New records of Diplomoceras (Ammonoidea, Diplomoceratidae) from Koryak Upland, North-West Pacific Province: taxonomic, biostratigraphical and palaeogeographical implications

The geographical and stratigraphical distribution of the heteromorph ammonite genus Diplomoceras within the North-West Pacific Province is briefly reviewed. Although Diplomoceras ranks amongst the more important, globally distributed taxa during the latest Cretaceous, its precise stratigraphical range still is uncertain. Moreover, the status of the various species assigned to this genus is a matter of debate; for instance, two commonly cited forms, Diplomoceras cylindraceum and D. notabile, are considered to be conspecific by some authors, but treated as discrete taxa by others. The majority of records of species of Diplomoceras are from Maastrichtian strata; however, several late Campanian finds bring some discrepancy into the discussion about their biostratigraphical significance. Here we record new specimens from the upper Maastrichtian Kokuy Unit, refer them to Diplomoceras cylindraceum and briefly consider the geographical, biostratigraphical and taxonomic issues surrounding the genus Diplomoceras.