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Boyer diss 2009 1046..

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Astragalus<br />

Description.—UM 87990 preserves a complete right and fragmentary left<br />

astragalus (Fig. 4.29A; Table 4.20A-C). The astragalus of plesiadapids is relatively well<br />

known (Szalay and Decker, 1974). Most descriptive features of these published<br />

specimens also apply to the astragalus of P. cookei.<br />

The most distinctive feature of the right astragalus is its mediolateral asymmetry.<br />

On the astragalar body, the dorsal facet for the tibia (the lateral tibial facet) slopes steeply<br />

medially. This occurs because the fibular facet (which meets the lateral margin of the<br />

lateral tibial facet) is much deeper dorsoventrally (4.9 mm) than the dorsoventrally<br />

oriented facet for the medial malleolus (medial tibial facet: 3.4 mm). As a further result<br />

of this asymmetry, the angle between the facets of the lateral side of the lateral tibial facet<br />

is slightly acute (84°), while the medial one is obtuse (115°).<br />

The lateral and medial margins of the lateral tibial facet are farthest apart distally<br />

and converge proximally, so that the facet is triangular. It forms an arc of ~90° and is<br />

shallowly grooved distally. The medial edge of the lateral tibial facet extends onto the<br />

neck of the astragalus. The medial and lateral margins of the lateral tibial facet also<br />

curve in the transverse plane so that they are laterally convex. The lateral convexity of the<br />

facet margins results in the medial tibial facet being concave, matching the convex<br />

surface of the medial malleolus of the tibia (see above).<br />

The medial tibial facet is triangular, with one of the apices of the triangle meeting<br />

the distal edge of the lateral tibial facet. Its dorsal and ventral margins thus diverge<br />

proximally and the whole surface curves ventrally in the sagittal plane. Thus the facet<br />

340

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