Gaudryceras izumienseMatsumoto and Morozumi, a Maastrichtian Ammonoid from Hokkaido and Alaska and Its Biostratigraphic Implications

Gaudryceras izumiense Matsumoto and Morozumi, 1980, an ammonoid of early Maastrichtian age, occurs frequently in the Izumi Group in the Izumi Mountains, Southwest Japan. Described herein are two specimens referable to this species that were recently collected from the Hakobuchi Formation of the Yezo Group in the Hobetsu area, Hokkaido and from the Matanuska Formation in the southern Talkeetna Mountains, Alaska. In the past, a precise biostratigraphic correlation of the uppermost Cretaceous in the North Pacific realm has been difficult because of the lack of common zonal-index taxa, but the discovery of G. izumiense in Hokkaido and Alaska has made it possible to correlate lower Maastrichtian strata in these localities with coeval strata in Southwest Japan. Gaudryceratid ammonoids are fairly abundant in the uppermost Cretaceous of the North Pacific realm, and consequently, they are ideal taxa for the establishment of a precise biostratigraphic correlation framework for the uppermost Cretaceous in this vast area.