Although Paleocene mammals have been known from western Canada for nearly 70 years, only during the last 15 years have concerted efforts been made to discover, develop, and describe collections documenting their evolution. Whereas much remains to be done, a faunal succession has been reconstructed, based on collections from 41 mammalian local faunas in Alberta and Saskatchewan, ranging from early Puercan to late Tiffanian age (the first 8 to 10 m.y. of the Tertiary). Latest Cretaceous mammalian local faunas in Saskatchewan show that evolution of progressive “Paleocene aspect” mammals, including condylarths, had begun during the North American Cretaceous as evidenced by the occurrence of fossils in stratigraphie settings free from the complexities that make uncertain the age of faunally similar assemblages in Montana. The Ravenscrag Formation, southwestern Saskatchewan, yields the oldest (early Puercan) Tertiary mammals known from Canada (Rav W-l: MHBT Quarry, Pine Cree Park and Croc Pot localities), including the first discovery of the ancestral primate Purgatorius outside of eastern Montana, and the oldest unarguable carnivoran. Torrejonian mammals are known from the Coalspur Formation (Rocky Mountain Foothills; Diss locality) south of Edson, Alberta. Cochrane 2 (Porcupine Hills Formation, west of Calgary) has yielded unexpectedly diverse earliest Tiffanian mammals, and early Tiffanian mammals have recently been discovered in the Paskapoo Formation near Drumheller (Hand Hills West, lower level), and east of Innisfail, Alberta (Aaron’s Locality). Middle Tiffanian localities occur in the Paskapoo Formation near Red Deer (DW 1 to 3, Mel’s Place, Joffre Bridge Road Cut, Mammal Site No. 1, Erickson’s Landing), and in the Hand Hills (Hand Hills West, upper level); the Police Point local fauna (Ravenscrag Formation), Alberta, appears to be late middle or early late Tiffanian in age. Late Tiffanian mammals at Roche Percée, Saskatchewan (Ravenscrag Formation), and Canyon Ski Lodge, Crestomere School, and Swan Hills, Alberta (Paskapoo Formation) conclude the Paleocene mammalian record known from Canada. Species lists for each locality are presented. The early and middle Tiffanian mammalian record from Alberta fails to show a decline in species numbers seen at several American localities representing this interval: instead of the global cooling sometimes hypothesized to account for this decline, it now seems to be a result of biological or sedimentological events acting on a local scale, sampling error, or some combination of these factors.

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.