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these species. Changing ecological niches among plesiadapids through the late Paleocene
are likely to track environmental changes experienced by these animals during this time
period (Gingerich, 1976).
Gingerich (1976) argued that one of the most dramatic ecological/evolutionary
transitions documented for plesiadapids occurred in a lineage leading from P.
tricuspidens to Platychoerops russelli to Platychoerops daubrei. In this hypothesized
lineage, the molar and incisor teeth exhibit a morphocline from bunodont and complex
(respectively) in P. tricuspidens to selenodont and simple in Pl. daubrei. Gingerich
(1976) suggested that this morphocline reflects an ecological/evolutionary transition from
a generalized diet to a highly folivorous diet.
Gingerich (1976) also stated that P. cookei is dentally very similar to Pl. russelli.
If P. cookei and Pl. russelli share a close phylogenetic relationship, their dental
similarities may represent a commonly inherited trait, or one may have inherited its
morphology more or less directly from the other. In the latter scenario, P. cookei could
be a member of the P. tricuspidens-Pl. russelli-Pl. daubrei lineage and may represent a
point on the morphocline described above. Alternatively, P. cookei and Pl. russelli may
have each evolved separately from more bunodont forms like P. tricuspidens in response
to similar ecological perturbations (e.g., changes in available food resources due to
climate change). Either way the cranium and additional detailed aspects of the dentition
of P. cookei might be expected to differ from those of P. tricuspidens in ways suggesting
a more folivorous diet.
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