EFFECT OF THE TETHYAN SEAWAY CLOSURE ON THE BIOGEOGRAPHY OF OLIGO-MIOCENE MARINE BIVALVES OF WESTERN INDIA

Tectonics shapes the biogeographic distribution of the marine community by changing the seaway connection over time. During Early Miocene (~19Ma), the African plate collided with the Eurasian continent forming the Gomphotherium landbridge. This landbridge separated the proto- Mediterranean Sea from the Tethyan sea. Using the shallow marine bivalves from the Oligo-Miocene formations of western India and comparing them with the global occurrence, we evaluated the biogeographic response of Western Indian Province (WIP) fauna to this closure.

We studied the marine bivalves from 11 time-constrained shell beds of the Kutch Basin from three formations—Maniyara Fort (Chattian), Khari Nadi (Aquitanian), and Chhasra (Burdigalian-Langian) representing a period of ~ 9 My (24.4–15 Ma). Our collection consists of ~ 2000 specimens representing 14 families, 30 genera, and 39 species. Ostrea latimarginata, Ostrea angulata, Talochlaarticulatelata, Anomia primaeva, and Placuna lamellata are the most abundant species across all the formations.

A global comparison of the Kutch fauna at the genus level with 3896 occurrences reported in the Paleobiology Database (PBDB), revealed a complex biogeographic pattern. Before Burdigalian, the Kutch fauna appears to be distinct from Eastern African-Arabian Province (EAAP) and the Mediterranean-Iranian Province (MIP). It, however, shows strong similarity across coeval faunas WIP including those reported from Pakistan in the west and northeast Indian province of Mizoram in the east. This demonstrates the presence of strong provincialism of the faunas of WIP even before the tectonic collision and seaway closure. Following the closure, the Kutch fauna displays similarity with the faunas from EAAP but not with MIP at the family level. Consideration of preferential preservation of calcitic fauna and taxonomic misidentification does not change the pattern. Our study supports a reduced faunal exchange between the Tethyan and proto-Mediterranean provinces even before the development of the Gomphotherium land bridge. It also indicates the development of a stronger regional faunal exchange within EAAP and WIP following the closure. This study underscores the role of tectonics in reconfiguring the seaways and influencing the distribution of marine communities.