Estimating humeral torsion on incomplete fossil anthropoid humeri

The high degree of humeral torsion (i.e., the head facing more medially) found in living hominoids is generally viewed as a component of increased mobility at the glenohumeral joint and the development of suspensory postures and locomotion. Humeral torsion, therefore, is an important character in the functional interpretation of primate fossil humeri. However, because the position of the humeral head is judged relative to the distal end of the humerus, it has been impossible to determine the degree of torsion on anything less than a complete humerus. The present study presents two methodologies for estimating the degree of torsion on incomplete fossil primate humeri. In the first method, humeral torsion measured relative to three alternative landmarks for a missing distal end of the humerus were combined with a variety of functional measurements on the proximal humerus in a multiple regression analysis. The derived equations were used to predict humeral torsion for a selection of fossil specimens missing their distal ends including humeri from Australopithecus afarensis and Australopithecus africanus. These estimates indicated that early hominids displayed less humeral torsion than either modern humans or African apes, which in turn suggests that the similar high degree of torsion found in the latter two forms evolved independently. The second method used the bicipital sulcus to estimate head position for humeri missing their most proximal portions. A strong correlation was found between true humeral torsion and predicted torsion using the estimated head position. With this method torsion was estimated for humeri of Proconsul heseloni, Dendropithecus macinnesi, and Dryopithecus fontani, and for the early hominid humerus, KNM-ER 739. All of the Miocene hominoids analyzed were found to possess a low degree of humeral torsion compared with living apes and humans. Although the estimate for KNM-ER 739 is problematic, the degree of torsion was probably similar to that of the other early hominids examined in this study.