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

Abstract

Major stages in the structural and functional evolution of the mammalian humero-ulnar joint are described on the basis of paleontological and cineradiographic 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.

Ecomorphological determinations in the absence of living analogues: the predatory behavior of the marsupial lion (Thylacoleo carnifex) as revealed by elbow joint morphology

“…The articular surface of the Thylacoleo elbow is also characterized by a large trochlear crest, which increases stabilization of the forearm (Jenkins 1973;Figueirido and Janis 2011;Figueirido et al 2015). Our results thus indicate that Thylacoleo was able to stabilize the forearm to a greater degree than the koala but less so than other more terrestrial taxa (excluding the wombat, see discussion below).…”

Remnants of an ancient forest provide ecological context for Early Miocene fossil apes

“…A relatively long, strong hallux with a well-developed flexor hallucis longus muscle indicates powerful grasping capabilities that would have facilitated arboreal locomotion 26,27 . Likewise, a rounded humeral head and lateral trochlear ridge on the distal humerus suggest mobility at the shoulder and elbow, and joint stability across the range of pronation and supination [27][28][29] . Without the aid of a tail for balance 30 , this combination of forelimb anatomy and powerful grasping would have accommodated Proconsul's habitual use of arboreal supports within a closed forest 31 .…”

Evolution of Large Carnivores During the Mid-Cenozoic of North America: The Temnocyonine Radiation (Mammalia, Amphicyonidae)

“…This type of humero-ulnar joint registration, which occurs in temnocyonines, was previously noted by Jenkins (1973) who observed that reduction of the medial epicondyle and its associated flexor musculature decreases torque at the elbow. These features are a prelude to a more erect forelimb stance, evident in both the Logan Butte T. altigenis and T. ferox.…”