An Overview of Japan from the Silurian Onwards from Viewpoints of Ostracods

Ostracoda are a kind of Crustacea with fossil records known from the earliest Ordovician. They live in all water environments, from polar regions to the equator, and from the deep sea to freshwater, but because they have a benthic lifestyle throughout their lives, they have poor dispersal ability and form biogeographical regions unique to each region. The foundations of the Japanese archipelago are thought to have been formed at least as early as the Cambrian, with the oldest fossils being a conodont from the Middle Ordovician period reported from central Japan. At present, ostracod fossils have been found since the Early Silurian period. Ostracods from the Early Silurian consist of the genera Hollinella and Clintiella, which were characteristic of the equatorial region at the time, and the endemic species Pauloloes supparata. Considering the co-occurring conodonts, it is believed that Japan at that time was near South China. Ostracoda discovered in Kyushu, southwestern Japan, from the Middle Devonian share commonalities with contemporaneous species from Yunnan Province, South China, such as Carinabeyrichia tripartita, Craspedobolbina (Mitrobeyrichia) yongshengensis, and Bairdia perisema. This suggests that Japan at that time continued to be located near South China. The Late Carboniferous ostracod assemblage of northeastern Japan includes endemic species such as Samarella, Kirkbyella, and Kirkbya, as well as Jordanites modica and Healdia lucida, which were reported from the Tarim Basin during the same period. This indicates that at least part of Japan at that time was located near a shallow sea area extending from the Tarim Basin to North China. From the Late Devonian to the Early Carboniferous, Japan's location changed dramatically from being close to South China to being close to North China. Shallow-marine ostracod assemblages from at least the Late Cretaceous and Paleogene have only been reported in Japan in East Asia, and these assemblages are significantly different from those in other regions on the basis of cluster analyses. The Recent ostracod assemblages of the Japanese archipelago are composed of species distributed in Far East Asia, East Asia, and parts of South Asia. In short, at least since the Early Silurian, Japan's ostracod assemblage has been an extremally unique fauna of the eastern margin of the continent, and is significantly different from that of other regions. Japan's ostracod assemblage is thought to play an important role in understanding the palaeozoogeography and palaeoenvironment of the Paleo-Tethys Ocean, the eastern edge of the Tethys Ocean, and the Panthalassa Ocean.