The effect of the Tethyan seaway closure on the Oligo−Miocene marine benthic diversity and distribution around Eurasia

Abstract

The Cenozoic evolution of the Tethyan seaway significantly shaped marine and terrestrial biota around Eurasia. The Tethys Ocean connected the Atlantic and the Pacific during the early Cenozoic, allowing marine faunal exchange. However, during the early Miocene, the ‘Gomphotherium Landbridge’ developed, restricting the marine connection between the proto-Mediterranean in the west and the provinces of the eastern Tethys Ocean. In contrast to the well-documented phenomena of terrestrial mammalian exchange through the land connection brought about by the closure, little is known about its impact on the marine fauna. To assess the overall effect of this separation on the distribution and diversity of marine organisms, we studied the Oligo−Miocene fossil record of marine benthos including Cnidaria, Echinodermata, Foraminifera and Mollusca. We compiled 15 894 reported occurrences from the early Oligocene to late Miocene comprising 1477 genera, 404 families and 85 orders from four faunal provinces and evaluated their palaeobiogeographic patterns. Our study demonstrates that the proportion of genera shared between the eastern and western provinces decreased after the early Miocene. The majority of the non-shared families developed during or after the early Miocene. High faunal diversity during the early Miocene and increasing endemicity among the eastern provinces are consistent with the development of a shallow seaway that supported speciation and reduced biotic exchange. The considerable variation in the magnitude and timing of the response across taxonomic groups reflects the combined effects of preservation bias and the differential responses of fauna. Our study supports an early Miocene initiation of the Tethyan seaway closure, followed by intermittent connectivity before the complete closure. This closure and its climatic consequences influenced the evolution of the Eurasian biota. Future studies should use a multidisciplinary approach combining the fossil record, genetic divergence and climate proxies to fully understand the effects of environmental changes and the seaway closure on marine benthos diversity dynamics.