Response of the Oligo-Miocene Bivalve Fauna of the Kutch Basin (Western India) to Regional Tectonic Events

Abstract

Tectonic changes have influenced the evolution of the marine community by changing the seaway configuration over time. Two such tectonic events during Oligo-Miocene, the closure of the Tethyan seaway leading to separation of the Arabian sea from proto-Mediterranean (~19 Ma) and significant uplift of the Tibetan plateau marking the initiation of the monsoon (~16 Ma), represent a classic case of tectonic shift influencing the regional environment of the Indian subcontinent. We investigated the taxonomic and body-size response of ~2000 marine bivalve individuals from 11 time-constrained shellbeds of the Kutch basin from three formations, Maniyara Fort (Chattian), Khari Nadi (Aquitanian), and Chhasra (Burdigalian-Langian), representing a time span of ~9 Ma (24.4-15 Ma). Principal coordinate analyses show a distinct species composition for Oligocene Maniyara Fort formation, differing substantially from the younger Miocene formations implying the possible effect of biogeographic separation due to the Tethyan closure. The absence of signature proto-Mediterranean taxa in Oligocene shellbeds also supports a limited faunal exchange as early as ~24.4Ma. Maximum likelihood estimation (MLE) analyses of richness and body size, however, reveal a dominantly conservative behavior supporting a stasis model without a strong directional trend through time. The absence of any persistent directional change in either diversity, body size, or species composition during an interval between ~17.2 Ma to ~15.7 Ma, an interval that has been claimed to have experienced the initiation of monsoon, points to the limited influence of this phenomena on the shallow marine ecosystem. Our results support limited faunal exchange due to the Tethyan closure and demonstrate little influence of the Himalayan upliftment on the evolutionary tempo of the Oligo-Miocene marine bivalves of the Kutch basin.