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Craniodental material of Plesiadapis cookei and its implications for paleobiological
distinction from P. tricuspidens
Chapter 3 provides the first extensive descriptions and comparisons of a skull of
Plesiadapis cookei, the largest, and one of the latest occurring North American
plesiadapid taxa (Gingerich, 1976). It is compared extensively to the similarly well
preserved skulls of P. tricuspidens from France. Previously known similarities between
P. cookei and P. tricuspidens include features such as large size, a penecontemporaneous
existence and dental features such as lack of a margoconid on the lower central incisor
(Jepsen, 1930; Gingerich, 1976). The skull of P. cookei possesses a unique mixture of
features, some representing similarities to other North American plesiadapids, as
described in Chapter 2, and others representing similarities to P. tricuspidens. P. cookei
is similar to P. tricuspidens in having a tube-like external auditory meatus and limited
exposure of molar tooth roots on the dorsum of its maxilla in its orbit, but otherwise it
looks more like other North American plesiadapids in features 2, 4 and 10 from above,
and in having proportionally smaller glenoid fossae than P. tricuspidens.
Quantitative comparison of 39 skull measurements confirms that P. cookei and P.
tricuspidens had skulls of virtually identical size. This constitutes strong evidence that
they had similar body sizes in life. This conclusion appears to be at odds with evidence
from molar teeth, which suggest that P. cookei was much larger than P. tricuspidens. An
investigation of the dentition was therefore undertaken. Metrics reflecting molar tooth
function, including orientation patch count and relief index, solidly support the
hypothesis that P. cookei was more specialized for eating leafy, fibrous foods than was P.
tricuspidens. If P. cookei was more specialized to folivory, then having larger teeth for
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