Analysis of postcranial skeletons of three species of the wolf- to bear-sized mesonychid Pachyaena from the early Eocene Willwood Formation, Bighorn Basin, Wyoming, indicates that it was a derived cursor combining specializations typical of modern Artiodactyla, Perissodactyla, and Carnivora. Skeletal anatomy supports the hypothesis, based on the dentition, that mesonychids evolved from Arctocyonidae. Numerous characters in the limb skeleton of Pachyaena are derived relative to Arctocyon, however, and resulted in restricted rotatory and mediolateral mobility at most joints, promoting predominantly sagittal movement. These characters, present in ungulates and cursorial carnivores Include a broad, caudally displaced humeral head; high greater tuberosity; proximal deltoid tuberosity; narrow, proximodistally elongate distal humeral articulation with moderately constricted capitulum; reduced epicondyles; deep olecranon fossa; anteriorly shifted radioulnar joint posteriorly concave ulnar shaft; high greater trochanter; deep distal femur; elongate distal tibiofibular syndesmosis; moderately grooved astragalar head; and mediolaterally compressed manus and pes with vestigial pollex and hallux. Some of these characters in Pachyaena are more carnivore-like in grade of cursorial modification. Distinctly ungulate-like specializations of Pachyaena include a wide and unevenly excavated radial head, a double-faceted radiocarpal articulation, an astragalar head with distinct cuboid and navicular facets, and hoofs. Pachyaena had relatively robust limbs and somewhat elongate distal limb segments like modern tapirs, but was functionally paraxonic like cursorial Carnivora and Artiodactyla. These features indicate that Pachyaena was a cursor adapted more for endurance than speed. Cursorial specializations in mesonychids probably arose independently from those in modern ungulates and cursorial carnivores.
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