The transition from the Eocene to the Oligocene epochs (from about 47 to 27 million years ago) was one of the most dramatic episodes of climatic and biotic change since the demise of the dinosaurs. The mild tropical climates that characterized the Paleocene and early Eocene were replaced by the beginning of modern climatic extremes, including glacial ice in Antarctica and modern deep-water oceanic circulation (summarized in Prothero, 1994). These changes were seen in plants and animals worldwide, both in the oceans and on land. Land floras changed from dense forests (found even at polar latitudes) to a mixture of woodland and scrubland. The land faunas responded with extinction of many forest-dwelling and leaf-eating animals, and replacement by snails, reptiles, and mammals tolerant of drier conditions and the more open vegetation.

The best terrestrial record of the Eocene-Oligocene transition is found in North America, in deposits which include the spectacular cliffs and spires of Big Badlands National Park in South Dakota, world-famous for its scenery and abundant fossils. Although the fossils and deposits have been studied since 1846, much critical new information has accumulated in the last twenty years. Enormous collections of fossil mammals from these beds were made by the Frick Laboratory of the American Museum of Natural History in New York, but only a small fraction of the studies on these fossils has been published.