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Boyer diss 2009 1046..

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axis of the calcaneum, but deviates distally by ~15°. A small fibular facet is located<br />

lateral to the ectal facet and faces laterally.<br />

The middle and anterior calcaneoastragalar facets (proximal and distal<br />

sustentacular) are located mediodistal to the ectal facet and separated from it by a nonarticular<br />

sulcus. The proximal sustentacular facet is shallowly cylindrically concave with<br />

an axis paralleling that of the ectal facet. The surfaces of the ectal facet and proximal<br />

sustentacular facet are not parallel but rather deviate from parallel by ~30° (i.e., they<br />

form an angle of ~150° with one another). Distally, the distal sustentacular facet is fairly<br />

flat and is continuous with the proximal one. Its entire surface is rotated medially with<br />

respect to the proximal sustentacular facet, so that it is comes close to paralleling the<br />

surface of the ectal facet. Given the change in sustentacular facet surface angle from<br />

proximal to distal, this facet can be described as helical in form, like its convex<br />

counterpart on the astragalus.<br />

The proximal end of the proximal sustentacular facet is confluent with a facet that<br />

covers the proximal side of the sustentaculum. This flat, square facet is the<br />

“sustentaculum tali” that would normally contact the proximal extension of the<br />

sustentacular facet of the astragalus in other plesiadapids, although it cannot do so in UM<br />

87990 (see above).<br />

The calcaneocuboid facet is saddle-shaped in UM 87990, being strongly concave<br />

mediolaterally, and slightly convex dorsoventrally due to a subtle pit in the facet on its<br />

plantar aspect, just dorsal to the anterior plantar tuberosity (see Beard, 1989). The<br />

calcaneocuboid is oriented oblique to the long axis of the calcaneum, and faces 15° or<br />

20° medially. The main notable features of the plantar surface are (1) the anterior plantar<br />

346

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