1996 Volume 1996 Issue 184 Pages 597-622
Mesodermochelys undulatus, a new genus and species, is described as the best Mesozoic representative of the Dermochelyidae (order Testudines, superfamily Chelonioidea). This new taxon is described from 15 specimens which include cranial fragments, lower jaw, axial and appendicular skeleton, and a nearly complete shell. These specimens were collected from the Lower Maastrichtian, Upper Cretaceous of Hobetsu-cho, Central Hokkaido, North Japan. Mesodermochelys shares with Cenozoic dermochelyids such derived characters as the reduced processus trochlearis oticum formed by prootic, the anterior process of surangular, and the elongate anterior projection of lateral process of humerus. The scute sulci of Mesodermochelys are nearly absent, only remaining in the neural region of the carapace. The bony shell of Mesodermochelys is well developed as a primitive condition. Some characters such as the unique configuration of the lower triturating surface with isolated simphyseal and lingual ridges, the undulating medial margin of broad peripherals, the narrow neurals, and the elongate iliac blade with outward curvature are considered as autapomorphic conditions of Mesodermochelys. Frequent occurrence of this primitive dermochelyid in the Upper Cretaceous of Japan suggests endemic distribution among Mesozoic chelonioid species.