Shells of Change: Gastropod Responses across the Eocene-Oligocene Boundary in the Kutch Basin

Roy, Supratim (2025) Shells of Change: Gastropod Responses across the Eocene-Oligocene Boundary in the Kutch Basin. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Supratim Roy (20MS142)) 20MS142_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

The Kutch basin is a peri-cratonic rift basin, located at the Western part of India (Biswas, 1992). This basin contains an almost entire Cenozoic sedimentary succession along with some stratigraphic gaps (Biswas, 1992; Saraswati et al., 2016). Faunistically this basin is rich in marine molluscan fauna, majorly Gastropods and Bivalves. The Eocene-Oligocene boundary (EOB) is also significant because during this time, a reducing environment was present at the Kutch basin and also there was a global cooling event occurred (Sarkar et al., 2003). This study documents the morphological disparity and the diversity of gastropods in terms of taxonomy and ecology of the Middle Eocene (Harudi formation and Fulra Limestone) and the Oligocene (Maniyara Fort formation) formations of the Kutch basin to understand how this reducing environment and cooling event have affected this organism. In this study, a total of 1363 gastropod species (53 genera and 27 families) are being used during the time interval from the Middle Eocene to the Early Oligocene. At the genus level, only four genera were able to cross the EOB, indicating a high extinction rate, however, the origination rate at the Oligocene is even higher than the extinction rate, which indicates an increase in Gastropod diversity. The same pattern is followed at the family level as well. In terms of body size, the Oligocene gastropods are relatively bigger. Also, in terms of the physical shape (Form) variation, Middle Eocene formations have a total of 6 forms with a high abundance of turriform gastropods, followed by globose and conic gastropods, whereas, the Oligocene gastropods have a total of 11 forms with high abundance of globose gastropods, followed by turriform, turbiniform and fusiform gastropods. Therefore, Oligocene gastropods are rich in both taxonomic diversity and morphological disparity. In terms of ecology, semi infaunal-facultatively motile-suspension feeders are the most abundant during the Middle Eocene, immediately followed by epifaunal-actively motile-carnivores. However, during the Oligocene, epifaunal-actively motile-herbivore grazers are most abundant, immediately followed by epifaunal-actively motile-carnivores. Even the number of Ecospaces is higher in the Oligocene making it ecologically more diverse than the Middle Eocene. Turritellids, which are semi infaunal-facultatively motile-suspension feeders, were the most abundant gastropods present during the Middle Eocene. Immediately after the turritellids, there were volutid and naticid gastropods, which were also significantly abundant. These gastropods normally prefer to stay in a warm, stable environment. Because of the cooling event and the fluctuating environment started to prevail from the EOB, the abundance of these gastropods might decrease and this might lead to an increase in the abundance of planktons, including diatoms and dinoflagellates. These are mostly phytoplanktons and along with these, there were also an significant abundance of coralline red algae in the Oligocene. These increase in the primary producers might lead to an increase in the abundance of herbivore grazers. This pattern is clearly evident in the study where, ampullinids, 1 which are epifaunal-actively motile-herbivore grazers, were the most abundant gastropods present during the Oligocene. Therefore, the period of the Eocene-Oligocene transition (EOT) witnessed a sharp faunal turnover in gastropods at both genus and family levels because of the global cooling event along with the reducing depositional environment.

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