The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


More than the sum of its parts? Multivariate analysis of locomotor behavior in Ardipithecus ramidus

CAMPBELL ROLIAN1, HOLLY M. DUNSWORTH2, KIERAN MCNULTY3, PIERRE LEMELIN4 and WILLIAM L. JUNGERS5.

1Department of Comparative Biology and Experimental Medicine, University of Calgary, 2Department of Anthropology, University of Rhode Island, 3Department of Anthropology, University of Minnesota, 4Division of Anatomy, University of Alberta, 5Department of Anatomical Sciences, Stony Brook University

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Locomotor behavior in the Pliocene hominin Ardipithecus ramidus has been inferred to be primitive, comprising mainly above-branch palmigrade quadrupedalism, with infrequent vertical climbing and suspensory behaviors. By extrapolation, initial analyses have concluded that the last common ancestor of African apes and humans was also a generalized arboreal quadruped. If correct, this interpretation implies extensive homoplasy in the evolution of the African ape postcranial skeleton. However, this interpretation is based only on univariate or bivariate analyses of the postcranial skeleton in haplorhines, many using uncommon variables and/or size corrections. To determine whether postcranial morphology in Ardipithecus is indicative of generalized arboreal quadrupedalism, we used discriminant function analysis (DFA) to assess the degree to which appendicular skeletal morphology correlates with locomotor behavior in primates. Data include segment lengths from stylopod, zeugopod and autopod elements of fore- and hindlimb in >800 specimens representing all extant primate groups, each assigned to locomotor categories based on published data.

The DFA accurately separates extant groups according to locomotor behavior based on postcranial morphology, correctly classifying >97% of cases. Among fossil taxa, Proconsul is unambiguously classified as an arboreal quadruped. In contrast, Ardipithecus occupies a unique morphospace intermediate between arboreal quadrupeds and vertical climbers/knuckle-walkers, although it is marginally closer in multivariate space to the latter. Our analysis indicates that Ardipithecus is not a typical arboreal, palmigrade quadruped. Instead, the total morphological pattern in its appendicular skeleton is consistent with frequent forelimb-dominated locomotor behaviors.

Funding was provided in part by the Natural Sciences and Engineering Research Council (Canada), and the National Science Foundation (DDIG BCS 0647624) to CR.

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