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primates, sharing many dental features with modern members of the group, but lacking
some of the key skeletal features (Simpson, 1935; Gingerich, 1976). Since gradistic
concepts of relationships have given way to cladistic ones, the key question regarding
“plesiadapiform” relationships to modern primates is the following: Do plesiadapiforms
and Euprimates share a special relationship to the exclusion of any other extant or
extinct order of mammals? If the answer to this question is “yes,” then morphological
and reconstructed behavioral differences between plesiadapiforms and euprimates may
illustrate an explicit sequence of adaptive changes, which equate to a description of
“how” euprimates acquired their morphological and ecological specializations from their
archaic “plesiadapiform” ancestors (Szalay and Dagosto, 1980, 1988; Cartmill, 1992;
Bloch and Boyer, 2002; Bloch et al., 2007).
Among “plesiadapiforms,” plesiadapids in particular have figured prominently in
formulation of hypotheses regarding relationships of plesiadapiforms to euprimates. This
is true primarily because plesiadapids were known from well-preserved skulls and
skeletons long before the discovery of such remains for any non-plesiadapid
plesiadapiform (Russell, 1964; Gingerich, 1971, 1975a; Szalay et al., 1975, 1987;
MacPhee et al., 1983; Beard, 1990; Bloch and Boyer, 2002). As the number of
plesiadapiforms known from non-dental remains has increased (Szalay, 1972; Kay et al.,
1992; Bloch and Silcox, 2006), and the phylogenetic significance of this material has
been evaluated (e.g., Bloch et al., 2007), it has been hypothesized that the Plesiadapidae
are members of the Plesiadapoidea (Fig. 1.1), the sister group to Euprimates. The
findings of these studies generate predictions regarding the primitive morphology and
behavior of Plesiadapidae. Thus, further documentation of this group’s evolution stands
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