DRILLING GASTROPOD PREDATION ON THE LOWER MIOCENE GASTROPOD ASSEMBLAGES FROM KUTCH, WESTERN INDIA: SPATIOTEMPORAL IMPLICATIONS

Spatiotemporal distribution of Miocene drilling predation is well known. Temporally, drilling frequency (DF) achieved its peak during the Miocene. Spatial distribution is widely spread, ranging from about 47^ºN to 5^ºN. We, here, present gastropod drilling predation on the lower Miocene gastropod assemblages (62 species belonging to 29 families) from Kutch, western India (20^ºN). The assemblage level DF from unbiased samples is 6.12% (N=5865 complete shells) which is very close to the lowest DF ever recorded from the Miocene. The present assemblages belonged to the Aquitanian and the Burdigalian stages which were represented by intertidal and subtidal environments respectively. The DFs are 5.60% (N=2393) and 6.48% (N=3472) for the two stages which are not statistically significantly different (p >0.05). This remarkable stasis of predation intensity exists for about 8 million years in spite of the fact that environment became relatively deep with time. Family/subfamily level DFs are also low and vary from 1.16% (Ampullinidae) to 7.89% (Rostellaridae) for the Aquitanian and 0.36% (Cerithiidae) to 7.58% (Turritellinae) for the Burdigalian. Out of 22 abundant species within the lower Miocene only 8 were drilled and the species level DFs are also low (0.36% to 10.71%) except one (25.77%; in Turritella kachchensis; N=97). Behaviorally, drill holes do not show any site stereotypy at least in the most abundant subfamily Turritellinae, but size stereotypy is observed in most of the turritelline species where a significant correlation exists between the predator size and prey size (p <0.05). Unsuccessful drilling is very rare and found only in the most abundant turritelline species, Zaria angulata where prey effectiveness (PE) is 1.51%.

Previous works suggested an increase in drilling frequency with latitude. Our result is consistent with this expectation – low DF has been found from the lower latitude. The reasons generally ascribed to explain this are less abundance of predators, competition among predators, low salinity environment etc. But in the present case, predatory gastropods were both diverse (6 species), abundant (N=542) and environments were normal marine. We argue that instability of environment and paleobiogeographic constraints might have caused this low DF values for the Indian gastropod assemblages.