hence inferred patterns of predation may reflect taphonomic as well as biological processes. Careful examination of shell fragments in an assemblage may permit recognition of this type of taphonomic bias.