栃木県塩原地域の上部中新統鹿股沢層より産する暖流系軟体動物化石 : 塩原動物群の再検討に向けて

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The upper Miocene Kanomatazawa Formation in the Shiobara area, Kanto region, yields shallow marine and brackish water molluscan assemblages currently called the Shiobara Fauna. The assemblages are the type of the Shiobara Fauna which developed in the middle to late Miocene temperate waters of northern Japan and the Japan Sea coast. This study reports an occurrence of a warm-water molluscan assemblage in the middle part of the Kanomatazawa Formation, which is dominated by the gastropods Umbonium (Suchium) miyagiense and Olivella cf. iwakiensis and the pelecypods Glycymeris cisshuensis and Callista chinensis. This assemblage suggests that the Shiobara Fauna lived under warm-temperate conditions that were intermittently influenced by warm-water currents in late Miocene. At this time, the tropical to subtropical Zushi Fauna flourished along the Pacific coast of central to southwestern Japan under the remarkable marine climatic warming of Climatic Optimum 3. The warm-temperate assemblage shows the following characters as a forerunner of the Pliocene warm-water Kakegawa Fauna. 1) Its generic composition is analogous to those of nearshore assemblages of the Kakegawa Fauna : 2) It contains ancestral forms of the characteristic species of the Kakegawa Fauna such as Umbonium (Suchium) suchiense and Glycymeris albolineata. In particular, U. (S.) miyagiense reported here is comparable to adult shell size to U. (S.) suchiense. These facts suggest that the transitional zone between the warm and cold water realms played an important role for isolation and speciation processes of the warm-water benthos.
日本古生物学会の論文
1996-12-15

栃木県塩原地域の上部中新統鹿股沢層より産する暖流系軟体動物化石 : 塩原動物群の再検討に向けて

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