Mixed assemblages of drilling predators and the problem of identity in the fossil record: A case study using the muricid gastropod Ecphora

Drillholes made by naticid and muricid gastropods are frequently used in evolutionary and ecological studies because they provide direct, preservable evidence of predation. The muricid Ecphora is common in many Neogene Atlantic Coastal Plain assemblages in the United States, but is frequently ignored in studies of naticid predation. We used a combination of Pliocene fossil, modern beach, and experimentally derived samples to evaluate the hypothesis that Ecphora was an important source of drillholes in infaunal bivalve prey shared with naticids. We focused on the large, thick-shelled venerid, Mercenaria, which is commonly drilled by naticids today. Laboratory experiments, modern beach samples, and the published literature confirm that naticids preferentially drill near the umbo (significant clumping of holes), show a significant correlation between prey size and predator size (estimated by outer borehole diameter), and prefer Mercenaria <50 mm antero-posterior width when other prey are present. Fossil samples containing Ecphora (with or without other large muricids) show no drillhole site stereotypy (no significant clumping, greater variability in placement), no significant predator: prey size correlation, drilled prey shells larger than the largest modern naticids could produce in an experimental setting, and drillholes larger in diameter than those estimated for the largest Pliocene naticids, thus supporting our hypothesis. Substantial overlap in the placement of holes drilled by naticids and muricids, however, made identifying predators from drillhole position problematic. The lack of overlapping ranges of prey shell thickness between fossil and other samples precluded the use of drillhole morphology to establish predator identity (e.g., ratio of inner borehole diameter to outer borehole diameter, drillhole angle). Whereas the difficulty in determining predator identity from drillholes limits the types of analyses that can be reliably performed in mixed-predator assemblages, recognizing Ecphora as a prominent drilling predator creates the opportunity to investigate previously unrecognized questions.