POSTCRANIAL SKELETON OF THE EARLY EOCENE MESONYCHID PACHYAENA (MAMMALIA, MESONYCHIA)

Analysis of postcranial skeletons of three species of the wolf- to bea r-sized mesonychid Pachyaena from the early Eocene Willwood Formation, Bighorn Basin, Wyoming, indicates that it was a derived cursor combin ing specializations typical of modern Artiodactyla, Perissodactyla, an d Carnivora. Skeletal anatomy supports the hypothesis, based on the de ntition, that mesonychids evolved from Arctocyonidae. Numerous charact ers in the limb skeleton of Pachyaena are derived relative to Arctocyo n, however, and resulted in restricted rotatory and mediolateral mobil ity at most joints, promoting predominantly sagittal movement. These c haracters, present in ungulates and cursorial carnivores, include a br oad, caudally displaced humeral head; high greater tuberosity; proxima l deltoid tuberosity; narrow, proximodistally elongate distal humeral articulation with moderately constricted capitulum; reduced epicondyle s; deep olecranon fossa; anteriorly shifted radioulnar joint; posterio rly concave ulnar shaft; high greater trochanter; deep distal femur; e longate distal tibiofibular syndesmosis; moderately grooved astragalar head; and mediolaterally compressed manus and pes with vestigial poll ex and hallux. Some of these characters in Pachyaena are more carnivor e-like in grade of cursorial modification. Distinctly ungulate-like sp ecializations of Pachyaena include a wide and unevenly excavated radia l head, a double-faceted radiocarpal articulation, an astragalar head with distinct cuboid and navicular facets, and hoofs. Pachyaena had re latively 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 m esonychids probably arose independently from those in modern ungulates and cursorial carnivores.