Paleontological Research

Abstract

Abstract.

Twelve species of middle and late Campanian (Late Cretaceous) ammonoids, of which one is a newly described species, are reported from the Chinomigawa Formation of the Yezo Group in the Urakawa area, southern central Hokkaido, northern Japan. Furthermore, two ammonoid biozones, the Metaplacenticeras subtilistriatum and Baculites subanceps in ascending order, are recognized. Zircon radiometric ages reveal that the ages of tuffs immediately below the B. subanceps Zone are 75.1±0.9 Ma and 76.0±1.3 Ma respectively, which infer an early late Campanian age. Therefore, the Metaplacenticeras subtilistriatum and Baculites subanceps zones correlate with the upper middle Campanian and lower upper Campanian, respectively. The latter zone is probably a correlative of the lower part of the Didymoceras sp. Zone of the Izumi Group in Southwest Japan because of the discovery of the herein newly described species, Didymoceras hidakense Shigeta sp. nov. Since Didymoceras and Baculites subanceps flourished in other regions during late middle Campanian time, their occurrence in the Urakawa area suggests that they extended their geographic distribution from other areas to the Northwest Pacific region in early late Campanian time.