Knowledge of plesiadapiform skeletal morphology and inferred ecological roles are critical for establishing the evolutionary context that led to the appearance and diversification of Euprimates (see Silcox, this volume). Plesiadapiform dentitions are morphologically diverse, representing over 120 species usually classified in 11 families from the Paleocene and Eocene of North America, Europe, and Asia (Hooker et al., 1999; Silcox, 2001; Silcox and Gunnell, in press). Despite this documented diversity in dentitions, implying correlated diversities in diets and positional behaviors, very little is known about postcranial morphology among plesiadapiforms. What is known has been largely inferred from a limited number of plesiadapid specimens, representing only a small sample of the known taxonomic diversity from North America and Europe (Beard, 1989; Gingerich, 1976; Russell, 1964; Simpson, 1935a; Szalay et al., 1975). While it has been suggested that plesiadapids may have been terrestrial, similar to extant Marmota (Gingerich, 1976), the consensus in the literature is that they were arboreal (Beard, 1989; Godinot and Beard, 1991; Rose et al., 1994; Russell, 1964; Szalay and Dagosto, 1980; Szalay and Decker, 1974; Szalay and Drawhorn, 1980; Szalay et al., 1975). While it has been further suggested that plesiadapids might have been gliders (Russell, 1964; Walker, 1974) or arboreal quadrupeds (Napier and Walker, 1967), they are now thought to have been more generalized arborealists with some specializations for vertical postures (Beard, 1989; Godinot and Beard, 1991; Gunnell and Gingerich, 1987; Silcox, 2001). Commenting on the need for a taxonomically broader sample of plesiadapiform postcranial skeletons, F. S. Szalay wrote: "It may be that once postcranial elements of the Paleocene primate radiation become more common, Plesiadapis might become recognized as a relatively more aberrant form than the majority of early primates" (Szalay, 1972: 18). In fact, this prediction has been validated in the course of the last 15 years of paleontological field and laboratory research. Since the early 1980s, field crews and fossil preparation labs of the University of Michigan Museum of Paleontology (UM), New Mexico State University (fossils housed at the U.S. National Museum of Natural History, USNM), and John Hopkins University (fossils also in the USNM) have recovered a number of plesiadapiform skeletons representing groups other than the Plesiadapidae. Several of these specimens with associated dentition and postcrania were collected from mudstones in the Bighorn Basin (Beard, 1989, 1990; Rose, 2001); however, the most complete specimens, including semi-to fully-articulated individuals, are derived from fossiliferous limestones in the Clarks Fork Basin (Bloch, 2001; Bloch and Boyer, 2001; 2002a,b; Bloch et al., 2001, 2003; Boyer and Bloch, 2000, 2002a,b; Boyer et al., 2001). Beard (1989, 1990, 1993a,b) studied postcranial specimens attributed to paromomyid and micromomyid plesiadapiforms and concluded that these taxa were very different from known plesiadapids in their locomotor repertoire. Specifically, Beard proposed that micromomyids and paromomyids were mitten-gliders and shared a sister-group relationship with extant dermopterans (=Eudermoptera of Beard, 1993a). Both the mitten-gliding hypothesis and the character support for Eudermoptera have since been questioned both with respect to the original evidence (Hamrick et al., 1999; Krause, 1991; Runestad and Ruff, 1995; Silcox, 2001, 2003; Stafford and Thorington, 1998; Szalay and Lucas, 1993, 1996) and based on new limestone-derived specimens that are far more complete and have more carefully documented dental-postcranial associations (Bloch, 2001; Bloch and Boyer, 2001; 2002a,b; Bloch and Silcox, 2001; Bloch et al., 2001, 2003; Boyer and Bloch, 2000; 2002a,b; Boyer et al., 2001). Despite doubt regarding Beards original arguments for gliding and a close relationship to Dermoptera, the observation that micromomyids and paromomyids are postcranially distinct from the better known plesiadapids is not disputed. Furthermore, a recent study of a carpolestid plesiadapiform skeleton (Bloch and Boyer, 2002b) indicates that these animals were different from plesiadapids, paromomyids and micromomyids in exhibiting capabilities for strong pedal grasping in a manner similar to euprimates (Bloch and Boyer, 2002a). Overall, these skeletons confirm the implications of the diverse dental remains by suggesting a commensurate diversity in positional behaviors among plesiadapiforms. This chapter includes: (1) a review of the methods for documenting postcranial-dental associations in freshwater limestone deposits from which most of the new significant plesiadapiform material is derived, (2) a summary of the postcranial anatomy and inferred positional behaviors of plesiadapiforms based on these new specimens, and (3) a discussion of the implications of the newly discovered postcranial anatomy for phylogenetic reconstructions and understanding primate origins and evolution. © Springer Science+Business Media, LLC 2007.