Sato T., Taketani Y. (2008) Late Jurassic to Early Cretaceous ammonite fauna from the Somanakamura Group in Northeast Japan // Paleontological Research. Vol.12. no.3. P.261–282.
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. (this species of course belongs to other genus than Subdichotomoceras, which is typical Boreal genus, but close to some Submeditterrean ammonites, such as Subplanitoides) hitherto unknown Haploceras sp. (characterized by fine falcoid ribbing) 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 former, is correlated to the late Kimmeridgian to early Tithonian, and the upper level, A4, to the Tithonian. The Nakanosawa fauna indicates therefore the Kimmeridgian to Tithonian interval as a whole. The ammonites from the Koyamada Formation, known only from the uppermost level A5, include a new species of Dalmasiceras muneoi, besides other berriasellids and neocomitids. The age of the formation is Berriasian. The ages hitherto assigned are not greatly modified by the new discoveries. Descriptions of the new species and some other interesting species are given in this paper.