Carbon and oxygen isotope analyses were used to investigate climatic changes that are hypothesized to have influenced the faunal turnovers that occurred during the Bridgerian-Uintan transition. Research within the Bridger Formation documents this large faunal turnover which includes the decline of perissodactyls and many arboreal species, including primates. These species are replaced throughout North America by artiodactyls, carnivores and broad-spectrum omnivores such as Pentacemylus, Tapocyon and Mytonomeryx. Although there is extensive indirect evidence from global climate proxy studies that a global temperature decrease, possibly accompanied by an increase in seasonality, was the ultimate cause of this faunal turnover, there is no direct evidence from the Bridger Formation, Wyoming. This study demonstrates there is a shift from dense forests to more open habitats across the Eocene-Oligocene transition accompanied by a decrease in temperature in the Bridger Formation. The temperature estimates determined by isotope analyses support the global proxies which find there is no temperature shift during the Bridgerian-Uintan transition. Finally, isotope evidence was found to support both an increase in aridity in the Pliocene and Recent epochs when compared to the middle Eocene.