shells of a species should have similar Taphonomic Bias in Analyses of Drilling Predation: Effects of Gastropod Drill Holes on Bivalve Shell Strength

The proportion of bivalve shells with drill holes i n 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 preserua-t ion wotentials. W e tested this as-sumption by measuring the mechan-ical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis. Under compressive loads, drilled valves are significantly weak-er than undrilled values, a difference we attribute to local stress concen-trations produced by the presence of the drill hole. Our results suggest that drilled valves may break preferen-tially and hence inferred patterns of predation may reflect taphonomic as well as biological processes. Care-ful examination of shell fragments i n a n assemblage may permit rec-ognition of this type of taphonomic bias