日本産Volutomitra属(腹足綱)化石の分類と分布

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  • ニホンサン Volutomitraゾク フクソクコウ カセキ ノ ブンルイ ト ブンプ

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新潟県の灰爪層下部から初めてVolutomira groenlandica alaskana Dallが採集された。また，千葉県の飯岡層産Mitra takii Ozaki，北海道の瀬棚層産Volutomira hahtaii Sawadaは本亜種のシノニムであることを明かにした。さらに，本亜種の出現が鮮新世- 更新世の境界付近であり，鮮新世末期における北半球の寒冷化に伴い南下した事も明かとなった。

One specimen of Volutomitra groenlandica alaskana Dall was firstly collected from the lower part of the Haizume Formation in Niigata Prefecture, central Honshu. This subspecies is now living in the northern Pacific region. The Haizume specimen has a small fusiform (shell height = 18.7mm, diameter = 8.0mm) , seven whorls sculptured by some week spiral threads, and four collumellar folds. Mitra takii was proposed by Ozaki (1958), based on the specimen from the Iioka Formation, Chiba Prefecture. Volutomitra hataii was newly described from the Chinkope (=Setana) Formation by Sawada (1962). However, both species are conspecific with V. groenlandica alaskana from their shell shape and their ornamentation. The fossil V. groenlandica alaskana occurrs from the Setana, Tomikawa, Iioka, Wakimoto, and Haizume Formations in Japan. The age of these deposits are assigned to the latest Pliocene to middle Pleistocene. Ogasawara (1996) and Gladenkov et al. (1991) pointed out that the distinct cooling event occurred at 2.5-2.0Ma. Some boreal species, including V. groenlandica alaskana, migrated southward to the Japanese Islands, corresponding with this cooling event.

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