Micromorphy Offers Effective Defense Against Predation: Insights From Cost-Benefit Analyses of the Miocene Microgastropod Predation Record from Kerala, India

Abstract

Predation, an important driver of natural selection, is studied in the fossil record using quantifiable traces like drill holes produced by gastropods and repair scars produced after durophagous attacks. Despite the abundance of such records in molluscan prey, predation records of micromolluscs (< 5mm) remain largely unexplored. Using a Miocene assemblage of microgastropods from the Quilon Limestone, India, we established the predator-prey dynamics with the help of costbenefit analyses. The overall predation intensity, measured by drilling frequency (DF) and repair scare frequency (RF) is low (DF = 0.06, RF= 0.04). The predation intensity does not depend on the relative abundance of prey families suggesting a non-random prey selection regardless of the encounter frequency. Predation is selective as revealed by higher predation observed in prey of specific family identity, ornamentation, and body size. The smallest size class has the lowest DF and RF supporting a negative size refugia. Higher frequency of incomplete drill holes (IDF) among prey in larger size classes and ornamented groups implies morphological defenses that result in higher failure. Microgastropods show a lower predation intensity than macrogastropods of the same family in a global comparison of coeval records. Results of the cost-benefit analyses explain this difference; the net energy gain from predatory drilling is found to increase monotonically with increasing prey size making the small prey less beneficial. Because the predators try to maximize net energy gain from a predatory attack, the microgastropod prey characterized by relatively low net energy yield would not be preferred in the presence of larger prey. Micromorphy, therefore, appears a viable strategy for the prey group to adopt as an evolutionary response against predation, especially in resource-limited conditions that fail to support large body size.