Mandibular Remains Support Taxonomic Validity of Australopithecus sediba Darryl J. de Ruiter, 1,2 * Thomas J. DeWitt, 3 Keely B. Carlson, 1 Juliet K. Brophy, 1,2,4 Lauren Schroeder, 5 Rebecca R. Ackermann, 5 Steven E. Churchill, 2,6 Lee R. Berger 2 Since the announcement of the species Australopithecus sediba, questions have been raised over whether the Malapa fossils represent a valid taxon or whether inadequate allowance was made for intraspecific variation, in particular with reference to the temporally and geographically proximate species Au. africanus. The morphology of mandibular remains of Au. sediba, including newly recovered material discussed here, shows that it is not merely a late-surviving morph of Au. africanus. Rather—as is seen elsewhere in the cranium, dentition, and postcranial skeleton—these mandibular remains share similarities with other australopiths but can be differentiated from the hypodigm of Au. africanus in both size and shape as well as in their ontogenetic growth trajectory. M andibular remains are well represented in the hominin fossil record and have been included among the holotypes of numerous early hominin taxa, including Austra- lopithecus sediba from Malapa (1, 2). The sug- gestion (3–5) that insufficient consideration was given to intraspecific variation when Au. sediba was diagnosed as a discrete taxon can be further investigated by using recently recovered mandib- ular remains. Most notable among these are two conjoined fragments, UW88-128 and UW88-129, that can be refit to the previously published man- dibular specimen UW88-54 ( 1) to form a near com- plete right hemi-mandible with complete dentition of an adult individual, Malapa Hominim 2 (MH2) (Fig. 1). These new portions allow us to examine the premolar and M 1 regions of the mandibular corpus of an adult probable female of Au. sediba and provide a first look at the mandibular incisors and premolars of this taxon. In addition, we can now directly compare the mandibular canines of probable male and female individuals of Au. sediba. As is seen in the cranium and postcranial skel- eton of Au. sediba (1, 6–14), the reassembled mandible of MH2 exhibits a mosaic of morpho- logical characters that align it with specimens at- tributed to both Australopithecus and early Homo. For instance, the mandible of MH2 shares with Au. africanus such features as a moderately de- veloped lateral prominence, moderately developed lateral superior and marginal tori, weakly de- lineated anterior and posterior marginal tubercles, a well-developed ectocondyloid buttress, and a well-developed lateral eminence of the ramus. However, it differs from Au. africanus and resem- bles specimens of early Homo by evincing relief between the alveolar prominence and the sub- alveolar fossa; possessing a small, steeply in- clined postincisive planum; and revealing weakly developed endocoronoid and endocondyloid buttresses, which result in a triangular planum that is ill-defined along its inferior extent (15). How- ever, gross morphology in hominin mandibles tends to be variable, as is seen in a direct compar- ison of MH1 and MH2; thus, we turn to size and shape analyses of mandibular corpora to assess the taxonomic validity of Au. sediba. Linear dimensions of mandibular corpora have been shown to be taxonomically informative with- in fossil representatives of the genus Homo (2). In absolute measures, the mandibular corpus of MH2 is slightly taller and narrower than that of MH1 (Fig. 2 and table S1). There is considerable over- lap in mandibular corpus height between Au. afarensis, Au. africanus, Au. sediba, H. habilis, H. erectus, and even H. sapiens. The corpus of MH1 is narrower than Au. africanus at the level of the P 4 ; it overlaps with the lower end of the range of Au. africanus at the level of the M 1 , al- though only a single specimen of Au. africanus, the otherwise large-toothed Stw 498, is narrower. MH1 overlaps with H. habilis and H. erectus in width at the level of the P 4 and M 1 , although again plotting near the lower end of the range of both taxa. The newly reassembled mandible of MH2 is narrower still than MH1 at the level of the P 4 and M 1 , plotting below the range for Au. africanus, H. habilis, and H. erectus; it appears among the narrowest in the African hominin fossil record. Calculating corpus cross-sectional area by using the formula for an ellipse, MH1 and MH2 have similar values at the level of both the P 4 and M 1 . The cross-sectional areas of the mandibular corpora of Au. sediba are small at the level of the P 4 and M 1 , plotting at or below the lowest end of the range of values for Au. africanus and within the ranges of H. habilis and H. erectus. In linear dimensions, mandibular corpus width—and to a lesser extent cross-sectional area—thus appear useful for dis- criminating Au. sediba from Au. africanus and for aligning the former taxon with specimens at- tributed to early Homo. This is particularly the case with the adult specimen MH2. To better understand the nature of differen- tiation between early hominin taxa in both size and shape of mandibular corpora, we replicated the approach of Lague and colleagues (2) using specimens attributed to species of Australopithecus and early Homo in Africa and Georgia ( 15). Seven of the eight measures originally used (2) could be replicated in the reassembled mandible of MH2 and form the basis of our analyses. Randomiza- tion of “distinctness values” (RDV) (2, 16) reveals that within-group variance is less than between- group variance; thus, it appears that the mandibles of Australopithecus and Homo that we examined do indeed reflect a taxonomic signal. We therefore investigated the size and shape of the mandibular corpus of Au. sediba in relation to other hominins. We performed a multivariate analysis of variance (MANOVA) ( 15), the overall results of which can be visualized as a plot of between-group dif- ferences, scaled to within group variances (Fig. 3A). The major result illustrated in this plot is that H. sapiens are clearly distinct from early hominins, especially the robust australopiths, whereas Au. sediba clusters with the nonrobust australopiths and early Homo. This major axis indicates that human mandibles are small over- all, have relatively narrow corpora, and relative- ly mesiodistally (MD) elongated (though small overall) canine alveoli (table S2). Among the early hominins, Au. sediba most closely matches this pattern. A planned contrast within the MANOVA results explicitly compar- ing Au. sediba and Au. africanus revealed a sig- nificant difference between them (F 8,23 = 2.55, P = 0.037). The difference between these two taxa primarily involved the smaller overall size of the mandible, the relatively deeper mandib- ular corpus, and the relatively MD elongated pre- molar row in Au. sediba when compared with that of Au. africanus (table S3). In other words, the mandibles of Au. sediba are not only scaled dif- ferently than those of Au. africanus, they are also shaped differently. Removal of humans from the ordination resulted in a primary axis that princi- pally defined differences between the robust austra- lopiths and the remaining early hominins (Fig. 3B). However, the secondary axis—approximate- ly one-third the magnitude of the first—separated Au. sediba clearly from all other early hominins (planned contrast, P = 0.034). This effect was due to Au. sediba having a relatively MD elongated canine alveolus (table S3). Excluding size as a variable results in ordinations similar to those in Fig. 3, demonstrating that size is not unduly in- fluencing the analysis (fig. S1). REPORTS 1 Department of Anthropology, Texas A&M University, College Station, TX 77843, USA. 2 Evolutionary Studies Institute, Univer- sity of the Witwatersrand, Private Bag 3, Wits 2050, South Africa. 3 Department of Wildlife and Fisheries Sciences, Texas A&M Uni- versity, College Station, TX 77843, USA. 4 Department of Anthro- pology, Loyola University, Chicago, IL 60660, USA. 5 Department of Archaeology, University of Cape Town, Rondebosch 7701, South Africa. 6 Department of Evolutionary Anthropology, Box 90383, Duke University, Durham, NC 27708, USA. *Corresponding author. E-mail: deruiter@tamu.edu www.sciencemag.org SCIENCE VOL 340 12 APRIL 2013 1232997-1 on April 11, 2013 www.sciencemag.org Downloaded from