THE FOSSIL RECORD OF DRILLING PREDATION IN BARNACLES

The fossil record of drilling predation has been documented in detail for a few major invertebrate groups, while other prey have been understudied. Here, we report on drilling predation using an extensive collection of Cretaceous – Quaternary barnacles. In total, a few hundred predatory drill holes were found in Eocene – Holocene wall and opercular plates of balanomorph and scalpellomorph barnacles. The drilled specimens originated from localities in the USA, The Netherlands, Belgium, Jamaica, Panama, Antarctica, South Africa, Chile, and Venezuela. Muricid gastropods are the inferred producers of the majority of the drill holes, whereas two drill holes may have been caused by octopods. Drilling frequencies appear low (< 10%), consistent with observations in modern ecosystems that muricids are facultative drillers and commonly kill barnacles without drilling. Drill holes are placed non-randomly in balanomorph wall plates: they occur preferentially between plates in the interplate region, on and around the rostrum, and in the middle part of shell (height-wise). Drill holes in opercular plates occur preferentially in scuta rather than terga despite a taphonomic bias (i.e., scuta appear overrepresented relative to the less robust terga). Drill holes in wall plates are commonly incomplete, but, as documented for extant barnacle prey, successful attacks can often be accomplished via non-penetrative drilling. Also, drill holes are significantly larger in larger barnacles. We show that drilling of barnacles is a worldwide phenomenon during the Cenozoic, occurred at low drilling frequencies, and lacks major temporal trends.