Samples of Oligocene fossil tortoise bone, paleosols and coprolites were collected from the “Turtle-Oreodon Layer” and elevations throughout the Scenic Member of the Brule Formation., White River Group, Paleogene (Badlands National Park, South Dakota) and fossil tortoise shell from Little Egypt Local Fauna, Vieja Group, Paleogene (Trans-Pecos Texas). These bones were analyzed for δ¹⁸O and δ¹³C isotope values of bone phosphate. Data generated from these samples are compared with contemporaneous deepsea drilling core isotope stratigraphy. Changes throughout the marine stratigraphic sections demonstrate a pulse-like transition of wet/dry seasonality with an increased aridity up section related to Paleogene glaciation of Antarctica. Climatic effects are mirrored lithologically by an up section sequence consistent with the drying climate (alternating coarse/fine sedimentation) of the late early Oligocene Scenic Member of the Brule Formation, White River Group. Coarse sediments of the Scenic Member replaced by finer grained, more chemically mature loess deposits of the Poleslide Member indicate a transition from a more moist climate This is accompanied faunally by replacement of large pulmonate gastropods by small pupoid gastropods and the divergence of Oligopherus and Stylemys from Hadrianus .

Oligocene (Rupelian and Orellan {NALMA}) temperature fluctuations during the early Cenozoic Antarctic glaciation and paleoclimatology associated with global events is mirrored in the isotopic oxygen and carbon integral to fossil tortoise bone formation. Analyses of fossil materials and their associated isotopic results can be applied to high resolution nonmarine stratigraphic correlation. The isotopic information from the tortoise bone strongly outlines a step like climatic change which documents the faunal and floral transition in the Paleogene such as indicated by replacement of turtles by tortoises, browsing by grazing ungulates, subtropical by savannah vegetation, and a strong link to Milankovitch climatic cycles.