Kappachelys okurai gen. et sp. nov., a new stem soft-shelled turtle from the early cretaceous of Japan

Abstract

Kappachelys okurai gen. et sp. nov. is named and described based on two isolated carapacial elements (right seventh costal and left seventh peripheral) from the Lower Cretaceous (?upper Neocomian) Akaiwa Formation of west-central Honshu, Japan. Kappachelys is a small turtle (shell length ~10 cm) that exhibits a unique combination of three features: coarse and deep vermiculate sculpture on carpace; no scute sulci; and well-developed peripherals. The form of the sculpture and lack of scute sulci both suggest affinities with the Trionychidae (soft-shelled turtles), whereas the plesiomorphic retention of well-developed peripherals indicates Kappachelys lies outside the Trionychidae. Given this combination of primitive and derived features, we interpret Kappechelys as a stem trionychid. In the same region of Japan, the overlying Lower Cretaceous (Aptian) Kitadani Formation contains some of the oldest known, unequivocal trionychid fossils. Based on its slightly older age, similar geographical distribution, and more primitive shell morphology, Kappachelys could be ancestral to the trionychids of the Kitadani Formation.