Revision of the triprojectate and oculate angiosperm pollen record in Japan, with new data from the Maastrichtian of the Hakobuchi Formation, Yezo Group, in the Hobetsu area, Hokkaido

Angiosperm pollen grains of the triprojectate and oculate groups are widely distributed in Upper Cretaceous strata of the Northern Hemisphere, being used as biostratigraphic markers from their high diversification rate. Most reports from northeastern Asia, however, concern terrestrial deposits, and the rich record existing in Japan from marine sequences, with a well-established bio-magnetostratigraphy, appears critical for correlating their range among the Circum Pacific region. Here we propose a revision of Japanese reports of these groups, constraining their diversity to 17 genera and 91 species. A selected array of 15 species of well-preserved triprojectate pollen was further obtained from an abundant sporo-pollen assemblage of a hadrosaurian dinosaur Kamuysaurus japonicus-bearing bone bed in the outer shelf deposits of the Upper Cretaceous Hakobuchi Formation, Yezo Group, recently discovered in the Hobetsu area of Mukawa Town in Hokkaido, Japan; the assemblage co-occurs with the ammonoid Pachydiscus (Neodesmoceras) japonicus belonging to the Nostoceras hetonaiense zone, supporting an early Maastrichtian age.