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The evolutionary place of El Sidrón Neandertals has been investigated in the context of possible Neandertal geographic patterning. The mandible, as an anatomical system with clear European lineage-derived features, has been largely used, maximizing the intra-El Sidrón variability (n = 3). The comparative samples were divided into regional subgroups according to north–south and east–west geographic polarities. Specimens coming from the southwest Asia and south European peninsulas (Iberian, Italian, and Balkans) are included in the southern subset. The east–west boundary is established by the Adriatic Sea, with the Balkans in the east.
Pairwise t test comparisons did not show differences between east and west. By contrast, a number of variables showed significant differences in a north–south division (Fig. 4). The position of the M3 in relation to other mandibular structures emerges as a determinant for the differences. Bi-M3 arcade width (t = 2,2711; df =14; P 3–lingula length (t = –2,1964; df = 19; P in a north–south polarity. Appreciable differences also hold for a variety of indices relating the latter variables with the M3–mental foramen length and corpus height (Fig. 4) (e.g., M3–lingula/M3–mental foramen, t test nonsignificant). Southern mandibles are wider at the level of the M3 and the coronoid process, and the corpus is located relatively closer to the ramus (e.g., shorter retromolar spaces). By contrast, northern mandibles are narrower, and the corpus–ramus distance is larger. This geographic patterning signal was further tested by geometric morphometrics.
The partial least-squares analysis allows visualization of the differences (Fig. 5A). The morphological pattern associated with northern Neandertal populations shows relatively higher prognathism (longer corpus). In addition, the southern populations are characterized by a relatively shorter corpus (reduced retromolar space) and vertically high posterior faces (see also Fig. 3). The height of the mandible, as recorded by linear measurements, is reflected by the height of the corpus as well as of the ramus. Overall, the geographic variation is not related to variation in overall mandibular size (centroid size is not statistically significantly different between northern and southern subsamples).
Procrustes distance of the three-dimensional (3D) data between northern and southern populations of Neandertals is d = 0.049, and it is statistically highly significant (F = 1.4565; df1 = 95; df2 = 1,615; P Fig. 5B). Simultaneously, mean shape differences between Neandertals and the EMP group were highly statistically significant. Interestingly, 3D procrustes distances between the EMP sample and those of southern and northern Neandertal subsamples show closer morphological affinities of the EMP to the northern subsample (EMP–northern Neandertals: d = 0.048; F = 1.84; df1 = 95; df2 = 1,615; P Neandertals: d = 0.044; F = 1.6949; df1 = 95; df2 = 1,805; P In all comparisons the 99th percentile of the F distribution was clearly below the observed F score (within Neandertals observed F = 1.4565, 99th percentile of permuted F = 1.37; between EMP and northern Neandertals: observed F = 1.8489, 99th percentile of permuted F = 1.7; and between EMP and southern Neandertals: observed F = 1.6949, 99th percentile of permuted F = 1.5763).
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