BIOERODED BELEMNITES AND DIFFERENTLY DISPOSED STEPHANOCERATID AMMONITES FROM KULDHAR MEMBER JAISLAMER FORMATION, RAJASTHAN, INDIA: A TOOL FOR SPECIFIC PALAEOENVIRONMENTAL REFINEMENT

Stephanoceratid ammonites and rostra of belemnites from Kuldhar member have been studied from the days of Spath (1927-33). But reports of bioerosion on belemnites have not yet been published from this member. The present study identified two types of bioerosions (borings) on belemnite rostra- one is cylindrical to ellipsoidal and the other is circular and both are perpendicular to surface. Elliptical one is Trypanites isp. and the circular one is Rogerella isp but both the two trace fossils are post-mortal (Prof. P. D. Taylor, BMNH, London 2015, Personal Communication). In most of the cases, they are concentrated in the apical region. It has been proved that the bioerosion producing organisms here, i.e., acrothoracid barnacles and polychaete worms, both can inhabit the environments of belemnites. The associated other common cephalopods here are stephanoceratid ammonites namely Macrocephalites (Macrocephalites flexuosus) and Subkossmatia (Subkossmatia flexicosta). They are embedded in rocks like oolitic limestone, at places, nodular limestone, gray shale and occasional red mud. Belemnite rostra in this ‘belemnite battlefield’ is highly scattered and have variety of orientations of the central groove and so also the ammonite moulds. These indicate that they were churned by activities of tracefossil producing organisms. However, clayey nature of most of the rock types have also played significant role for the final disposition of the fossil moulds, e.g., at places, it has been observed that horizontal and vertically disposed ammonite moulds occur side by side which is an artefact of thixotropic property of clay. According to Westermann (1990) both these cephalopods can thrive at a maximum depth of 110 m. The rock types in the present area can also form at this depth. Hence from lithology as well as fossil clasts of nektobenthic organisms, the actual environment of formation of this Kuldhar Member can be regarded as upper part of more distal shelf. This precise depth connoted view of environment is more refined than existing published view shallow shelf environment by Pandey et.al. 2010.