Ammonoid Fauna of the Upper Olenekian Osawa Formation in the Utatsu Area, South Kitakami Belt, Northeast Japan

A late Olenekian (Early Triassic) ammonoid fauna is described from the Osawa Formation in the Utatsu area, South Kitakami Belt, Northeast Japan. The fauna comprises 14 species belonging to 13 genera: Hemilecanites discus (Arthaber), Albanites sheldoni (Kummel), Pseudosageceras? sp., Pseudokymatites tabulatus sp. nov., Columbites parisianus Hyatt and Smith, Tardicolumbites aff. tardicolumbus Guex et al., Yvesgalleticeras sp., Subcolumbites sp., Hellenites elegans Guex et al., Metadagnoceras sp., Procarnites sp., Leiophyllites wakoi sp. nov., Leiophyllites? sp., and Nordophiceratoides bartolinae Guex et al. Among them, genera Hemilecanites, Albanites, Pseudokymatites, Tardicolumbites, Yvesgalleticeras, Hellenites, and Nordophiceratoides are described for the first time from the Japanese Triassic. The generic composition of the ammonoid fauna of the Osawa Formation has thus increased to 25 genera by adding these to the 18 existing genera. This high taxonomic diversity indicates that the nektonic faunas had already recovered from the end-Permian mass extinction by the beginning of the late Olenekian in the South Kitakami Belt. The ammonoid fauna of the Osawa Formation is similar to that of the late Olenekian of Idaho, North America, and in addition, it has some faunal similarity to those in the western Tethys province. These faunal characteristics are likely to reflect the paleogeographic position of the South Kitakami Belt, which was located at the low-latitude area of the Tethys-Panthalassa border during the Triassic.