Boyer diss 2009 1046..

CHAPTER 3:
DESCRIPTION OF THE FIRST KNOWN CRANIUM OF PLESIADAPIS
COOKEI, COMPARISON TO P. TRICUSPIDENS, AND CONSIDERATION OF
SOME FUNCTIONAL ASPECTS OF DENTITION
ABSTRACT
I describe the first known skull of the plesiadapid Plesiadapis cookei UM 87990 from the
middle Clarkforkian North American Land Mammal Age of the Clarks Fork Basin,
Wyoming. This taxon has been described from its dentition as morphologically
intermediate between the European taxa Plesiadapis tricuspidens and Platychoerops
russelli, with which it existed penecontemporaneously. I compare skull morphology and
aspects of the dentition of P. cookei with samples of P. tricuspidens and other
plesiadapids. The cranium of P. cookei is very similar to that of P. tricuspidens in most
respects and appears to have been of roughly the same size. However, the skull of P.
cookei differs in (1) having larger teeth, (2) having a more molariform P 4 , (3) lacking a
P 2 , (4) having broader nasal bones, (5) having smaller glenoid fossae, (6) possibly having
a less expanded annular component to its ectotympanic bone, and (7) having a more
posteriorly projecting nuchal crest. Features 3-7 make P. cookei more similar to earlieroccurring
North American plesiadapids such as Plesiadapis anceps, Nannodectes
intermedius, and Pronothodectes gaoi. The proportionally and absolutely larger teeth of
P. cookei, compared to those of P. tricuspidens, are a curious distinctive feature.
However, further consideration of other dental features, including orientation patch
counts and relief index generated from scan data, suggests that large teeth in P. cookei
reflect a specialized folivorous diet. Specifically, as compared to P. tricuspidens, P.
cookei has a dentition with higher molar relief, greater molar complexity, more advanced
molarization of premolars, and simpler central incisors. Therefore, despite being similar
in their large body size, P. cookei in North America and P. tricuspidens in France
probably inhabited different dietary niches.
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