Record of intense predatory drilling from Upper Jurassic bivalves of Kutch, India: Implications for the history of biotic interaction

The study of past biotic interactions is important not only to understand the paleoecological history of a community, but also to test the evolutionary role of such interactions. Drill holes in invertebrate exoskeletons provide one of the very few scenarios where biotic interaction could be studied directly and the related hypotheses could be tested with statistical rigor. Hence, the documentation and interpretation of the spatio-temporal patterns of drill hole frequencies has been the subject of extensive paleontological research. The two main gastropod groups responsible for the drilling predation in modern marine environment arose in Cretaceous as supported by their body fossil. However, the drill holes have been reported from the fossils of as far back as Precambrian age. The trend shows an overall low but variable intensity in Paleozoic and Mesozoic and a significant increase in Cenozoic. There are few reported cases of drilled bivalves from the Mesozoic, although in those instances, frequencies are fairly low. Most of the previous records of drilling predation during Paleozoic and Mesozoic come from North America and Western Europe. Here we report 148 drilled bivalve specimens of a single species from the Upper Jurassic horizon in western India. This is highest in number for any taxon ever recorded since Precambrian to Cretaceous. The frequency of drilling constitutes 30% of shells of the same species examined. This drilling frequency is highest when compared to all the reported Mesozoic drilling frequencies in Bivalves. The shapes of the drill holes are indicative of gastropod predation. The drilling gastropods responsible for these lethal attacks are also similar to their modern counterparts in terms of their highly selective prey choice and site-specificity. These results suggest that (1) Mesozoic bivalves were preyed upon by drilling gastropods, often with high intensities, and (2) the specialized characters of modern drilling predators were also present in Mesozoic. This largely unexplored record of bivalve drill holes from the Middle Mesozoic contradicts the general trend of “Mesozoic quiescence” as claimed by most researchers.