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KSD-VP-1/1: the “Big Man” just got bigger

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author WebbN, KrennV, FornaiC, 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) 198
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 3.6 Ma old Australopithecus afarensis skeleton KSD-VP-1/1 known as “Kadanuumuu” from Woranso-Mille, Ethiopia [1], is pivotal for evaluating morphological variation and sexual dimorphism within the East African australopithecines given its relative completeness and large body size. Its robustness in combination with a narrow greater sciatic notch suggest that KSD-VP-1/1 was probably male [1]. The diameter of its hip joint previously generated a body mass estimate of 59.5 kg, which is nearly double that of the 28.9 kg calculated for the supposed female A. afarensis specimen A.L. 288-1 [2]. Here, we reassess the morphological vari- ation and sexual dimorphism within A. afarensis based on our novel reconstruction of the KSD-VP-1/1 hipbone. We performed a 3D geometric morphometric analysis based on 24 homologous landmarks with a sample of pelves from extant humans (n=60), chimpanzees (Pan troglodytes; n=35), gorillas (Gorilla gorilla, G. beringei; n=45) and orangutans (Pongo pygmaeus; n=30), as well as restored hipbones of A.L. 288-1 (A. afarensis) [3], Sts 14 [3] and StW 431 (both A. africanus), and MH2 (A. sediba) [4]. Our reconstruction of the KSD-VP-1/1 hipbone was done by virtually reassembling the individual pieces of the fractured specimen af- ter removing the adhering matrix. Then, we utilized landmark registration methods coupled with thin-plate spline-based warping of A.L. 288-1 to estimate the missing regions of the hipbone such as the iliac crest, the ischiopubic ramus and the pubic symphyseal body. Additionally, least squares best-fit regions were fitted to the undistorted sections of the acetabulum to estimate acetabular size. Using published regression equations [2], we predicted a femoral head diameter of 40.9 mm, which translated into a body mass range of 64-68 kg. These values are slightly larger than those previously published [2], thereby confirming the enormous body size variation within these early hominins. The principal component analysis of the Procrustes shape coordinates for the hipbone landmark configurations showed that KSD-VP-1/1 was closer to Sts 14 (A. africanus) rather than A.L. 288-1 (A. afarensis). This trend was maintained even when size-adjusted residuals were plotted to control for allometric influences on the shape data. Hu- mans and australopithecines had different trajectories when residual shape components were compared to the common allometric component, suggestive of unique scaling trends for each group, even among different reconstructions of the same specimen (Pro- crustes ANOVA; P < 0.01). This adds to our previous geometric morphometric analysis of the KSD-VP-1/1 sacrum [5], which demonstrated the presence of a surprisingly narrow, and likely primitive, shape of the sacroiliac joint that distinguished it from that of A.L. 288-1 and Sts 14. Procrustes distance-based pairwise permutations confirmed this morphological disparity with differences between KSD-VP-1/1 and A.L. 288-1 exceeding 98% of pairwise comparisons within modern humans. Thus, our reconstruction and geometric morphometric analysis of the KSD-VP-1/1 hipbone contextualizes its unique morphological variation and its large body size relative to other australopithecines. These findings highlight the morphological heterogeneity within the A. afarensis hypodigm. The peculiar shape of the KSD-VP-1/1 sacroiliac joint in combination with the robustness of the pelvis itself suggest possible functional and taxonomic diversity within the East African australopithecines that further research will need to elucidate.
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