Fort Union and Willwood formations of the northern Bighorn Basin, Wyoming, preserve a 1500-meter thick sequence of continental Paleocene strata on Polecat Bench, much of it late Paleocene in age, with an exceptionally complete record of vertebrate faunas and mammalian evolution. This is the type area of the Clarkforkian land-mammal age. Discovery and study of the Clarkforkian fauna in 1912-1914 was crucial in convincing vertebrate paleontologists to recognize a distinct Paleocene epoch separate from the Eocene. Mammalian faunas of the northern Bighorn Basin have been collected and studied for nearly a century. In recent years study of individual taxonomic groups and fossil quarries has been supplemented by stratophenetic analyses of the more common mammals, and by increasingly precisely-constrained biostratigraphy, magnetostratigraphy, and stable isotope stratigraphy. One of a sequence of late Paleocene ashes in the lower part of the Polecat Bench section has been dated radiometrically, and temporal calibration in the upper part of the section is possible using Milankovitch climate cycles affecting development of paleosols.

Study of well-sampled mammalian faunas from fossil quarries in the northern Bighorn Basin shows that virtually modern diversity was achieved by the mid-Paleocene (Torrejonian land-mammal age), but diversity decreased in the late Paleocene (mid-Tiffanian land-mammal age). The Paleocene ended with the onset of the Paleocene-Eocene carbon isotope excursion (CIE), which is well documented in the Polecat Bench section. Here three mammalian faunas (Clarkforkian Cf-3 and Wasatchian Wa-M and Wa-0) are known succeeding each other rapidly within the first 1-1/2 precession cycles of the CIE. The Wa-0 fauna with the first horses and dwarfed Ectocion parvus persisted through the remaining four precession cycles of the main body of the CIE. Normal Wasatchian mammals (Wa-1) appeared in the recovery phase of the CIE. These changes correlate well with changes in global temperature.