During the transition from the Eocene to the Oligocene epochs, the mild tropical climates of the Paleocene and early Eocene were replaced by modern climatic conditions and extremes, including glacial ice in Antarctica. The best terrestrial record of the Eocene-Oligocene transition is found in North America, including the spectacular cliffs and spires of the Big Badlands National Park, in South Dakota. The first part of this book summarises the latest information in dating and correlation of the strata of late middle Eocene through early Oligocene age in North America, including the latest insights from argon/argon dating and magnetic stratigraphy. The second part reviews almost all the important terrestrial reptiles and mammals found near the Eocene-Oligocene boundary in the White River chronofauna, from the turtles, snakes and lizards to the common rodents, carnivores, artiodactyls, and perissodactyls. This is the first comprehensive treatment of these rocks and fossils in over sixty years and will be an invaluable resource to vertebrate palaeontologists, geologists, mammalogists and evolutionary biologists.