Stereotypic boring behaviour inferred from the earliest known octopod feeding traces: Early Eocene, southern England

Abstract

A bulk sample of 267 disarticulated valves of the bivalve Venericor clarendonensis (Wood) collected from the Lower Eocene London Clay (southern UK) yielded 38 individuals that had been perforated by small drill holes (0.70–2.14 mm in outer diameter). These drill holes had more or less circular plan views, with slightly irregular openings, and taper as they pass through the valve, conforming to the ichnotaxon Oichnus simplex Bromley. They show stereotypic positioning, being concentrated in the posterior region on the prey, moreover there is remarkable preference for perforating the sites of muscle attachment (principally the posterior adductor). We consider the most likely culprits to be octopods. As such these are the oldest octopod drill holes yet recorded. They provide the only evidence of these important top predators in this shallow marine community and also demonstrate that the sophisticated predatory behaviour shown by modern octopods had been evolved by at least the Early Eocene. □Eocene, octopod behaviour, Oichnus, stereotypic boring. Although predator prey interactions have arguably been a very important force in evolution (Vermeij 1987; Stanley 2008), quantitative data from the fossil record are frustratingly difficult to obtain. Many types of predatory techniques (such as swallowing prey whole or prising hardparts open) leave no discernible trace and even where the method used does cause damage to fossilizable hardparts, by crushing and biting, it may be difficult to distinguish such traces from post-mortem breakage or to attribute it to any particular predator. However, one of the more fruitful lines of research has come from the study of predatory drill holes in shelled prey (Leighton & Aronowsky 2003). In the case of drill holes it may be possible identify the potential predator and to collect data concerning aspects of predatory behaviour such as size selection of prey, stereotypic attack positioning and measures of success rates (see Harper 2003 for a review). Here we report on a large sample of an Early Eocene bivalve, Venericor clarendonensis, in which a number of valves have been punctured by distinctive small holes. We present evidence that they are likely to have been made by predatory octopods, which record a remarkable stereotypic behaviour.