Boyer diss 2009 1046..

CHAPTER 2:
A REEVALUATION OF CRANIAL STRUCTURE IN PLESIADAPIDAE
(PLESIADIFORMES: MAMMALIA) BASED ON NEW SKULL MATERIAL
ABSTRACT
Newly discovered skulls and petrosals attributed to a species of the most basal genus of
Plesiadapidae, Pronothodectes gaoi, and a skull of Nannodectes intermedius, the
geologically oldest known skull of a plesiadapid, are described comprehensively for the
first time. Additionally, new observations and thorough photographic documentation of
morphology are provided for the best preserved cranial specimen of Nannodectes gidleyi,
and several specimens of Plesiadapis tricuspidens. These new specimens and
observations will ultimately allow a more detailed and better supported reconstruction of
the features characterizing the cranium of the ancestor of the Plesiadapidae, than was
previously possible. All specimens observed have enlarged premaxillae that contact the
frontals. However, in P. tricuspidens the premaxillae have a relatively larger contact
with the frontals. As a consequence, the nasals of P. tricuspidens are exceptionally
narrow at their caudal end. A reassessment of the evidence for bony contacts in the
orbital mosaic reveals that existing specimens do not unambiguously support the previous
conclusion that the maxilla and frontal had a sutural contact in this vicinity. A
reassessment of the morphology of the orbitotemporal region of specimens of P.
tricuspidens leads to the conclusion that it lacks foramen rotundum. All specimens
except that representing N. gidleyi appear to have a laterally-positioned intratympanic,
transpromontorial route for the internal carotid plexus. New evidence from the
promontorium of the petrosal bone, its tympanic processes, and bones forming the
auditory bullae of plesiadapids, rodents, treeshrews, and euprimates leads to the
conclusion that the bony composition of the plesiadapid bulla is still ambiguous.
However, previously undocumented similarities to the bullae of some euprimates suggest
that the plesiadapid bulla is petrosally derived. Pr. gaoi and N. intermedius have a
mediolaterally narrow external auditory meatus, whereas P. tricuspidens has a
mediolaterally expanded, tube-like external auditory meatus. Pr. gaoi and N. intermedius
appear to have proportionally smaller glenoid fossae than P. tricuspidens and N. gidleyi.
Previous perceptions of plesiadapid cranial morphology were based primarily on
P. tricuspidens. Therefore the foregoing observations (as well as others) that illustrate P.
tricuspidens to differ from most other plesiadapids and to exhibit different morphological
features than previously supposed, may have implications for hypothesized relationships
of plesiadapids to other plesiadapiforms and euarchontans.
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