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SUMMARY AND CONCLUSION
The skull of Plesiadapis cookei exhibits a laterally positioned internal carotid
plexus groove like other plesiadapids. It lacks distinctive sutures delimiting the auditory
bulla from either the ecotympanic or the petrosal bone. Furthermore, P. cookei exhibits a
number of similarities to North American plesiadapids, not shared by P. tricuspidens, and
several similarities to P. tricuspidens not shared by other North American plesiadapids.
While P. cookei appears to have been the same size as P. tricuspidens, based on overall
cranial measurements, its teeth are bigger. Larger teeth and a number of other dental
features in P. cookei suggest that it had a more specialized folivorous diet than did P.
tricuspidens. An inferred specialized folivorous diet in P. cookei makes it similar to
Platychoerops, whose species have previously been hypothesized to be descendents of P.
tricuspidens.
The cladistic relationships of P. cookei, P. tricuspidens and Platychoerops should
be reanalyzed to determine whether the morphological features of P. cookei are 1)
convergent similarities to Platycheorops evolved for a similarly folivorous diet, 2)
features inherited from a common ancestor with Pl. russelli, the plesiadapid with which it
shares the most dental similiarities (Jepsen, 1930; Gingerich, 1976), or 3) a reflection of
it being a member of a lineage (and a point along a morphocline) leading from P.
tricuspidens to Pl. daubrei (Gingerich, 1976). Such an analysis is presented in Chapter 5.
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