Mesodma primaevus

One calcaneum from the Judith River Formation, AMNH 77176, is similar to UMVP 1432, a right calcaneum of M. thompsoni, described by Deischl (MS, pl. 9, fig. 36). The shaft of AMNH 77176 is almost one-half the length of the calcaneum and is expanded at its proximal terminus (fig. IOJ). The posterior astragalar facet is a large, knoblike, and highly convex articulating surface. It is separated from the smaller, oval sustentaculum by a deep groove, the interosseous fossa. The sustentaculum is narrowly bridged to the distomedial slightly concave cuboid facet, ventral to which is the prominent hook-shaped trochlear process. Length of the calcaneum is 4.9 mm.