Five ammonoids from the Turonian/Coniacian (Upper Cretaceous) boundary horizon in the Ashizawa Formation, Futaba Group, Fukushima, Japan

抄録

Five species of Late Cretaceous ammonoids, Yezoceras miotuberculatum, Yezoceras elegans, Eubostrychoceras indopacificum, Pseudoxybeloceras sp., and Yabeiceras orientale, have been identified from the Obisagawa Member of the Ashizawa Formation, Futaba Group, Fukushima Prefecture, northeastern Japan. Notably, Yezoc.miotuberculatum, Yezoc. elegans, and the genus Pseudoxybeloceras, previously known from the Yezo Group in Hokkaido, northern Japan, were identified for the first time from the Futaba Group. The presence of Yezoc.miotuberculatum and Yezoc. elegans indicates faunal similarities between the Ashizawa Formation and the Yezo Group in Hokkaido. Co-occurrence of age-diagnostic inoceramids suggesting latest Turonian to earliest Coniacian age allows us to provide a detailed biostratigraphic correlation between the Futaba and Yezo groups.