A New Species of the Heteromorph Ammonoid Phylloptychoceras from the Lowest Maastrichtian of Hokkaido, Japan

A new Cretaceous heteromorph ammonoid, Phylloptychoceras horitai sp. nov., is described from the lowest Maastrichtian of Hokkaido, Japan. Its shell is ornamented with very weak, broadly rounded ribs and its suture line is characterized by a deeply incised, trifid dorsal lobe and three bifid lateral saddles with minor indentations. This occurrence suggests that Phylloptychoceras evolved in the North Pacific during late Campanian or early Maastrichtian time and then achieved worldwide distribution during late Maastrichtian time.