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Australopithecus variability at Sterkfontein: evidence from the pelvic remains

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author FornaiC, KrennV, WebbN, HaeuslerM,
Project Birth and human evolution - implications from computer-assisted reconstructions
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Original article (peer-reviewed)

Journal Proc Europ Soc Hum Evol
Volume (Issue) 8
Page(s) 63
Title of proceedings Proc Europ Soc Hum Evol

Open Access

URL https://www.eshe.eu/static/eshe/files/PESHE/PESHE_2019_OnlinePESHE.pdf
Type of Open Access Publisher (Gold Open Access)

Abstract

The taxonomy of Southern African Australopithecus has been a matter of intense debate. Traditionally, all hominin fossils from Sterkfontein Member 4 (ca. 2.6 – 2.1 Ma) have been attributed to A. africanus despite a high morphological variability in both craniodental and postcranial elements [1]. The interpretation of the Sterkfontein fossil material is complicated by its fragmentary and incomplete state of preservation and by the uncertainty surrounding its chronology. The adult, supposed male StW 431 and the subadult female Sts 14 – also retrieved from Sterkfontein Member 4, are represented by partial skeletons and preserve a fairly complete pelvis and vertebral column. For these pelvic fragments differing reconstructions have been generated via various methods [2]. Based on their general shape and on a recent geometric morphometric analysis of the first two sacral elements [3], we previously suggested that the morphological differences between Sts 14 and StW 431 might exceed the expected range of variation for a male and female of the same species with reference to the human variation. Here, we perform new virtual reconstructions of the StW 431 and Sts 14 pelves to elucidate key aspects of their variation. The restoration of the StW 431 hipbone was performed by virtually reassembling the preserved right and mirrored left fragments. The alignment of the elements was facilitated by several contact points and by the absence of patent asymmetry. Except for the ischiopubic ramus and the pubic body, which are not preserved, the missing parts of the hipbone were filled by alternatively warping the Sts 14 and A.L. 288-1 hipbones, obtaining the same outcome. The pelvic girdle could be reassembled confidently thanks to the presence of well-preserved auricular iliac and sacral surfaces. Sts 14 was restored by virtually removing the areas reconstructed by Robinson with plaster of Paris and by reassembling the fractured ischiopubic region of the right hipbone. The missing areas of the left hipbone were implemented using mirror-images of the right side. Finally, the pelvic girdle was recomposed. The sacrum was completed based on the preserved first two sacral elements. The virtual protocol was supported by manual reconstructions using 3D prints of the original fossils. The new virtual models of StW 431 and Sts 14 pelves are generally compatible with the earlier reconstructions by Haeusler [2,4], even though in StW 431 local differences appeared with respect to the orientation of the iliac blades, and in Sts 14 the sacrum might be less curved than previously thought. Compatible with previous observations, StW 431 presents a very peculiar morphology which differs dramatically from that of Homo and great apes possessing extremely flared iliac blades. Moreover, the StW 431 pelvis is markedly different from Sts 14, the latter being reminiscent of the Homo pelvis with its more verticalized iliac blades. The Stw 431 sacrum is significantly less entrapped in the pelvis than in Sts 14, implying a greater mobility of the lumbar spine. Moreover, its iliac blades are much wider and more laterally flaring. The pelvic inlet is sagittally oval in both individuals, rather than being platypelloid. Since pelvic shape reflects functional adaptation to locomotion and birthing, it is reasonable to assume that the profound morphological differences observed between StW 431 and Sts 14 might indicate different biology and taxonomic affiliations. Our conclusions agree with the observations by Crompton et al [5] that StW 431 might belong to a taxon other than A. africanus. However, further studies are needed to elucidate the relationship between StW 431 and StW 573, a third specimen from Sterkfontein Member 2 attributed to A. prometheus that was likened to StW 431 and distinguished from Sts 14 based on some pelvic features.
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