EFFECT OF CLIMATIC SHIFT ON BIODIVERSITY OF BIVALVES: A CASE STUDY FROM LOWER MIOCENE OF KUTCH, INDIA

Shallow marine areas are extremely sensitive to sea level changes that are often caused by climatic fluctuations. The molluscan fauna that lives in these settings possess a unique combination of accreting shells preserving climatic signature and a history of rapid ecological and evolutionary response to climatic shifts. However, often taphonomic processes destroy the original composition of the shells obscuring the paleoclimatic signatures. The shallow marine deposits from Lower Miocene of Kutch, India provides an unique situation to reconstruct paleoclimate with a dominance of durables calcitic shells of Ostreidae that retains the original isotopic composition; hence, it is an ideal scenario to test the relationship between climatic shift and the biodiversity of shallow marine fauna.

Our study primarily focuses on the shallow marine bivalve community from ten different localities representing two formations of Lower Miocene, namely, Khari Nadi formation (Aquitanian) and Chhasra formations (Burdigalian). We reconstructed the paleotemperature using the δ¹⁸O value analysed from accretionary growth sections of bivalve Ostrea angulate from the studied localities. On comparing the δ¹⁸O value of both the formations, we found a significant difference between the two; younger Chhasra formation revealed a much more negative value compared to those of older Khari Nadi formation. This indicates a climatic shift towards higher temperature in Lower Miocene of India. This climatic warming coincides with change in bivalve diversity and paleoecology of shallow marine deposits from this area. The rarefied family diversity of bivalves shows a decrease in overall diversity from Khari Nadi to Chhasra formation. While Khari Nadi formation is represented by families including Pectinidae, Ostreidae, Veneroidae, Tellinidae, Arcidae, Cardiidae, Plicatulidae, Glycymeridae, Chhasra formation only has Pectinidae, Ostreidae, Tellinidae, Cardiidae, Plicatulidae, Mellidae. The community structure also changes with nearly a four fold increase in the dominance of Ostreidae from Khari Nadi to Chhasra Formation. This coupling of decreasing diversity and increasing dominance could have been triggered by the change in temperature as indicated by the isotopic signature.