Lower jaws of two species of Menuites (Pachydiscidae, Ammonoidea) from the middle Campanian (Upper Cretaceous) in the Soya area, northern Hokkaido, Japan

The lower jaws of two pachydiscid ammonites Menuites soyaensis (Matsumoto and Miyauchi) and Menuites sp. are described on the basis of two specimens from the middle Campanian (Upper Cretaceous) in the Soya area of northern Hokkaido, Japan. They are preserved in situ in the body chamber and characterized by a widely open outer lamella with a nearly flat rostral portion. The outer lamella consists of an inner “chitinous” layer sculptured by a median furrow in the anterior to mid-portions and an outer calcareous layer with prismatic microstructure. The lower jaw features of the two Menuites species are shared by other species of the Pachydiscidae described in previous works, indicating that they are diagnostic characters of this family. In view of the shovel-like shape without a pointed rostrum, the lower jaws of the two Menuites species were likely used as a scoop to feed on microorganisms. Key words: pachydiscid ammonites, middle Campanian, lower jaw, Soya area (Hokkaido)