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Aim Cenozoic dynamics of large-scale species diversity patterns remain poorly understood, especially for the Western Pacific, in part, because of the paucity of well-dated fossil records from the tropics. This article aims to reveal the spatiotemporal dynamics of species diversity in the Western Pacific through the Cenozoic, focusing on the tropical Indo-Australian Archipelago (IAA) biodiversity hotspot.Location Tropical and north-western Pacific Ocean.Methods We analysed well-preserved fossil ostracodes from the tropical Western Pacific and combined their diversity data with other published data from the region to reconstruct Cenozoic dynamics of species diversity in the tropical and north-western Pacific Ocean. We fitted generalized additive models to test for differences in richness over time and across geographical regions while accounting for sample-size variation among samples.Results Low-, mid-and high-latitude regions all show a similar diversity trajectory: diversity is low in the Eocene and Oligocene, increases from the Early Miocene to the Plio-Pleistocene but then declines to the present day. Present-day high biodiversity in these regions was established during the Pliocene with a remarkable diversity increase at that time. Latitudinal diversity patterns are relatively flat and never show a simple decline from the tropics to higher latitudes.Main conclusions Western Pacific Cenozoic ostracodes exhibit a spatiotemporal pattern of species diversity that is inconsistent with the commonly reported and persistent pattern of declining diversity from the tropics to the extratropics. While this inconsistency could be interpreted as evidence that ostracodes are a contrarian clade, Atlantic ostracodes display a standard latitudinal species diversity gradient. Contrasting patterns between oceans suggest an important role for regional factors (e.g. plate tectonics and temporal geomorphological dynamics) in shaping the biodiversity of the Western Pacific.

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Temporal changes of ostracodes during the last 100 years observed in three sediment cores from Hiroshima Bay, the Seto Inland Sea, Japan, provide valuable information about influences on ostracodes caused by anthropogenic pollution. This is the first detailed report of historical records of the relationship between ostracodes and pollution established from core samples drilled in the polluted inner bay. At least 38 ostracode species were identified from 40 samples. Based on biofacies, the density of ostracodes and the faunal structure, it is elucidated that industrialization combined with the effects of the second world war caused a decrease in the density and a increase in the equitability of ostracodes, and that anthropogenic pollution caused a simplification of ostracode assemblages in Hiroshima Bay. The response of two particular ostracode species to anthropogenic pollution is also demonstrated. Callistocythere alata was sensitive and Bicornucythere bisanensis has a strong resistance for anthorogenic pollution in ostracode species. Thus, the relative frequencies of these two ostracodes can be used as an indication of such pollution. We discuss the limitations of using recent ostracode assemblages in the analysis of the palaeoenvironment, resulting from the changes induced by anthropogenic pollution during the last 100 years.

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The Executive Committee of the International Union of Geological Sciences on January 17, 2020 ratified the Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage/Age and Middle Pleistocene Subseries/Subepoch at the Chiba section of the Chiba composite section, Japan. The Chiba composite section is a continuous and expanded marine sedimentary succession in the east-central Japanese archipelago facing the Pacific Ocean. It contains well-preserved pollen, marine micro-and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, two geomagnetic field paleointensity proxies, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework. Its open-ocean continental slope setting has captured both terrestrial and marine environmental signals from upper Marine Isotope Stage (MIS) 20 to lower MIS 18. The M-B reversal serves as the primary guide for the Lower-Middle Pleistocene boundary, yield-ing an astronomical age of 772.9 ka. The GSSP is positioned 1.1 m below the directional midpoint of the reversal, at the base of a regional lithostratigraphic marker, the Ontake-Byakubi-E (Byk-E) tephra bed, in the Chiba section. The GSSP has an astronomical age of 774.1 ka and occurs immediately below the top of Marine Isotope Substage 19c.

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