What's your angle? Size correction and bar-glenoid orientation in "Lucy" (A.L. 288-1)

There has been much debate as to the locomotor repertoire of Lucy (A.L. 288-1) and other specimens of Australopithecus afarensis, ranging from fully committed bipeds to species that spent a significant time in the trees as well as on the ground. We examined the bar-glenoid angle, a character purported to indicate arboreal propensities, and its implications for this specific debate and the more general challenge of extracting behavioral information from fossils. We examined the bar-glenoid angle in ontogenetic samples of Pan paniscus, Pan troglodytes, Gorilla gorilla gorilla, Gorilla gorilla beringei, Pongo pygmaeus, Homo sapiens, and A.L. 288-1 ("Lucy"). We found that there is no allometry in the bar-glenoid angle for the great apes, but a weak correlation for humans. Moreover, the data scatters for the African apes and humans converge at the smaller size ranges, and Lucy's value for bar-glenoid angle falls precisely in this area of overlap. Therefore, we conclude that the bar-glenoid angle is not tightly correlated with function and, as such, cannot be used as a morphological signal of arboreal behavior, especially in the smaller size ranges, at which arboreal and nonarboreal species overlap. Our work does not resolve issues concerning Lucy's precise locomotor repertoire but adds new information to consider. The total morphological pattern, plus an appreciation of the underlying variance in morphological and behavioral characters in extant species, is key for making functional inferences from the morphology of fossils.