Abstract
Well-preserved diatoms are reported from the Omagari Formation (Santonian–Early Campanian) of the Upper Yezo Group in the Teshio-Nakagawa area of northern Japan. The Unit has a diatom floral assemblage that is rich in diverse species of Hemiaulus spp. and Triceratium spp., in addition to a few other genera of diatoms and silicoflagellates. Although valve microstructures are generally somewhat dissolved, preservation of these diatoms is significantly better than those described from previous research on Cretaceous siliceous phytoplanktons in Japan. The diatom floral assemblage in the study area is one of the oldest recorded in Japan, and should provide important insights into the Late Cretaceous evolutionary radiation of diatoms around the northwestern Pacific margin, an area where detailed information on primary producers is significantly lacking.
