Manubriosternal Morphology of Anthropoid Primates.

Objectives: The purpose of this paper is to examine the proportions of the manubrium and sternebrae across anthropoid primates to explore variation hypothesized to be related to thoracic shape and locomotor specialization, and to determine whether the sternoclavicular joint orientation in hominoids reflects hypothesized differences in shoulder joint positioning relative to the thorax.

Materials And Methods: Metric data and sternoclavicular joint orientation data were collected from calibrated photographs of manubria and sternebrae from a large sample (n = 244) of extant anthropoid primates, as well as a small sample of fossil taxa. Manubriosternal and rib cage metric data were also collected from CT scans of an additional 52 extant anthropoid torsos. The data were analyzed using descriptive statistics, regression analyses, and linear correlations.

Results: Manubriosternal morphology varies among anthropoids and appears to track thoracic shape and positional repertoire. Hominoids tend to have broader manubria and sterna than monkeys. Ekembo and Equatorius appear to most closely resemble colobines and Alouatta in sternebral shape, suggesting a thorax most similar to these taxa. Neandertals and early anatomically modern humans are most similar to humans, providing no evidence for a relatively broad rib cage. Sternoclavicular joint orientation also differs among hominoids, with all hominoids having more cranially inclined joint surfaces than humans. The human-like value observed for Australopithecus sediba supports the hypothesis that the clavicle in this species was not inclined laterally in resting posture.

Conclusions: Given the correlation between manubriosternal breadths and thoracic form across anthropoids, this study suggests that manubriosterna may provide useful information for interpreting skeletal form and positional repertoires in fossil anthropoids.