South America, Central America, the southeastern United States, Arctic Canada, Europe, Asia, and Africa all have been suggested as possible or probable biogeographic sources for taxa that appeared in the Western Interior of North America during the late Paleocene and early Eocene. Recent compilations of the geographic and temporal distributions of Paleocene and Eocene mammals and new data, derived primarily from recent collections from early Tiffanian (late Paleocene) quarries in the Crazy Mountains Basin of south-central Montana, permit tests of these hypotheses, particularly those involving a southern New World origin. Significant first appearances of mammalian higher taxa in the Western Interior occur in the earliest Tiffanian, late Tiffanian, earliest Oarkforkian, and earliest Wasatchian. Those that appear in the earliest Tiffanian probably were derived from late Torrejonian forms in the same region. It appears, therefore, that there was not a pronounced geographic shift in North American mammalian faunas across the Torrejonian-Tiffanian boundary as suggested in some southern New World origin hypotheses. It has been suggested that Palaeanodonta, Dinocerata, and Notoungulata (represented by Arctostylopidae), which appear in the late Tiffanian in the Western Interior, originated in South America, but the evidence is inconclusive and highly controversial. New higher taxa that appear in the Western Interior at the beginning of the Clarkforkian, particularly Rodentia and Tillodontia, probably originated in Asia and dispersed across Beringia. Most of the suprageneric taxa that first appear at the beginning of the Wasatchian in the Western Interior (Perissodactyla, Artiodactyla, Adapidae, Omomyidae, and Hyaenodontidae) also probably appeared in Asia and Europe at essentially the same time; there is no evidence for heterochrony. Recent paleontological discoveries and paleogeographic evidence suggest that the ultimate origins of some or all of these taxa lay in either Africa or the Indian subcontinent. The latter biogeographic source has not been seriously considered previously.

North American Paleocene land mammal ages are the Mantuan, Puercan, Torrejonian, Tiffanian, and Clarkforkian. These ages (and associate stages) are subdivided into 16 zones or subzones, varying in duration from 0.1 to 2.9 m.y., defined by widespread species. Although gross evolutionary changes during the first four of these ages are about equal, their durations are very unequal. As defined by magnetostratigraphy and fossil occurrence, the Mantuan is about 0.2 m.y., the Puercan about 1.1 m.y., the Torrejonian about 3.1 m.y., the Tiffanian about 6.1 m.y., and the Paleocene part of the Clarkforkian about 1.3 m.y. in duration. Puercan encompasses normal magnetozone 29, Torrejonian zones 28 and 27, and the Tiffanian–Clarkforkian boundary falls in zone 25. The type Rio Chico Formation of Patagonia is of mid-Tiffanian to Clarkforkian age. Problems in the identification of magnetozones in the San Juan Basin have arisen because an unconformity is present between the Kirtland Shale and the Ojo Alamo Sandstone, and for some years an extra normal chron was falsely identified. When this hiatus is taken into account, marine and terrestrial fossil correlations agree with magnetozone correlations throughout Upper Cretaceous and Paleocene rocks. The Danian stage in marine rocks in North Dakota is equivalent to Mantuan through early Tiffanian; the Thanetian is exactly equivalent to mid-Tiffanian to early Clarkforkian. The rate of Paleocene sedimentation in the major basins of North America does not depart from linearity much more than the contemporary rate of seafloor spreading. Terrestrial rates of sedimentation vary from a peak of 568 bubnoffs (b) (meters per million years) for the Hoback Formation at the Rocky Mountain front to 99 b in the San Juan Basin and 15 b in the Black Peaks Formation in Texas, compared to 2.7 b at Gubbio, Italy. Sedimentation rates along a transect through the Bighorn, Powder River, and Williston basins follow the equation Y = 200X −0.25 , where Y is the sedimentation rate in bubnoffs and X is the radial distance in kilometers from the Absaroka thrust. Absolute taxonomic and morphologic rates of evolution of the most rapidly evolving mammals during the Bugcreekian–Mantuan interval across the Cretaceous–Paleocene boundary peak at 5 genera per m.y. and 3.85 darwins (a rate of measurement defined in the text), the fastest rates known in the fossil record, and decline exponentially to more normal rates of 1 species per m.y. and 0.5 darwins by the Tiffanian. Range charts of 299 species of ungulates, primates, and multituberculates permit ready identification of zones. Seven new species of multituberculates are described, and shape and metrical properties of latest Cretaceous–Paleocene neoplagiaulacid multituberculates are summarized for ease in identification.