Plesiadapidae

Plesiadapid postcrania are currently known from a wider geographic and temporal range and from a greater diversity of species than are those of any other plesiadapiform group. Not surprisingly, they exhibit greater morphological diversity than seen in any of the other three families considered here. A skeleton of Plesiadapis cookei (Figure 3D; Gunnell and Gingerich, 1987; Gunnell, 1989; Gingerich and Gunnell, 1992, 2005; Hamrick, 2001), a large species known exclusively from western North America, is in the process of being described (Boyer, in preparation). Plesiadapids obtain a large size rather early in their evolutionary history, and this may explain many of their characteristic features (Gingerich, 1976).

Clinging and climbing on large diameter substrates appears to be a major feature of the locomotor repertoire of Plesiadapis (Gingerich and Gunnell, 1992; Gunnell, 1989). The ability to grasp small-diameter supports with the hands and feet is reduced, and agile pronograde bounding would probably have been infrequent (Gunnell, 1989).

The unguals of plesiadapids differ from those of other plesiadapiforms in having a shaft that is relatively long, an extensor tubercle that is reduced and proximally extended such that the articular surface is more plantarly oriented, and a flexor tubercle that faces plantarly instead of proximally. As a consequence of reduction in the extensor tubercle, the dorsal margin of the ungual shaft is generally convex for its entire length. The digit ray as a whole is most comparable to that of semi-arboreal new world porcupines such as Erethizon and Sphiggurus and thus, consistent with the hypothesis of clinging and climbing on large diameter vertical supports. The proximal ends of the unguals of at least Plesiadapis cookei are, however, strikingly similar to those of sloths and the pedal unguals of Pteropus (Megachiroptera: Pteropodidae), possibly indicating some suspensory behaviors. Godinot and Beard (1991) illustrate the digit ray for Plesiadapis tricuspidens showing it to not have this suspensory feature. Furthermore, Beard (1989) illustrated the phalanges of another plesiadapid, Nannodectes intermedius demonstrating that it is more like P. tricuspidens in this regard.

Although the pollex of plesiadapids is divergent and probably fairly mobile (Beard, 1989, 1990), they have been described as lacking prehensile phalangeal proportions (Beard 1990; Hamrick, 2001), suggesting a reduction in their ability to grasp small diameter supports in a euprimate-like way (Beard, 1990; Boyer and Bloch, 2002; Hamrick, 2001). However, Godinot and Beard's (1991) reconstruction of the P. tricuspidens ray shows it to have a short metacarpal, making it more similar to other plesiadapiforms (e.g., different from P. cookei) in this respect.

The humerus of Plesiadapis cookei suggests less emphasis on euprimate-like grasping (Gunnell and Gingerich, 1987) and might be more similar to that of sloths and dermopterans in features that represent suspensory tendencies. This is distinctly not the case for Nannodectes intermedius in which the humerus is more like that of other plesiadapiforms (Beard, 1989).

The close similarity of some plesiadapid unguals to those of sloths and bats, the similarity of at least some plesiadapid humeri to sloths and dermopterans, and the lack of prehensile phalangeal proportions, indicate more frequent use of underbranch clinging. Whereas smaller-bodied plesiadapiforms could navigate small branches using strong grasping (similar to extant Ptilocercus and callitrichines) and pronograde postures, plesiadapids were also likely capable of some grasping, but may have also relied more on suspensory behaviors to distribute their weight and avoid torques when moving on small branches, as large-bodied platyrrhine and hominoid primates do today.

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