Prey selection by drilling predators: A case study from Miocene of Kutch, India

The fossil record of predatory drill holes in shelled invertebrates provides valuable evidence to understand the evolutionary role of biotic interactions in deep time. It is hypothesized from modern studies that predatory gastropods do not randomly attack molluscan prey; rather they select their prey in order to maximize the energy gain. We have tested this hypothesis using bivalves from Miocene marine deposits of Kutch, India. The prey group consists of Chlamys sp., Placuna lamellata and four species of oyster bivalve namely Ostrea latimarginata, Ostrea angulata, Crassostrea gigensis, and Hyotissa hyotis. The overall drilling frequency is 20% and the species level frequency is as high as 35%. There is quite a high incidence of incomplete drill holes; while the assemblage level frequency is 41%, the species level frequency is as high as 57%. Our assemblage demonstrates preferred selection of prey in terms of taxonomy, size, site and valve by the predatory gastropod. Such selections are guided by the energy maximization strategy of the predator. Moreover, the high incidence of incomplete drill hole makes the dynamics even more intriguing since it shows a different pattern of selectivity compared to that of successful attacks. The success rate of an attack differs with size of the predators, hence indicative of an ontogenetic improvement in predatory skills. The overall predation intensity, although comparable to a few reports from other continents, is largely different from the global average of drilling frequency of Miocene.