Variation in parietal bone thickness and structural arrangement in Eastern African erectus-like Homo: comparative evidence from late Early Pleistocene Uadi Aalad and Mulhuli-Amo, Danakil depression of Eritrea.

Cranial vault thickness behaves as a highly variable feature in extant and fossil humans. While traditionally used to define H. erectus s.l. because of its relatively thick-walled bones, recent estimates show that, along the id-sagittal plane and at the frontal and parietal eminences, bone thickness does not markedly differ between H. erectus s.l. and anatomically modern humans, nor does it reliably distinguish African and Asian erectus-like representatives. However, compared to the parasagittal areas, paleoanthropological information on thickness variation along the lateral parietal wall is scarce. By using direct, CT- and microCT-based observations, we quantified the tabular and diploic thickness in two circa 1 Ma parietal bones from the 4.7 km apart but stratigraphically correlated sites of Uadi Aalad and Mulhuli-Amo, Eritrean Danakil depression. The first belongs to the nearly complete UA 31 cranium, noted for its blend of erectus-like and derived morphoarchitectural features. The second parietal bone is part of an incomplete cranium preliminarily labelled MA 64-88-378, assembled from nine elements collected on the surface in 2011-2014, whose global shape closely resembles UA 31. While our records do not systematically sample homologous landmarks but rather comparable regions in UA 31 and MA 64-88-378, their relative thickness at various sites is nearly invariably diploe >outer>inner table, even if proportions vary site-specifically. In all cases, intra-individual thickness variation tends to exceed that commonly observed in extant humans. We found a similar pattern, but expressing even more contrasted values, in the chronologically slightly younger parietal specimen from Gombore, at Melka Kunture, Ethiopia. Funding support provided by: Italian Ministry for University and Research, Univ. "La Sapienza" of Rome ("Grandi Scavi"), Italian Ministry for Foreign Affairs, French CNRS.