| Summary: | The Paleocene and early to middle Eocene Family Phenacodontidae includes three species--Tetraclaenodon puercensis, Phenacodus vortmani, and P. primaevus--with available postcranial material for an investigation of postural and locomotor capabilities. The details of bone and joint morphology are compared within the phenacodontids and with several extant analogues whose postural and locomotor capabilities and morphological correlates are known better. Several aspects of the postcranial morphology of the phenacodontids, especially P. vortmani and P. primaevus, suggest cursorial capabilities. The anterior thoracic
vertebral column has limited flexibility dorsoventrally and mediolaterally, but the articular surfaces in the lumbar section, especially in the larger P. primaevus, tend to allow dorsoventral movement while restricting mediolateral movement, thus potentially adding to total stride length. The digitigrade limb posture, again especially developed in P. vortmani and P. primaevus, adds to limb length and stride length. Several features of the joints in the forelimb and hindlimb restrict motion to the parasagittal plane or enhance thrust against the ground in the parasagittal pIane, thus contributing to forward motion. The curvatures of the glenoid fossa and the humeral head enhance flexion and extension and restrict abduction. The mediolateral width and shapes of the articular surfaces in the elbow joint prevent supination. The flexor hinge in the wrist allows a powerful thrust against the ground during push-off. The keels in the distal metapodials maintain that thrust in the parasagittal plane. In the hindlimb, the high greater trochanter and the third trochanter increase the mechanical advantage of some extensors of the femur. The deep patellar groove suggests strong extension at the knee. Similarly, the deep grooves and distinct condyles of the distal tibia and dorsal surface of the astragalus,
along with the relatively long calcaneal tuberosity, suggest powerful flexion at the ankle joint and confine motion at the upper ankle joint to flexion and extension. The phenacodontids and especially Phenacodus vortmani and P. primaevus were good runners, particularly compared with their Paleocene and Eocene contemporaries. Within the genus Phenacodus, the more slender P. vortmani, with its more elongate distal elements of the hindlimb, probably was the more efficient runner.
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