PALEOBIOLOGICAL IMPLICATIONS OF A VARIABLE DRILLING BEHAVIOR DISPLAYED BY THE MURICID GASTROPOD NUCELLA LAMELLOSA PREYING ON THE BIVALVE MYTILUS EDULIS

The fossil record left by drilling predators is one of the most powerful empirical systems used to study the evolutionary history of predator-prey interactions. This is because drilled fossils provide numerous quantifiable data on drilling predators (morphology, size, position, and frequency of drill holes) and their prey (taxonomic identity, size, morphology, and taphonomy of drilled specimens). The cognitive strength and validity of paleobiological interpretations based on drill holes can be augmented by observing modern drill-hole producing predators. In this study, a controlled experimental design was used to evaluate the variability in drill-hole parameters within a highly-constrained system involving mono-specific predators preying on mono-specific prey. Over 50 specimens of the drilling gastropod Nucella lamellosa and several hundred specimens of its natural prey, the bivalve Mytilus edulis, were collected from a single site (Argyle Creek, San Juan Island, WA, USA) and transferred to the Friday Harbor Laboratories (University of Washington). Each snail was marked and allowed to hunt freely. Valves of each killed prey were collected and imaged, whereas the snail responsible for the kill was released to continue its hunt. The results indicate that (1) the studied predator is a facultative driller (13% mussels were killed and eaten without drilling); (2) individual gastropods may produce drill holes of variable size and morphology in consecutive attacks on prey of the same type and similar size; and (3) the drill-hole diameter is a poor predictor of the predator size. The ‘fossil record’ produced in the experiment underestimates the frequency of successful attacks by drilling predators and obscures the relationship between the size of the predator and its prey. Moreover, the highly variable morphology of drill holes could mislead a researcher into postulating that more than one predatory species drilled the mussels. This case study shows that drill-hole parameters can be highly variable even within single populations of drilling predators and illustrates the fact that drill holes can yield biased estimates of predatory behavior right at the outset, even before passing through various taphonomic filters associated with their fossilization.