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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.

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