The Ammonoid Desmoceras In The Upper Albian (Lower Cretaceous) Of Japan

The Upper Albian Desmoceras species of Japan are described, based on material from the Yezo Supergroup in Hokkaido, north Japan, and from the Fujikawa Formation in Shikoku, southwest Japan. Based on comparative analysis of morphologic variation and shell growth of relevant specimens, we assign the present material to two species: Desmoceras (Desmoceras) latidorsatum and Desmoceras (Pseudouhligella) poronaicum. Japanese examples of the latter species have been previously referred to 'Desmoceras (Pseudouhligella) dawsoni,' a Canadian species present on Queen Charlotte Islands, British Columbia, but not represented in the Japanese Islands. Desmoceras (Pseudouhligella) poronaicum is also known from British Columbia. theta he subgenus Pseudouhligella presumably originated in the eastern Pacific during the Late Albian and subsequently migrated to the western Pacific during latest Albian to earliest Cenomanian time. Desmoceras (D.) latidorsatum is a Tethyan form; its presence in the Upper Albian of the North Pacific probably reflects a northward excursion of warm-water currents from the tropical region during Late Albian time.