Of the eleven turtle taxa recognized from the White River Group, eleven occur in the Chadronian, five in the Orellan and three in the Whitneyan. Chrysemys inornata Loomis 1904 and Graptemys cordifera Clark 1937 are synonymized and placed in Pseudograptemys, new genus. Trachemys? antiqua Clark 1937 is transferred to Chrysemys. Testudo praeextens Lambe 1913, T. quadratus Cope 1885, T. thomsoni Hay 1908 are included in Gopherus (Oligopherus, new subgenus) laticuneus (Cope 1873). Despite the addition of records of aquatic turtles to the Orellan and Whitneyan, the general pattern of reduction of the aquatic turtle fauna between the Chadronian and Orellan (Eocene-Oligocene) transition remains marked.
The White River Group occupies an historical and geographic center for studies of fossil turtles in North America. The first described North American nonmarine turtle (Emys nebrascensis Leidy 1851) was based on fossils from the White River Group. The group also transits a critical time in the record of North American turtles, recording the demise of the great Eocene turtle diversity in the continental interior (Hay, 1908; Hutchison, 1992). The major change occurs across the Chadronian/Orellan (Eocene/Oligocene) boundary interval (Hutchison, 1992; Prothero and Swisher, 1992) and sets the stage for the evolution of the modern turtle fauna.
The scope of this paper is centered on the White River Group in the strict sense, that is, the Chadron and Brule formations and correlatives in North and South Dakota, Nebraska, Wyoming, and Colorado (Emry et al., 1987).