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The Vertebral Column of Australopithecus Sediba

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The Vertebral Column of Australopithecus Sediba RESEARCH ARTICLES Gauteng, South Africa (24, 25) preserve 23 pre- sacral vertebrae and a partial, five-element sacrum (Fig. 1). Here, we describe vertebrae assigned to The Vertebral Column of the juvenile male holotype [Malapa Hominin 1 (MH1)] and the adult female paratype (MH2), Australopithecus sediba in addition to the previously published sacrum of MH2 (26). Scott A. Williams,1,2* Kelly R. Ostrofsky,3 Nakita Frater,4 Steven E. Churchill,3,5 The MH1 partial vertebral column consists of Peter Schmid,4,5 Lee R. Berger5 four cervical, six thoracic, and two lumbar verte- brae; that of MH2 is composed of two cervical, Two partial vertebral columns of Australopithecus sediba grant insight into aspects of early hominin seven thoracic, and two lumbar vertebrae, as well spinal mobility, lumbar curvature, vertebral formula, and transitional vertebra position. Au. sediba as a sacrum. Specimen numbers and identifica- likely possessed five non–rib-bearing lumbar vertebrae and five sacral elements, the same configuration tions are provided in table S1. Vertebrae belong- thatoccursmodallyinmodernhumans.Thisfinding contrasts with other interpretations of early ing to MH1 are readily distinguished from those hominin regional vertebral numbers. Importantly, the transitional vertebra is distinct from and above of MH2 by their lack of apophyseal rings. As the last rib-bearing vertebra in Au. sediba, resulting in a functionally longer lower back. This with modern human juveniles and the Nariokotome configuration, along with a strongly wedged last lumbar vertebra and other indicators of lordotic juvenile skeleton (KNM-WT 15000) (1), MH1 posture, would have contributed to a highly flexible spine that is derived compared with earlier possesses short vertebral centra relative to superior- members of the genus Australopithecus and similar to that of the Nariokotome Homo erectus skeleton. inferior interfacet distance, consistent with its juvenile status (27). In contrast, MH2’s centra he vertebral column plays a central role to Au. africanus (Sts 14 and Stw 431) and Homo demonstrate ratios similar to those of adult mod- in posture, locomotion, and overall trunk erectus (KNM-WT 15000). Thoracic and lum- ern humans. Tstability and mobility in vertebrate ani- bar vertebrae have been defined in two ways in mals. The hominin vertebral column is distinct the recent literature: (i) based on the presence Cervical Vertebrae in its role in force transmission and weight- and absence of ribs (i.e., the last thoracic verte- A small cervical body fragment of unknown se- bearing in an upright torso balanced over two bra is identified as the last rib-bearing vertebra) rial position, two fragmentary mid-cervical verte- legs. Owing to these biomechanical demands, (8, 15, 16), and (ii) based on the orientation of brae, and a nearly intact last cervical (C7) vertebra the hominin spine has several specialized traits the articular facets (i.e., the last thoracic vertebra (fig. S1) are associated with MH1. Two mid-level not seen in other mammals, including sigmoid is identified as the transitional vertebra, one that cervical vertebrae are attributed to MH2: a nearly curvature; a pyramidal configuration of articu- bears thoracic-like, coronally oriented superior ar- complete C3 or C4 (fig. S2) and a partially com- lar facets with descent through the lower lum- ticular facets and lumbar-like, curved and laterally plete C5 or C6. These vertebrae do not articulate bar column; and a wide, curved sacrum (1–6). directed inferior articular facets) (6, 17, 18). In and, thus, likely represent either C3 and C5 or C4 Whereas these adaptations are present in modern modern humans and other extant hominoids, the and C6. humans and currently known extinct hominins, last rib-bearing vertebra and the transitional ver- other aspects of the modern human vertebral tebra generally occur at the same vertebral level Rib-Bearing Vertebrae column—for instance, large relative lumbosacral (i.e., are the same vertebral element) and, there- Six rib-bearing vertebrae are attributed to MH1: body size—are not present in early hominins such fore, yield identical numbers of thoracic and lum- two upper thoracic, one mid-thoracic, and three as Australopithecus afarensis and Au. africanus bar vertebrae under either definition [(15, 18, 19), lower thoracic vertebrae. An upper thoracic verte- (2, 3, 7), suggesting that the vertebral column but see (20)]. bra (fig. S3) demonstrates superior articular facet evolved in a mosaic fashion. Therefore, an under- In contrast, recent evidence indicates that shape and orientation and relationships among standing of which adaptations characterized early fossil hominins may not follow this pattern the body, transverse process, and superior articu- various fossil species and when the full com- (9, 15). Newly identified vertebral and rib frag- lar facets diagnostic of a second thoracic vertebra plement of modern humanlike features evolved is ments associated with KNM-WT 15000 show (T2). The middle thoracic vertebra, which likely integral to reconstructing postural and locomotor that the transitional vertebra is cranially adjacent represents T7, is complete but pathological: The evolution in the hominin lineage. to the last rib-bearing vertebra in this specimen dorsal surface of the right lamina exhibits a rounded Recently, two key components of the hominin (9). This configuration yields contrasting num- triangular lytic lesion extending ventrally into vertebral column—its numerical composition and bers of lumbar vertebrae when different opera- the lamina (fig. S4). Poor preservation precludes degree of lumbar lordosis—have been at the tional definitions are employed: five nonribbed certainty of articulation between the three lower forefront of discussion (4, 6, 8–17). However, lumbar vertebrae and six vertebrae with sagittally thoracic vertebrae, although spinous and trans- adequately preserved vertebral columns that lend oriented upper articular facets (posttransitional verse process morphologies and orientations and themselves to reconstructions of regional verte- vertebrae). Sts 14 also seems to demonstrate this costal facet configurations suggest the associa- bral numbers are rare in the hominin fossil record, pattern (8, 15), although the last rib-bearing ver- tion and possible affiliation of two of these verte- particularly before 50 thousand years ago, and tebra in this specimen is somewhat anomalous brae as T9 and T10. are best represented by partial skeletons assigned in that it bears a lumbar-like structure on one side, There are seven rib-bearing vertebrae asso- giving it an intermediate (half-thoracic-, half-lumbar) ciated with MH2 (table S1), including a block of appearance (8, 10, 15, 21, 22). Dissociated verte- four consecutive upper–to–mid-thoracic vertebrae 1Center for the Study of Human Origins, Department of An- thropology, New York University, 25 Waverly Place, New York, bral bodies and neural arches at the thoraco-lumbar and a block of three articulating lower rib-bearing NY 10003, USA. 2New York Consortium in Evolutionary Pri- border in Stw 431 have led to contrasting recon- vertebrae. The upper block contains variably com- matology, New York, NY 10024, USA. 3Department of Evolu- structions of this specimen (8, 23), precluding a plete fragments of what likely represent either tionary Anthropology, Box 90383, Duke University, Durham, definitive assessment of the relationship between a T3-T6 or T4-T7 series. The lower block is of NC 27708, USA. 4Anthropological Institute and Museum, Uni- versity of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, last rib-bearing and transitional vertebrae. Clearly, particular interest because it preserves in artic- Switzerland. 5Evolutionary Studies Institute, University of the additional specimens that preserve this region ulation the penultimate and ultimate rib-bearing Witwatersrand, Private Bag 3, Wits 2050, South Africa. of the vertebral column are needed. The 1.977 vertebrae (Fig. 2). The antepenultimate rib-bearing *Corresponding author. E-mail: [email protected] Ma partial skeletons of Au. sediba from Malapa, vertebra is also preserved but is displaced slightly EMBARGOED UNTIL 2PM U.S. EASTERN TIME ON THE THURSDAY BEFORE THIS DATE: www.sciencemag.org SCIENCE VOL 340 12 APRIL 2013 1232996-1 Australopithecus sediba from articulation with the penultimate vertebra. the articular facet joints onto the laminae and nial body of S1 through the caudal body of S5; The penultimate vertebra bears a full costal facet habitual lordotic posture (fig. S7). Both the pe- five distinct sacral bodies are recognizable. The at the body-pedicle border and lacks an inferior nultimate and ultimate lumbar vertebra are wedged sacrum is incomplete medio-laterally, with the facet or demi-facet. The zygapophyses bear flat, dorsally (–1.5° and –10.9°, respectively) (Fig. 4, alae broken at its lateral aspects on both sides. dorsally (and slightly laterally) directed superior fig. S8, and table S2). The left ala and sacral foramina are missing en- articular facets and curved, laterally directed in- tirely, whereas the right side retains four large and ferior articular facets; that is, the penultimate rib- Sacrum complete sacral foramina and part of the auricu- bearing vertebra is the transitional vertebra. The The MH2 sacrum is cranio-caudally complete lar surface. The preserved right zygapophysis is ultimate thoracic vertebra bears a full costal facet along the anterior midline (except for the de- curved and postero-medially oriented at
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