Bathyal molluscs from Upper Pleistocene methane seeps in Krishna-Godavari Basin, offshore eastern India

Compared to other ocean basins there are few reported Recent methane seep communities from the Indian Ocean, with records from offshore Indonesia and Pakistan, and, more recently the east coast of India, in the Krishna-Godavari and Mannar Basins in bathyal water depths. Also from the former area, Upper Pleistocene aged fossil methane seep assemblages have been recovered from sediment cores. Here we describe systematically bivalves, gastropods and scaphopods from a methane seep assemblage penetrated by two sediment cores, drilled in 1045 m and 1050 m water depth, at horizons dated to between 40 and 52 kyrBP. The fossil molluscs comprise 29 taxa: 15 gastropods, 12 bivalves and two scaphopods. Of these, nine are new species: six gastropods (Paralepetopsis bathyalus Hoffman & Little sp. nov., Mesopelex godavariensis Hoffman & Little sp. nov., Anatoma sahlingi Hoffman & Little sp. nov., Cirsonella aperta Hoffman & Little sp. nov., Dikoleps? magnarota Hoffman & Little sp. nov., and Alvania axistriata Hoffman & Little sp. nov.) and three bivalves (Ledella favus Hoffman & Little sp. nov., Yoldiella umbostriata Hoffman & Little sp. nov., and Vesicomya prashadi Hoffman & Little sp. nov.). Six of the molluscan taxa likely had chemosymbionts: (Acharax sp., Gigantidas cf. platifrons, Conchocele sp., Pliocardia cf. solidissima, Callogonia cf. leeana, and Archivesica cf. kawamurai), representing 21% of the diversity in the seep assemblage. Apart from Acharax sp., all these putative chemosymbiotic taxa were likely obligate to seeps, as was probably the case for Paralepetopsis bathyalus Hoffman & Little sp. nov. and Anatoma sahlingi Hoffman & Little sp. nov. The other bivalve, gastropod and scaphopod species in the assemblage have living relatives common in bathyal habitats and can thus be considered as facultative or ‘background’ fauna. The fossil seep assemblage shares some taxa with recent seep communities in the east coast of India and elsewhere in the Indian Ocean, although additional systematic work is needed on the living taxa for a full comparison to be made.