First discovery of Eocene coastal-estuarine ostracods from Japan, with the geological history of the migration of estuarine genera in the Far East

Abstract

The genus Neomonoceratina, which dominated the ostracod assemblage in the uppermost part of the Akasaki Formation (Miroku Group) of the Kyushu Island, Japan, is associated with the benthic foraminifer Ammonia cf. beccarii, mollusca Terebralia? sp. and Anomia sp., and the oogonium of Charophyceae, indicating a marine-to-brackish estuarine environment involving inflowing freshwater. This is the first record of Eocene coastal-estuarine ostracods from the eastern margin of the Eurasian continent and demonstrates that different pre-Neogene coastal-estuarine ostracod assemblages flourished in this region. The assemblages comprised five ostracod species (including one novel species) assigned to the genera Neomonoceratina, Paijeiborchella, Propontocypris and Parakrithella. These species exhibited genus-level links with the Eocene borehole cores along the continental shelf of the East China Sea and other areas of Kyushu. Fossil data of characteristic Eocene coastal-estuarine genera collected worldwide indicate that different characteristic genera inhabited each region. For example, Neomonoceratina originated on the Indian subcontinent by the Early Paleocene period at the latest, along with the northern drift of the Indian subcontinent. These species subsequently diversified west and east with the equatorial current and counter-current via the Tethys and reached the eastern margin of the Eurasian continent among the various eastwards-migrating species, where one genus ultimately adapted to the coastal-estuarine environment. Notably, the coastal-estuarine ostracod assemblage of the eastern margin of the Eurasian continent differs completely from that of the Tethys during the Eocene period. Our results suggest that coastal-estuarine ostracod assemblages are a powerful tool for palaeogeographic reconstruction.

Cenozoic history of the tropical marine biodiversity hotspot

“…In addition, we counted diversity per genus as the number of species in a genus and calculated the mean diversity per genus of all genera occurring within each geological interval. Among all genera that were relatively abundant and speciose throughout the Cenozoic in the IAA, we identified typical Tethyan genera that originated in the broad Tethys biogeographic area during the Palaeogene (that is, Neomonoceratina in the Palaeocene and Stigmatocythere, Pokornyella, Keijella and Tenedocythere in the Eocene) 52,53,[78][79][80] ; cosmopolitan genera that had a broad geographical distribution across the Cenozoic (Cytheropteron, Krithe, Argilloecia, Loxoconcha and Xestoleberis) 54,55 ; and IAA genera that first appeared in the central Indo-Pacific during the Neogene (that is, Pistocythereis, Lankacythere, Atjehella, Hemikrithe and Hemicytheridea in the Miocene [54][55][56]81,82 ). Finally, we compared the trends of their respective diversity changes per genus throughout the Cenozoic.…”