Pliosaurid (Reptilia: Sauropterygia) remains from the Upper Cretaceous of Japan, and their biostratigraphic and paleogeographic significance

Abstract

Four indeterminate pliosaurid remains from the Cenomanian and Turonian of the Yezo Group in Hokkaido, northern Japan, are described/re-described. Although their fragmentary nature does not allow identification to lower taxonomic levels, two specimens represent large individuals whose skull size approaches that of Megacephalosaurus from North America. The late Turonian age of the youngest specimen indicates that the Pliosauridae survived in the northwestern Pacific to least the time when the entire clade was nearing extinction. Along with the known stratigraphic distributions of the Elasmosauridae and Polycotylidae in the Japanese Cretaceous, the described pliosaurid specimens demonstrate that all three major plesiosaurian clades of the Late Cretaceous crossed the Cenomanian–Turonian boundary at least in this region.