The jaw apparatuses of two species of Late Cretaceous Phylloceratina (Ammonoidea), Hypophylloceras subramosum and Phyllopachyceras ezoensis, are described on the basis of well‐preserved in situ material from Hokkaido, Japan. Gross morphological and X‐ray CT observations reveal that the upper and lower jaws of the two species are essentially similar in their overall structure. Their upper jaws consist of a shorter outer lamella and a pair of larger, wing‐like inner lamellae that become narrower and join together in the anterior portion, as in those of other ammonoids. The upper jaws of the two phylloceratid species are, however, distinguishable from those of other known ammonoids by the presence of a thick, arrowhead‐shaped calcified rostral tip. The lower jaws of the two species consist of a short, reduced inner lamella and a large, gently convex outer lamella covered with a thin calcareous layer, the features of which are common with the rhynchaptychus‐type lower jaws of the Cretaceous Lytoceratina. In the presence of a sharply pointed, thick calcareous tip on upper and lower jaws, the jaw apparatuses of the Phylloceratina resemble those of modern and fossil nautilids, suggesting that they were developed to serve a scavenging predatory feeding habit in deeper marine environments. This and other studies demonstrate that at least some Mesozoic rhyncholites and conchorhynchs are attributable to the Phylloceratina and Lytoceratina.

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Plotosaurus is a highly aquatically adapted mosasaur, which is supposed to inhabit the deep ocean basin. The geographic occurrence of this genus has been limited only to the west coast of North America. In this study, two Plotosaurus-type mosasaur caudal vertebrae derived from the Upper Cretaceous Nakaminato Group in Ibaraki Prefecture, Japan, are described with discussion on the paleobiogeographic significance of the Late Cretaceous mosasaur fauna in the Northwestern Pacific region. The two specimens are an intermediate caudal vertebra found in a beach cobble, which presumably originated from the Hiraiso Formation (upper Campanian), and a terminal caudal vertebra found in situ in the lower Isoai Formation (lower Maastrichtian). Because their relative centrum lengths (ratio of centrum length/centrum height, ~0.7) are very close to that of Plotosaurus, the specimens are referred to cf. Plotosaurus sp. The two specimens provide the first evidence that highly specialized Plotosaurus-type mosasaurs inhabited the Northwestern Pacific Ocean, suggesting that such forms had a wider distribution than previously recognized and might have existed since the late Campanian in the Northwestern Pacific Ocean.