A new geoemydid (Testudines, aff. Rhinoclemmydinae) from the upper Eocene Chadron Formation (White River Group) of northwestern Nebraska

Abstract

Notapachemys oglala, gen. et sp. nov., is described based on carapace and plastron fossils collected from upper Eocene (Chadronian NALMA, White River Group) exposures in northwestern Nebraska, U.S.A. It is recognized as having a thick, smooth, partly fused, acarinate, egg-shaped carapace with strong plastral hindlobe kinesis. Many of these traits are present in some ptychogastrine geoemydids, a group known from the early Eocene to late Miocene of Europe. However, other aspects of shell morphology such as a six-sided Neural 1, medial gular notch, lack of distal gular tubercles, absence of inguinal scutes, and elongate kinetic inguinal buttresses resemble semi-aquatic to semi-terrestrial species of the New World tropically distributed genus Rhinoclemmys. The new taxon is one of the oldest plausible stem rhinoclemmydine geoemydids that possesses a mosaic of ptychogastrine and rhinoclemmydine features, suggesting the possibility of a close but unresolved relationship between these two groups. A convoluted history of ptychogastrines and rhinoclemmydines in North America is further exemplifed in the early Miocene of the Panama Canal Basin, where ptychogastrine fossils from the Las Cascadas Formation (~22–20 Ma) and the oldest Rhinoclemmys fossils from the Cucaracha Formation (~18–16 Ma) have been collected in a region where Rhinoclemmys still lives today.