Boyer diss 2009 1046..

INTRODUCTION
Among North American plesiadapids, one of the latest occurring and largest
species is Plesiadapis cookei (Jepsen, 1930; Gingerich, 1976). The only known skull of
P. cookei was discovered with an associated skeleton in late Paleocene strata of the
Clarks Fork Basin in 1987 (Gunnell and Gingerich, 1987; Gingerich and Gunnell, 1992,
2005; Bloch and Silcox, 2001). The specimen has not yet received a thorough description
or analysis despite the fact that the skull is arguably the most complete known for a North
American plesiadapid. In some respects this new skull is even better preserved than the
nearly complete skulls of P. tricuspidens, an apparently slightly smaller species (e.g.,
Fleagle, 1999) from the Paris Basin in France (Russell, 1964; Gingerich, 1976).
In light of its large body size, one might expect P. cookei to be evolutionarily
derived in many respects compared to basal members of Plesiadapidae, which appear to
be much smaller. However, the skull of P. cookei may still retain features that were
present in the ancestral plesiadapid. Thus, a thorough understanding of the morphology
of P. cookei is relevant to further assessing predictions generated by cladistic hypotheses
that postulate plesiadapids as a sister group to carpolestids and as a close relative of
anatomically modern primates (= Euprimates: Hoffstetter, 1977) (Bloch and Silcox,
2006; Bloch et al., 2007).
Another reason to study P. cookei relates to inferred environmental and ecological
changes that bracket its existence in North America (e.g., Zachos et al., 2001).
Understanding how this taxon differs morphologically from earlier (North American) and
later (European) plesiadapids may reveal the nature of ecological differences among
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