Effectiveness of small size against drilling predation: Insights from lower Miocene faunal assemblage of Quilon Limestone, India

Abstract

The nature of drilling predation, although well documented for molluscan fossils, is understudied for micromolluscs (<5 mm). Studying predation in micromolluscs is especially critical in evaluating the adaptive significance of small size against predation and assessing the evolutionary implication of predator-prey size relationship (PPSR). This study documents a drilling predation event in microbivalves from the lower Miocene (Burdigalian) fossil assemblage of Quilon Limestone from Kerala, India. Our sample of over 2000 valves represents nine families with an average drilling frequency (DF) of 0.06 and an incomplete drilling frequency (IDF) of 0.26. The characteristic drillhole morphology and occurrence of five genera of modern drilling gastropods (Naticid: Natica, Tanea and Polinices; Muricid: Triplex and Dermomurex) from the same locality reveal the predator identity. Predation in the studied assemblage is found to be highly selective in terms of prey taxa, size, mobility and site selection. Six out of nine families show predatory drilling indicating taxon selectivity. Poor correlation between DF and abundance further supports this view. Failed attacks are strongly correlated with morphological features such as surface ornamentation (Lucinidae) and presence of conchiolin layers (Corbulidae). Drilling occurs primarily on medium size class; prey outside this size range show lower rates of attack. This indicates the existence of a “negative size refugia” for extremely small prey along with the classical size refugia existing for large prey. Mobility is found to be an effective defense against drilling predation. Microbenthos of the Quilon Limestone shows a lower predation intensity in comparison to the Miocene macrobenthos worldwide including coeval formations of the Kutch Basin. The interaction in the microbenthos is more strongly size-dependent in contrast to the Kutch fauna. Reduced predation intensity in the microfauna and the existence of “negative size refugia” support the claim of micromorphy acting as a defense mechanism and highlights the role of size-dependent predation in marine benthos.