DRILLING PREDATION IN EXTREME SIZE CLASS: INSIGHT FROM EARLY MIOCENE MICROMOLLUSCAN ASSEMBLAGE OF KERALA, INDIA

The nature of predation traces is relatively poorly known from micromolluscs. The Early Miocene (Burdigalian) fossil assemblage of Quilon limestone of Kerala, India provides a unique opportunity to study such predation events from a micromolluscan assemblages. The depositional environment of this assemblage has been interpreted to be a seagrass habitat. The average body size of the bivalve specimens recovered from this assemblage is extremely small, ranging between 0.46 to 4.76 mm.

Our sample of ~1000 valves representing nine families with four families carrying record of predatory drilling, shows an average drilling frequency (DF) of 8.42% and an incomplete drilling frequency (IDF) is 0.04%. The majority of the complete drillholes represent wall drilling (87%) with some edge drilling (13%). Undrilled bivalves are significantly larger than the drilled ones (p<0.05) indicating a possible size refugia. There is no strong correlation between prey and predator size (Pearson rho= 0.10, p=0.49). Drilling is primarily observed in anomidae (DF=19%), donacidae (11%), cardiidae (9%) and lucinidae (5%) families. There is no evidence of drilling in glycymerididae, veneridae, tellinidae, mytilidae and arcidae. Lucinidae showed highest incomplete drilling frequency (IDF=79%) and a strong size refugia where the undrilled specimens are significantly larger than the drilled bivalves (p<0.05). A strong surface ornamentation in Lucinidae might be contributing to the higher incidence of incomplete drillholes. Majority of the drillholes have naticid characteristics. This corresponds to the higher dominance of small naticid individuals in the sample. Only in donacidae, majority of the attacks show characteristics of a muricid predator.

Our study documents the first case of drilling predation from Miocene micromolluscs of west Indian province (IWP) fauna during the time of Tethyan Seaway closure. This assemblage shows a significantly lower drilling frequency in comparison to other reports of drilling from bivalves of WIP from coeval formations in the north. Such change in drilling pattern highlights the role of size dependent predation pattern among marine invertebrates.