Late Jurassic to Early Cretaceous ammonite fauna from the Somanakamura Group in Northeast Japan

ABSTRACT Over 300 specimens of ammonites of Late Jurassic to Early Cretaceous age were collected by the members of local fossil research associations from various localities in the Soma area, Northeast Japan. The new collection of ammonites is confined to two formations, Nakanosawa Formation in the lower and Koyamada Formation in the upper part of the Somanakamura Group. Four ammonite-bearing levels (A2 to A5) are ascertained in these formations, and each level was dated by the ammonites discovered. The lower three levels (A2, A3 and A4) are contained in the Nakanosawa Formation. The newly collected ammonites from this formation include a well preserved adult specimen of Aulacosphinctoides tairai sp. nov., relatively well preserved specimens of Subdichotomoceras chisatoi sp. nov., hitherto unknown Haploceras sp. and Hybonoticeras sp., besides many already known species. The lowest level A2 represents a certain age within the Kimmeridgian - early Tithonian. The next lower, A3, somewhat later than the forme...