Willwood Formation paleosols are ranked on a scale of 0 to 5 on the basis of their relative maturity (= relative time required to form). In the lateral dimension, the least mature soils were developed more proximal to ancient channel belts, whereas the more mature paleosols formed in areas more distant to channel belts. Quantitative study shows that both mammalian taxonomic composition and taphonomic completeness vary systematically with the maturity of these paleosols. Species-level differences in taxonomic composition are identified for pedofacies sequences located at the 442-m and 546-m levels of the Willwood Formation. At 442 m, Cantius frugivorus and Hyopsodus sp., cf. H. minor account for practically all of the adapiform primate and hyopsodontid condylarth faunas in stage 3 to 4 paleosols (which are distally located with respect to the ancient channel belt). Laterally adjacent and stratigraphically equivalent stage 1 to 2 paleosols (proximally located with respect to the ancient channel belt), are instead dominated by Cantius sp. nov. and Hyopsodus sp., cf. H. miticulus. Intermediate proportions of these taxa occur at localities in paleosols of intermediate maturity (stage 2 to 3 paleosols) at the 442-m level. At 546 m, the otherwise relatively rare species Hyopsodus powellianus makes up nearly 50 percent of the hyopsodontid fauna at some localities developed in stage 1 paleosols; elsewhere in this pedofacies the species Hyopsodus minor and H. lysitensis make up the overwhelming majority of the Hyopsodus. Also at 546 m, the adapiform primates Cantius abditus and “Copelemur”feretutus exhibit reversals in relative abundance from proximal to distal localities across the pedofacies; Cantius is more abundant in proximal localities and “Copelemur” is dominant in distal localities. Ordinal-level differences in taxonomic composition were detected at localities in two distinct pedofacies lying at or slightly above Biohorizon C (= “Graybullian-Lysitean” boundary). There, Condylarthra and Artiodactyla are more common in immature (stages 1 to 2) than mature (stage 4) paleosols, whereas the reverse is true for Primates, Carnivora, Rodentia, and Perissodactyla. Lateral controls on completeness of skeletal elements, as related to lateral variation in sedimentation rate, are also evident. Proportions of less complete skeletal elements are considerably higher at localities developed in mature paleosols, where sedimentation rates were low. These findings underscore the inherent relatedness of geographic distribution of taxa, taphonomy, and sedimentology and suggest that intrabasinal differences in microhabitat had a significant effect on the local taxonomic composition of the Willwood mammalian fauna.

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.

Abstract The Bridger Formation is located in the Green River basin in southwest Wyoming, and the Uinta and Duchesne River formations are located in the Uinta basin in Utah. These three rock units and their diverse fossil assemblages have great scientific importance and are also of historic interest to vertebrate paleontologists. Notably, they are also the stratotypes for the three middle Eocene North American Land Mammal “Ages,” the Bridgerian, Uintan, and Duchesnean, from oldest to youngest. The fossils and sediments of these formations provide a critically important record of biotic, environmental, and climatic history spanning ~10 million years (49–39 Ma). This article features a detailed field excursion through portions of the Green River and Uinta basins that focuses on locations of geologic, paleontologic, and historical interest. In support of the field excursion, we also provide a review of current knowledge of these formations with emphasis on lithostratigraphy, biochronology, depositional and paleoenvironmental history, and the history of scientific exploration.