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 assemblage s is commonly used as a measure of the intensity of predation by drill ing gastropods. Previous studies have assumed that drilled and undrill ed bivalve shells have equal preservation potentials. We tested this a ssumption by measuring the mechanical strength of drilled and undrille d valves of the Recent bivalve Mulinia lateralis. Under compressive lo ads, drilled values 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 m ay break preferentially and hence inferred patterns of predation may r eflect taphonomic as well as biological processes. Careful examination of shell fragments in an assemblage may permit recognition of this ty pe of taphonomic bias.