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.

The Bighorn Basin has produced the richest and most diverse early Eocene mammalian faunas in the world and is the principal source of our knowledge of skeletal anatomy in these mammals. Until recently, most of our information on postcranial anatomy in early Eocene mammals came from the works of Matthew and his contemporaries. Considerable new evidence has been unearthed in the last 25 years, but very little of it has yet been described or even reported in the literature. Since 1979, a USGS–Johns Hopkins project working in the Wasatchian part of the Willwood Formation has collected more than 150 skeletal associations (representing more than 25 genera in 20 families), varying from several bones to virtually complete, articulated skeletons. Among these are important new specimens—some of them the first or the most nearly complete skeletons known—of Palaeanodon, Alocodontulum, Microsyops, Phenacolemur, Cantius, Chriacus, Anacodon, Oxyaena, Prototomus, Didymictis, Vulpavus, Miacis, Phenacodus, Hyracotherium, Homogalax, Wasatchia, and Diacodexis. Comparison of characters such as limb proportions, long bone and joint structure, and ungual shape with those in extant forms whose behavior is documented enables inferences of locomotor capabilities in extinct mammals. A wide range of terrestrial adaptations is apparent in Willwood mammals, which include fossorial palaeanodonts, a large digger/rooter ( Ectoganus ), ambulatory ( Oxyaena, Didymictis) or graviportal forms ( Coryphodon ), incipient cursors ( Phenacodus, Pachyaena ), more specialized cursors ( Hyracotherium ), small cursorial/saltatorial types ( Diacodexis, Wasatchia), and small saltatorial mammals (leptictid insectivores). Arboreal locomotion was of at least two types: quadrupedal climbing and leaping (adapid primates), and scansorial claw-climbing (small arctocyonids and miacid carnivorans) that involved extreme tarsal mobility. Some postcranial modifications are strikingly similar to those in extant relatives of these Eocene mammals, suggesting that modification of skeletal form occurred well in advance of dental evolution.

Changes in sample size may confound interpretation of faunal change in the fossil record. The record of early Eocene mammals from the Wasatchian Land-Mammal Age in the Clark’s Fork Basin, Wyoming, shows a high correlation between the square root of sample size and species richness. This correlation suggests that Schankler’s (1980) Biohorizon A, a faunal turnover composed mainly of disappearances, is largely an artifact of sampling fluctuation. Within the interval of Biohorizon A, sample size drops from record high to record low values. Monte Carlo simulation of the drop in sample size across Biohorizon A demonstrates the role of sampling variation in producing artifacts of faunal change. The distribution of missing species resulting from the simulations provides a reliable estimate of the number of species that are likely to be missing at a specified sample size, even if they were present in the original population. Results of the simulations indicate that most of the 16 disappearances observed in the 200-m interval above Biohorizon A can be explained by low sample size alone. For each species that disappeared in the actual record, the frequency of absence in the simulations is a basis for comparing the likelihoods of two hypotheses: (1) that the species was present but not represented by fossils, and (2) that the species was absent Evaluation of the likelihood ratio for these hypotheses indicates that Arctodontomys wilsoni-A. nuptus, Phenacodus vortmani, and Homogalax n. sp.-H. semihians are the lineages most likely to have disappeared over the interval of low sample size. Reconsideration of the biostratigraphic correlation between the central Bighorn Basin and the Clark’s Fork Basin based on the first appearances of Homogalax protapirinus and Tetonius matthewi/steini supports a higher stratigraphic position for the interval corresponding to Biohorizon A in the Gark’s Fork Basin than its original placement.

The Buck Spring Quarries, located in the southern part of the type area of the Lost Cabin Member of the Wind River Formation, Wind River Basin, Wyoming, provide one of the richest assemblages of fossil vertebrates known from the latest Wasatchian Land-Mammal Age (ca. 50.5 Ma, Lostcabinian, early Eocene) of North America. More than 100 species of mammals, reptiles, birds, amphibians, and fishes are known. The quarries uniquely preserve associated skeletal remains, and complete skulls and dentitions of a large percentage of the vertebrates. The fossils come from a 2-m-thick sequence, which is composed primarily of mudstones, bioturbated limestone lenses, and laminated limestone/mudstone couplets. These sediments were deposited in a well-drained swamp or ponded area between 250 and 600 m away from a low-sinuosity stream. The fossils are especially common in limestones, where they accumulated as a result of (1) natural death (articulated specimens), (2) predator activity (coprolites and kill sites), and (3) very limited hydraulic transport of smaller bones. Rarefaction estimates from surface and quarry collections suggest similar patterns of species richness, which are among the highest known for the Paleogene and compare favorably with penecontemporaneous Lostcabinian assemblages and slightly younger Gardnerbuttean (early Bridgerian) ones. The mammalian assemblage is dominated by small species of mammals, and has a body-size distribution and species diversity similar to modern tropical communities. The abundance of arboreal mammals indicates that a multistoried woodland habitat was in close proximity to the quarry area.