An early polycotylid plesiosaur (Reptilia : Sauropterygia) from the Cretaceous of Hokkaido, Japan

A partial skeleton of a short-necked plesiosaur excavated from the Upper Ce nomanian of the Middle Yezo Group of Hokkaido, Japan, includes disarticulat ed vertebrae, the right half of the pectoral girdle, fragments of the pelvi c girdle, ribs, gastralia, and gastroliths. Gastroliths are unusual in shor t-necked plesiosaurs. Skeletal characters indicate that the specimen belong s to the Family Polycotylidae, well known from North America, the former So viet Republics, and possibly from New Zealand. They are rare in East Asia a nd hitherto unknown from Japan. Extensive ossification indicates that this specimen is an adult individual, yet it is smaller than the adult specimens of other known polycotylids. The elongated epipodial bones are a unique ch aracter of the specimen but are probably plesiomorphic. The fossil is evide nce of biogeographical diversification of the family at the beginning of th e Late Cretaceous.