Implications of A.L. 444-2 for the Taxonomic and Phylogenetic Status of Australopithecus afarensis

Abstract

A.L. 444-2 is the first specimen to preserve the cranium and mandible of a single adult individual of A. afarensis. Pairing this specimen with A.L. 417-1, which includes a mandible and maxilla, enables us to compare comprehensively the craniofacial morphology of male and female individuals of the species for the first time. The occluded mandibles and maxillae of A.L. 444-2 and A.L. 417-1 reveal a distinctive hominoid snout contour, combining a strongly inclined, convexly sloping nasoalveolar clivus with a relatively upright mandibular symphysis, a straight to slightly rounded anterior symphyseal outline, and an anteriorly placed gnathion. Both A. afarensis specimens feature a very deep mandibular corpus, whose height occupies close to 70% of the orbitoalveolar height of the face. In the African great apes, this value ranges from 36% to 54%, and in modern humans, it is 66%. The high value in humans is due to a short orbitoalveolar region rather than to a deep mandible. A. afarensis appears to share a relatively deep corpus with A. robustus (the only robust species in which the feature can be determined for a single individual) but not with A. africanus. Relative to the calvarial length, the A.L. 444-2 braincase height is apelike, falling between the tall modern human braincase and the low braincase of A. boisei and A. aethiopicus. In A. africanus (Sts. 5) and H. habilis (KNMER 1813) the relative braincase height is like that of A.L. 444-2 and the great apes. According to Le Gros Clark’s (1950) index expressing the height of the calvaria above the roof of the orbit as a percentage of total calvarial height, Sts. 5 and KNM-ER 1813 have tall, “humanlike” braincases, whereas A.L. 444-2, A. boisei, A. aethiopicus, and the African great apes group together with low braincases. In contrast to the rounded, nearly circular midsagittal outline of the chimpanzee calvaria, the posterior parietal/ occipital arc in A.L. 444-2 is steep and deviates anteriorly from the circle. This is also true of the A. boisei calvaria. As expected from the calvarial height comparison, the slope of the A.L. 444-2 frontal squama is smaller than that of A. africanus and H. habilis.