Entobian bioerosion of Miocene oysters, Kachchh, Gujarat

Idiomorphic clionid borings were discovered on some of the oyster shells occurring in limestone intercalations within the Claystone Member of the Chhasra Formation (Burdigalian, Miocene), Kachchh, Gujarat. Epoxy resin was injected in a fragmentary shell of Crassostrea gingensis, which subsequently was completely dissolved in dilute hydrochloric acid. Examination of artificial casts of borings, thus obtained, revealed presence of three ichnospecies of Entobia. An open camerate system with long, cylindrical chambers is diagnostic of E. cateniformis, while camerate system of E laquea consists of lace-like network of small, globose, equidimensional chambers. Entobia isp. A shows limited exploratory threads and small irregularly distributed apertures. Entobian borings suggest clear, well-illuminated, shallow, sublittoral waters with little sediment input. However, proliferation of these borings is not very extensive in the limestone intercalations under consideration, suggesting that the spell of nondeposition must have been short. It is interesting to note that out of seven oyster species known from the Miocene strata of Kachchh, clionid borings were observed on only two species viz., Hyotissa hyotis and Crassostrea gingensis.