DIVERSITY AND COMMUNITY STRUCTURE OF MARINE BIVALVES IN RELATION TO CLIMATIC FLUCTUATION: INSIGHTS FROM A TROPICAL SETTING

The climatic variation has complex effect on biological communities. However, predicting the long-term response of ecological community based on modern observation has its own limitations. Data on climatic changes and resulting biotic response from deep time enables us to evaluate the species vulnerability to modern climate change. Early Miocene has often been considered a close replica of modern climatic scenario and is marked by a number of climatic fluctuations. Although studied in temperate zones, the extent of climatic variability and associated response of tropical ecosystem during this time remains poorly studied. A tropical marine molluscan assemblage of lower Miocene is preserved in Kutch, India. Using the bivalves of this assemblage, we evaluated the diversity and community structure of Miocene tropics in relation to the prevailing climatic condition.

The lower Miocene of Kutch is classified into Khari Nadi (Burdigalian) and Chhasra (Aquitanian) formations. A total of 1975 bivalve specimens from 18 localities representing 23 species are used for this study. Both the formations are dominated by Ostrea angulata followed by Talochlamys articulata. Shells of O. angulata and T. articulata are used for geochemical analyses to reconstruct the age (Sr dating) and climatic variation (δ¹⁸O and δ¹³C). The Early Miocene of Kutch records a decrease of 1.6‰ (~ -2.4‰ to -4.0‰ ) in δ¹⁸O value corresponding to an increase in temperature from ~23°C to 34°C during a span of 5 Ma (21.9 to 16.9 Ma). The increase in temperature between Khari Nadi and Chhasra formation is consistent with the global trend during Early Miocene. The warming trend was associated with an increase in species abundance and richness of bivalve assemblage. Multivariate analysis on the data revealed a decrease in the within-habitat diversification from older Khari Nadi to younger Chhasra formation. More than 49% of the bivalves from both the formation belonged to stationary epifaunal suspension feeding type, however facultatively mobile epifaunal suspension feeder dominated in Chhasra formation. Our study shows a significant change in species richness as a response to climatic fluctuation from the tropics, an area characterized by an ecosystem that is generally considered to be less affected by climatic variability.