Tectonic and eustatic controls on the palaeobiogeographic distribution of Eocene bivalves

ABSTRACT The Eocene epoch experienced important global events—a warming and transgression at the beginning, and a cooling and regression at the end. Crucial tectonic rearrangements were also taking place, such as the northward movement of the Indian plate and closure of the Tethys Sea. Here, the global palaeobiogeography of all Eocene bivalve genera is studied to see the effects of these tectonic and eustatic changes. We have established formal palaeobiogeographic provinces and explored their evolving relationships based on statistical analyses of global distribution data of all bivalve genera for each stage of the Eocene. The late Paleocene (Thanetian) and early Oligocene (Rupelian) stages have also been included to understand potential effects of climate and sea-level changes. Bivalve diversity increased at the beginning and fell at the end of the Eocene, apparently in response to opposite climatic and eustatic changes at these boundaries. The palaeobiogeographic provinces also expanded and shrank, respectively. The tropical and subtropical basins of northwestern Europe and eastern and western America accommodated maximum bivalve diversity and yielded temporally stable provinces. These provinces shared many bivalve genera. However, similarity of the western American fauna with that of eastern America dwindled with time as the Western Interior Seaway disappeared. The distribution sequence of shared diachronous genera between the provinces revealed that (1) the Western American Province later developed affinity to the Japan and Eastern Russian Province facilitated by the Alaska and Oyashio currents, (2) the Northern Equatorial Current and North Atlantic Gyre were primarily responsible for the similarity between the Tethyan and Atlantic provinces, and (3) the Tethys seaway was active until the middle Eocene, which allowed westward migration of the western Indian fauna leading to its similarity with the European and northern African faunas. This link was severed in the late Eocene.