Limb movements in a monotreme (Tachyglossus aculeatus): a cineradiographic analysis

In a walking echidna the principal movement of the humerus is long-axis rotation. The humerus remains approximately perpendicular to the sagittal plane, but the femur is directed anterolaterally at angles from 35 degrees to 50 degrees . In addition to long-axis rotation, the femur elevates and depresses in an arc which usually varies between 40 degrees and 90 degrees . The femoral angle, the femoral elevation and depression, and the plantar contact of the manus beneath the glenoid are features found also in generalized therians.