Boyer diss 2009 1046..

CHAPTER 4:
THE FIRST KNOWN SKELETON OF PLESIADAPIS COOKEI AND BASIC
FEATURES OF THE SKELETON OF PLESIADAPIDAE
ABSTRACT
The only evidence for the existence of most Paleocene mammals consists of the remains
of their dentitions. Paleocene primates are no exception, with only a handful of nondental
specimens in existence. I describe the most completely known skeleton of a
Paleocene primate, Plesiadapis cookei UM 87990, from the middle Clarkforkian North
American Land Mammal Age of the Clarks Fork Basin, Wyoming. Previously described
partial skeletons and postcranial material attributed to other plesiadapid species,
particularly that of P. tricuspidens from France, are compared to the skeleton of P.
cookei. HRxCT data of bones are used to reconstruct articulated fore- and hind limb
complexes more completely than previously possible. I evaluate functional implications
of the skeleton and CT reconstructions, including the implications for the hypothesis that
P. cookei utilized suspensory postures more than other plesiadapids.
The skeletal morphology of P. cookei is similar to what is known of other noncarpolestid
plesiadapiforms in many respects. The forelimb is interpreted to have
exhibited a habitually flexed elbow, a physiologically abducted forearm, and a partly
supinated manus. Examination of articular surfaces and HRxCT reconstructions of the
articulated hand and wrist indicate a habitually dorsiflexed manus that was capable of
more extensive dorsiflexion than palmarflexion. This is consistent with a capacity for
pronograde or orthograde quadrupedal locomotion. It is not a specialization for
suspensory postures. Furthermore, the articulated wrist indicates a mobile pollical
metacarpal, contrary to previous assessments. New information on metacarpal
associations indicates that the phalanges are significantly longer relative to the
metacarpus than previously thought. Regarding the hind limb, plantarflexion of the
astragalotibial joint results in conjunct inversion and medial rotation of the foot by almost
90°. Furthermore, the foot was capable of an additional 90° of inversion-eversion and
45° of conjunct plantarflexion-dorsiflexion at the astragalocalcaneal joint. The nature of
the mobility at the hip, knee and foot together would have facilitated clinging and
climbing on relatively large tree trunks. The vertebral column exhibits proportions
similar to those of callitrichid primates and phalangerid marsupials. The neck is
relatively short, the thorax is slightly longer than the lumbus, and the tail is elongate. The
position of the anticlinal vertebra near the end of the thoracic region suggests that P.
cookei could use a bounding gait. Various other features suggest the use of orthograde
postures. Findings that suggest arboreality and specialization for vertical postures are in
agreement with previous conclusions based on studies of other plesiadapids and other
regions of the postcranium of P. cookei. However, further unambiguous support for the
suggestion that P. cookei relied on suspensory postures was not discovered.
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