THE RECORD OF OLDEST NATICID GASTROPOD FROM THE OXFORDIAN (UPPER JURASSIC) OF KUTCH, INDIA: PALAEOECOLOGICAL IMPLICATIONS

Predatory drilling naticid gastropods play a significant role in predator-prey dynamics since the late Cretaceous when their evident body fossils were found to be diverse and abundant. There were some recent reports of high intensity (≈30%) naticid-like drill holes from Jurassic. They were attributed to some unknown predatory gastropods in the absence of true naticid body fossils. Globularia and Ampullina recorded from Triassic- Jurassic beds which also yield naticid-like drillholes, have been previously believed to be the possible drillers. But, extant ampullinids are herbivorous in nature and thus not related to naticids. Moreover, Globularia and Ampullina do not have the typical naticid protoconch.

We here report the unambiguous fossils of naticid gastropods from the Oxfordian (Upper Jurassic) Dhosa Oolite Member of Kutch, western India. The Dhosa Member is an oolitic fossiliferous limestone and represents a condensed deposit which resulted from a relative sea level high stand. Dhosa Oolite is richly fossiliferous, mainly dominated by turritelline species and includes typical Oxfordian ammonites and naticids. Naticids are globular, low spired with large aperture, thus resembling Gyrodes and SEM study reveals typical smooth and depressed protoconch. Coeval turritelline prey species bear characteristic beveled naticid drill holes.

We recently reported intense predatory gastropod drilling (about 30%) on an astertid bivalve from the same Dhosa Oolite in other area. We could not ascertain possible drillers in absence of true naticids. Now the predator has been found to co-occur with the drilled prey. Thus many Jurassic drillholes may be ascribed to naticids. Besides, we have recently documented another higher incidence of naticid drilling (≈25%) on a Late Cretaceous turritelline lineage, which attained the Cenozoic value. All these records of high drilling frequency refute the early claim of low-intensity drilling frequency during the Jurassic to Late Cretaceous. We further assert that the Mesozoic marine revolution has been established already during the Jurassic.