The functional anatomy and evolution of the mammalian humero‐ulnar articulation

TLDR

Early pelycosaurs had a horizontally projecting humerus that rotated about its axis, producing torque on the flexed elbow and a primarily stabilizing joint, while cynodonts introduced an ulnar condyle and early mammals retained a spiral configuration. The study aims to delineate the major structural and functional stages of the mammalian humero‑ulnar joint using paleontological and cineradiographic evidence. A spiral joint permits ulna extension in the sagittal plane as the humerus rotates, adducts, and elevates, and the primitive therian trochlea evolved through enlargement of the intercondylar groove and retention of part of the ulnar condyle mechanism. Cineradiography reveals how different mammalian humero‑ulnar joint morphologies correlate with limb posture and excursion patterns.

Abstract

Abstract Major stages in the structural and functional evolution of the mammalian humero‐ulnar joint are described on the basis of paleontological and cineradiograhic evidence. In pelycosaurs (the earliest known fossil reptiles with mammalian affinities), the humerus projected laterad and more or less horizontally; locomotor movements were principally rotation about its proximodistal axis. Because the forearm moved in a plane perpendicular to this axis, the flexed elbow was subjected to substantial torque. The humero‐ulnar joint consisted of two pairs of facets that engaged upon humeral rotation and was principally a stabilizing rather than a flexion‐extension mechanism. Cynodonts (advanced mammal‐like reptiles ancestral to mammals) possessed an ulnar condyle rather than a trochlea. A condylar humero‐ulnar articulation, usually with a spiral configuration, was retained by early mammals and persists in slightly modified form among modern prototherians. The spiral joint allows the ulna to extend in a sagittal plane as the humerus rotates, adducts, and elevates. The primitive therian trochlea evolved by enlargement of the intercondylar groove separating the ulnar and radial condyles and by retention of part of the ulnar condyle mechanism. Cineradiography demonstrates the relationship of diverse types of mammalian humero‐ulnar joints to limb posture and excursion characteristics.