Late Campanian-early Maastrichtian heteromorph-dominated ammonoid assemblages of the Nakaminato Group, central Honshu, Japan: biostratigraphic and paleontological implications

A late Campanian and early Maastrichtian heteromorph-dominated ammonoid fauna is reported for the Nakaminato Group. The mudstone-dominated offshore Hiraiso Formation and the overlying sandy turbidite-dominated Isoai Formation are exposed along the Pacific coast of Hitachinaka City, Ibaraki Prefecture, central Honshu. This paper describes five taxa of nostoceratid and diplomoceratid heteromorph ammonoids from these strata. Among the five lithostratigraphic units of the Hiraiso Formation, the lowest unit contains Didymoceras sp., and the two overlying units yield the late Campanian index species D. awajiense, together with Diplomoceras sp., a few planispiral ammonoids, and several specimens of Inoceramus (Endocostea) shikotanensis. The Isoai Formation contains a few indeterminate nostoceratid ammonoids as well as Baculites spp. in its upper part, and the middle to late early Maastrichtian index species “Inoceramus” kusiroensis. These taxa provide important information for the Campanian and Maastrichtian biostratigraphy and palaeoecology of the northwest Pacific region.