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Boyer diss 2009 1046..

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Figure 4.33. Surface reconstruction of astragalus (Ast), calcaneum (Cc) and foot of<br />

Plesiadapis cookei (UM 87990) based on HRxCT data showing the detailed movements<br />

and degree of inversion accomplished at the astragalocalcaneal joint [HRxCT resolution<br />

= 0.018 mm (cubic voxels)]. See Szalay and Decker (1974) for detailed discussions of<br />

comparisons of mobility among various primitive eutherians. A, distal view of everted<br />

(top) and inverted (bottom) astragalocalcaneal joint. Calcaneum orientation held<br />

constant. B, dorsal view of everted (top) and inverted (bottom) astragalocalcaneal joint.<br />

C, same dorsal views as in “B” with astragalus shown transparent to allow visualization<br />

of astragalocalcaneal articular surface. D, same dorsal views as in “C” with astragalus<br />

calcaneal facets highlighted. E, same dorsal views as in “D” with astragalus calcaneal<br />

facets shown in relation to astragalus facets of calcaneum. Inversion is accomplished by<br />

(1) plantar-flexion and medial translation of the calcaneum at the posterior<br />

astragalocalcaneal joint (comprised of ectal facets) and (2) medial translation and lateral<br />

rotation of the calcaneum at the anterior astragalocalcaneal joint (comprised of<br />

sustentacular facets). F, dorsal view of calcaneum with astragalus removed and astragalar<br />

facets highlighted. G, dorsal view of astragalus showing change in foot position with<br />

inversion (left to right) at astragalocalcaneal joints. H, lateral view of astragalus showing<br />

same change in articulation at astragalocalcaneal facet. Note that inversion also leads to<br />

substantial conjunct plantar-flexion of the foot.<br />

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