Evolution of Cambay Basin, western India during the Eocene from the study of gastropods and shell beds

Abstract

During the early Palaeogene western India was flooded with extensive transgressions. The first marine sediments were deposited in the western marginal basins during the Ypresian, i.e., early Eocene. Molluscs, including gastropods, bivalves and scaphopods, appeared and flourished in these basins, within the transitional habitats between sea and land. The early Cenozoic time was important for the evolutionary history of gastropods because several of the gastropod groups, abundant in modern-day seas, either first appeared or went through rapid diversifications and distributions during this time. While comprehensive monographs about the Paleocene-Eocene gastropods are available from the European and North American basins, and even the adjacent Pakistan basins, the Indian basins remained relatively less explored. Among the western Indian basins, most of the published reports came from the Kutch Basin, a few from the Subathu and Rajasthan basins and none from the Cambay Basin. The gastropods present in the Ypresian Cambay Shale of the Cambay Basin are studied here. 32 caenogastropods and 9 heterobranchs are reported and described. 32 of them (24 caenogastropods and 8 heterobranchs) are new. 13 of them are microgastropod (< 4 mm). 3 new genera and 2 new subgenera are introduced. The fossils were collected from four lignite mines at Mangrol, Vastan, Tadkeshwar and Valia, Gujarat. The Camaby Shale is an argillaceous formation with intercalations of lignite seams and marine fossil bearing limestones. Glauconitic green shale layers and shell beds dominated by only one or a few opportunist molluscs yielded gastropods of eurytopic or marginal marine affinity. A change from the freshwater-dominated coastal swamp to a relatively open-marine condition in an overall marginal marine set-up could be deciphered from the sequence of lithologies and fossils. The composition, diversity-abundance pattern, environmental preference and trophic structure of the molluscs, especially gastropods, elucidate this transgressive sequence. The Cambay gastropod assemblage predominantly comprises endemic species and widespread genera. A similar pattern was unveiled by the Paleocene-Eocene gastropod faunas reported from all Indian and Pakistan basins, situated within the Western Indian Province (WIP). The wide distribution of the genera took place due to the faunal exchanges via the circum-tropical equatorial currents along the relict Tethys, Atlantic and Pacific oceans. The westerly flowing somewhat weak Tethys current was important for migration of western Indian faunas. However, the specific endemism is enigmatic because a large part of the fauna had planktotrophic development. The restricted nature of the basins might be a significant factor. The early developmental mode was not found to be a decisive factor in the geographic distribution of the WIP gastropods at both generic and specific levels. The maximum geographic distribution and also, temporal duration of these genera were found to be not significantly different among the WIP genera characterized by planktotrophy, non-planktotrophy (lecithotrophic/direct), and a combination of these. With greater time, the maximum geographic distribution became broader. Also, majority of these genera were found to attain their maximum geographic distribution at the early stages of their temporal ranges. The changes in the maximum latitudes of these genera support their migration towards higher latitudes during the warming and towards lower latitudes during cooling periods, in response to the climatic turnovers of the Palaeogene.