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The Toarcian Oceanic Anoxic Event (OAE) is known as a global environmental change based on the worldwide distribution of organic black shale. This event has been studied in detail in Europe from various viewpoints including lithostratigraphy, biostratigraphy and chemostratigraphy. However, contributions from the Asian regions are still limited. The aims of this study are to present the biodiversity fluctuation in ammonoid from southwest Japan, and to discuss the relationship between the diversity crises and the OAE in the Early Toarcian.
Two ammonoid mass-extinction events were recognized in both the NW European and the Mediterranean provinces around the Upper Pliensbachian-Lower Toarcian. The first event took place at the Pliensbachian/Toarcian (P/T) boundary, while the second one recorded at the uppermost part of the Semicelatum Subzone of the European zonation. The second event corresponds to the beginning of the Early Toarcian OAE. It is characterized by the diversity fall and the increasing value of extinction. In addition, the diversity of endemic species significantly fell at this horizon.
In Japan, the existence of the Toarcian OAE was discussed in the Nishinakayama Formation, the middle part of the Toyora Group. The Toyora Group is Lower to Middle Jurassic continental shelf sediments exposed in southwest Japan. The Nishinakayama Formation, which is known as the most major source of Early Jurassic ammonoid fossils in Japan, is mainly composed of black mudstone. The mudstones in the middle part of the formation were considered to compare with the facies of the global OAE based on the evidence of sedimentary facies and bivalve fauna. Ammonoid biostratigraphic work was carried out along the Sakuraguchi-dani Valley. The following four successive ammonoid zones were recognized in the formation in ascending order; the Fontanelliceras fontanellense Zone, Protogrammoceras nipponicum Zone, Dactylioceras helianthoides Zone, and Harpoceras inouyei Zone. The P/T boundary corresponds to the base of the D. helianthoides Zone.
The analysis of ammonoid diversity trend is done along the North Valley of the Sakuraguchi-dani Valley. The evolutionary trends of diversity, origination and extinction are discussed at species level and genus level. As a result, a diversity fall is recognized in the uppermost D. helianthoides Zone. This event is characterized by the minor decrease of diversity, no major value of extinction, the diversity fall of Harpoceratinae, and that of endemic species. Moreover, the diversity fall corresponds to the uppermost Semicelatum Subzone of the NW European zonation. Consequently, it is coeval with the second mass-extinction event which is related to the Toarcian OAE in Europe. The effect of the Toarcian OAE seems to have reached to the coastal areas of East Asia.
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