Boyer diss 2009 1046..

een flexed, and the leg slightly abducted. The foot would have been dorsiflexed and
slightly inverted with the capacity for a large range of inversion.
Abducted limbs result in a posture with sprawled hands and feet. Such a posture
would have made P. cookei adept at navigating relatively large diameter substrates. A
physiologically supinated hand and inverted foot would have accommodated cylindrical
substrates well (i.e., tree trunks and branches). Jenkins (1974) presents data on Tupaia
glis showing it to adopt such supinated hand and inverted foot postures during
quadrupedal locomotion on relatively narrow branches.
The nature of mobility, in the hind limb especially, would have been useful in
ascending vertical substrates, because the hind limb has much of its mobility
concentrated in an anterposteriorly extending plane with a mediolateral, rather than
dorsoventral orientation. Thus, extension of the hind limb would not tend to push the
body farther away from the substrate. The femur of Plesiadapis extends and flexes in the
transverse plane. Because the knee would have been laterally rotated during much of the
support phase of locomotion, flexion and extension of the knee also occurred mainly in a
transverse plane. Finally, as explained in the Results section, although flexion and
extension of the astragalotibial joint is limited, abduction-adduction movements and
inversion-eversion movements are emphasized. It is important to keep the body close to
the substrate during vertical climbing because the moment created by gravity on body
mass with respect to a vertical substrate increases when the center of mass moves away
from the substrate.
To be clear, what suggests orthograde postures and locomotion in Plesiadapis is
the unusual degree to which its limbs appear to have been sprawled. For instance,
382