Special Issue: Australopithecus sediba --- The Skull of Australopithecus sediba

Abstract

Australopithecus sediba presents a mosaic of both australopith-like and Homo-like characters. The preliminary ac-count of the skull of Au. sediba provided in Berger et al. (2010) is augmented here to include a comprehensive de-scriptive and comparative analysis of both qualitative and quantitative characters of the craniodental remains of the holotype and paratype specimens, Malapa Hominins 1 and 2 (MH1 and MH2). Newly recovered mandibular material attributable to the holotype specimen (MH1), including two unworn premolars, is also presented here. Australopithecussediba shares several cranial characters with other australopiths, most prominent of which include its small brain size, marked glabellar block, robust zygomatics with steeply inclined zygomaticoalveolar crest, de-gree of prognathism, patent premaxillary suture, topography of the entrance to the nasal cavity and the insertion of vomer, and narrow palate. Combined with postcranial evidence, these confirm that the Malapa skeletons reflect an australopith adaptive grade. At the same time, Au.sediba shares numerous characters with specimens of early Homo, most prominent of which include its limited postorbital constriction, widely spaced temporal lines, medi-ally positioned mandibular fossa, moderately-defined supraorbital torus and supratoral sulcus with expanded supraorbital trigon, unflared zygomatics, anterolaterally oriented lateral orbital margins, anteriorly positioned anterior nasal tubercle, raised intermaxillary suture, small mandibular symphysis and corpus, well excavated subalveolar fossa, steeply inclined lingual alveolar plane, and weak superior transverse torus with absent in-ferior transverse torus. The potential for homoplasy in adaptively significant features in late australopiths and basal members of the Homo clade, combined with probable reticulate evolution, renders the identification of the ancestor of Homo difficult. In addition, the transition from australopith to Homo likely took place piecemeal over hundreds or perhaps thousands of generations, thus the combination of traits that characterize early Homo are rec-ognizable largely as a result of the imperfect nature of our available fossil record. Notwithstanding this, given the similarities shared between Au.sediba and Homo, on present evidence we favor the hypothesis that the Au. sediba lineage represents the most likely ancestor of the genus Homo, or a close sister group to that ancestor. This special issue is guest-edited by Scott A. Williams (Department of Anthropology, New York University) and Jeremy M. DeSilva (Department of Anthropology, Dartmouth College). This is article #2 of 9.