Taxonomy and stratigraphic distribution of the Upper Cretaceous marine reptiles from Japan are reviewed. Remains of the Chelonioidea (sea turtles), Mosasauridae, and Plesiosauria are known in various parts of Japan, including the holotypes of the dermochelyid . Mesodermochelys undulatus, mosasaurine . Mosasaurus hobetsuensis and . M. prismaticus, tylosaurine . Taniwhasaurus mikasaensis, and elasmosaurid . Futabasaurus suzukii. Less diagnostic materials of other groups such as protostegiids, plioplatecarpines, polycotylids, pliosauroids, were also collected. . Mesodermochelys dominates the chelonioid fauna, and in comparison with European and North American faunas, suggests a rather restricted geographical distribution of chelonioid species during the Late Cretaceous. The mosasaurid records support the world-wide trend of increasingly mosasaurine-dominated post-Santonian assemblages, and demonstrate suprageneric-level compositional changes in the northwestern Pacific through time. Elasmosaurid fossils are known from all stages of the Upper Cretaceous in Japan and indicate their continuous presence in the northwestern Pacific. Polycotylid remains are fewer in number and limited to the lower Upper Cretaceous. Pliosauroid specimens are even rarer but raise the upper limit of the stratigraphic range of the group in Northern Pacific to the Turonian.
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