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Original article (peer-reviewed)

Journal Proc Europ Soc Hum Evol
Volume (Issue) 8
Page(s) 116
Title of proceedings Proc Europ Soc Hum Evol

Open Access

Type of Open Access Publisher (Gold Open Access)


In 1987/1988 Alun R. Hughes and his team discovered the hominin skeleton StW 431 from Sterkfontein Member 4. At the time of its first description in 2003, Toussaint and colleagues [1] assigned this specimen to the genus Australopithecus ”...although there [was] little direct morphological evidence as to the species...” (p. 222). Fifteen years later, as part of their announcement of the almost complete StW 573 Little Foot skeleton from Sterkfontein Member 2 (3.67 ± 0.16 Ma) [2], Clarke and colleagues placed StW 431 in the A. prometheus hypodigm [3]. Importantly, they used aspects of StW 431 anatomy, e.g. the pelvis, which is badly crushed in StW 573, to make inferences about the positional behaviour, locomotion and palaeobiology of A. prometheus [3]. Here we comprehensively review and re-analyse the StW 431 morphology. Besides the pelvis, StW 431 preserves the last ten vertebrae, a rib head, and fragments of the right scapula, clavicle and elbow. Skeletal regions that can be compared to those of StW 573 Little Foot are therefore rather limited. However, a new reconstruction of the StW 431 pelvis suggests a different morphology and functional adaptation from that of Sts 14, a partial skeleton classically attributed to A. africanus. For example, the position of the sacrum within the pelvis is significantly higher positioned and the iliac blades are wider. The strong muscle markings on the iliac crests have previously been interpreted as showing a human-like development of the abdominal muscles and the m. latissimus dorsi [4]. Alternatively, the m. latissimus dorsi could also have had a much broader, chimpanzee-like origin extending to the anterior superior iliac spine. The elbow morphology, on the other hand, is comparable to that of other australopiths, both from East and South Africa. Hence, when viewed in its entirety it is clear that StW 431 is distinct from A. africanus sensu stricto. The fact that morphological differences between StW 431 and other postcranial fossils from Sterkfontein are generally more marked in the pelvic girdle than they are in the rest of the skeleton cautions against inferences about locomotor behaviour based on isolated skeletal elements. Evidently, Plio-Pleistocene hominins from Sterkfontein had a unique mode of locomotion and positional behaviour, but they also retained the ability to engage in arboreal activities. On the basis of our analyses we provisionally concur with Crompton et al. [3] that StW 431 probably belongs to A. prometheus. Confirming this possibility is not straightforward however, as A. prometheus is mainly defined on the basis of craniodental traits [5]. Regardless, the distinctiveness of StW 431 from A. africanus throws new light on the behaviour and palaeobiology of Plio- Pleistocene hominins from South Africa, including the palaeobiogegraphy of early hominins. Clarke has consistently argued that the morphology of A. prometheus and, by implication, StW 431 resembles that of the much younger Paranthropus robustus (ca. 2.1 - 0.62 Ma) and has implied an ancestor-descendent relationship between these Plio- Pleistocene South African hominins. Such a suggestion is not supported by our analyses of the postcranial skeleton. Rather, simi- larities in morphology could be the result of homoplasy, i.e. plastic responses of the skeleton to similar ecological conditions. Such homoplasies may have been exacerbated through niche partitioning of sympatric hominins, e.g. A. prometheus and A. africanus in case of Plio-Pleistocene hominins, and P. robustus and early Homo in case of Pleistocene hominins. Theoretical and palaeoeco- logical considerations similarly argue against Clarke s proposition. The prevailing palaeoecological and palaeoclimatic conditions make it unlikely that A. prometheus and P. robustus are part of the same evolving lineage (e.g. anagenesis).