Late Maastrichtian (latest Cretaceous) Ammonoids from the Naiba Area, Southern Sakhalin, Russian Far East

Six early late Maastrichtian (Late Cretaceous) ammonoid taxa are reported from the Krasnoyarka Formation of the Yezo Group in the Naiba area, southern Sakhalin, Russian Far East. These taxa are grouped into "immigrant species", i.e., those that migrated from other regions (Pachydiscus subcompressus, Anagaudryceras mikobokense, Gaudryceras seymouriense and Zelandites varuna) and "indigenous species"with a North Pacific distribution (Anagaudryceras matsumotoi). It is unclear to which group Tetragonites sp. belongs. Zelandites varuna and G. seymouriense occur in both the lower upper Maastrichtian as well as the upper lower Maastrichtian in southern Sakhalin, but they have never been found in the middle Maastrichtian. The appearance of these two species in the cold-water regions, i.e., North Pacific and Antarctic, as well as intermediate southern mid-latitudes regions suggests that cooling events occurred during the late early and early late Maastrichtian in the Northwest Pacific region. Their disappearance during the middle Maastrichtian may indicate that the Northwest Pacific region was affected by the greenhouse Middle Maastrichtian Event (MME). This hypothesis suggests that the influx (e.g. P. subcompressus and A. mikobokense) and reappearance (e.g. Z. varuna and G. seymouriense) of many immigrant species into the Northwest Pacific region during late Maastrichtian time may have been associated with the post-MME cooling.