MICROMORPHY OFFERS EFFECTIVE DEFENSE AGAINST DRILLING AND DUROPHAGOUS PREDATION

Predator-prey interaction is one of the important drivers of natural selection. The fossil record of drilling and durophagous predation on molluscan prey has contributed significantly in assessing the evolutionary effect of predatory interactions. The interaction, however, has remained poorly explored in the micromolluscan assemblage. The microgastropods from early Miocene (Burdigalian) fossil assemblage of Quilon limestone of Kerala, India provides a unique opportunity to study such predation events.

Our sample of ~1300 gastropod specimens representing 25 families with 14 families carrying traces of predatory attack, shows an overall drilling frequency (DF) of 0.06, repair frequency (RF) of 0.04 and incomplete drilling frequency (IDF) of 0.14. The relative abundance of families does not explain the variation in predation intensity across family; the prey selection appears to be determined based on taxonomic identity, ornamentation and body size. Predation is primarily observed in Buccinidae, Cerithiidae, Eulimidae, Naticidae, Obtortionidae, Phasainellidae, Pyramidellidae, Rissoinidae, Scaliolidae and Turbinidae families. Rissoinidae and Turbinidae show the highest DF (0.22) and RF (0.18), respectively. IDF is significantly higher in ornamented families (p-value=0.03), implying a predation resistance due to ornamentation. Both DF and RF are relatively low in the smallest size class indicating an “inverse size refugia”. However, the largest size class is found to have higher IDF, indicating a higher degree of failure. A global comparison shows that the intensity of predation among the microgastropods is lower than their regular-sized counterparts for both drilling and durophagous predation. Cost-benefit analysis of drilling predation in microgastropods demonstrates a monotonic increase in the net energy gain with the increasing size of the prey. Although the attacks on microgastropods yield a positive benefit: cost ratio, the low value of net energy gain is likely to make the predator prefer available larger prey over microgastropods. Micromorphy, therefore, may have been adopted by prey groups as an evolutionary response against predation, especially in resource-limited conditions that fail to support large body size.