Middle and late Maastrichtian (latest Cretaceous) ammonoids from the Akkeshi Bay area, eastern Hokkaido, northern Japan and their biostratigraphic implications

Nine ammonoid species are reported from the Maastrichtian Senpohshi Formation exposed along the western coast of Akkeshi Bay, eastern Hokkaido, and their respective chronologic assignments are discussed on the basis of a previous magnetostratigraphic study. Pachydiscus flexuosus occurs in the lower and middle parts of the formation (= polarity chron C31n, i.e., middle to upper middle Maastrichtian). Gaudryceras makarovense, Anagaudryceras matsumotoi and Diplomoceras cf. notabile and P. flexuosus as well occur in the lower part (middle middle Maastrichtian). The uppermost part of the formation (= probably the lower part of polarity chron C30n, i.e., lower upper Maastrichtian) is fossiliferous and yields a diverse ammonoid assemblage including Neophylloceras sp., Pseudophyllites sp., Zelandites varuna, A. matsumotoi, Gaudryceras cf. seymouriense, Gaudryceras sp., and D. cf. notabile. Integration of bio- and magnetostratigraphy in the Senpohshi Formation makes it possible to determine precise and detailed chronologic assignment of strata containing similar faunas in the Northwest Pacific realm.