Boyer diss 2009 1046..

Function.—The functional features of the astragalus have been discussed at length
for plesiadapids and plesiadapiforms (Szalay and Decker, 1974; Szalay, 1984). A few
observations and interpretations can, however be added here. Beard (1989) speculated on
the degree and nature of mobility between the astraglus and tibia but the occurrence of
these elements from a single individual allows a better constrained assessment of
mobility here. It can be demonstrated by manipulation of the astragalus and tibia that at
full dorsiflexion of the tibia on the astragalus, the head and neck of the astagalus project
anteriorly and form an angle of slightly less than 90° with the shaft of the tibia. When the
astragalus is plantarflexed by rotating it through the full 90° of arc formed on the lateral
tibial facet, there are two other conjunct motions that occur because of the slanting
surface of the lateral tibial facet, and laterally concave surface of the medial tibial facet
(Fig. 4.30): (1) the astragalus inverts (rotates laterally on its proximodistal axis) by a full
90°. (2) The distal end of the astragalus rotates medially (around a dorsoplantar axis) by
a full 90°. The angle between the astraglar neck and body changes by somewhat less
than 90°. Thus the act of “plantar-flexion” of the astragalus on the tibia, results in
relatively litte true plantar flexion (see discussion below for the broader significance of
these features for positional behaviors).
Comparison.—To compare P. cookei to other plesiadapids and other mammals,
18 linear and six angular measurements were taken on the astragalus (Table 4.20A-D;
Figs. 4.31, 32). The linear measurements were standardized against absolute size in the
usual way (see Materials and Methods) using a geometric mean of all 18 linear
measurements. The angular measurements are reported in degrees, but were analyzed in
radians.
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