Unveiling a ~290 Kya hominin partial parietal bone from the "Ruidera-Los Villares" paleoanthropological site: filling the gap of the Middle Pleistocene record of the Iberian Peninsula

Abstract

After a road construction in the 1960s, faunal fossils were surface collected by locals at the "Los Villares" house state (Ruidera, Ciudad Real). The study of the fossils showed the presence of cutmarks on some herbivore bones dated around 300-400 ka by a combined ESR-Uranium series approach [1]. A systematic excavation of 10 m 2 was carried out in 2023 to clarify the spatial and stratigraphic extension of the fossil assemblage. We observed that the site is formed by red breccias with speleothem fragments, interpreted as the sedimentary infill of an eroded rock shelter, some of them being in a secondary position. The faunal analysis showed that small bovids dominate the association, although the greatest diversity is found among carnivores. The mammal community is compatible with other Iberian sites from the late Middle Pleistocene, suggesting a rocky ecosystem with scattered vegetation. The reduced lithic tools assemblage consists of quartzite cores and flakes with few extractions, possibly attributed to an ancient industry. Finally, a partial human parietal bone from a large hominin stands out among the fossil remains found during this campaign. The fossil, about 8x7x1 centimeters, preserves enough morphological features to make its anatomical assessment as part of the coronal suture, the temporal line, and a "human-like" middle meningeal artery imprint [2]. OSL dating yields a preliminary age of 290 ± 24 ka. As the fossil remains were not found in situ, establishing the sedimentary context and dose rate calculation was complicated. A thin layer of sediment was extracted from the inner part of the fossil, under controlled red light conditions. The inner core was used to extract the quartz for equivalent dose calculation. The external part that had been exposed to light was removed and kept for ICP-MS analysis to determine the beta dose rate. Gamma dose rate was derived from a sediment sample which apparently corresponded to the same unit where the fossil belonged. Comparing the anatomy of the fossil remains, RV'23-005 has a maximum thickness of 11.37 mm in the area above the temporal bone squama, whereas other fossils from the Iberian Peninsula such as the Skull 5 from Sima de los Huesos have a maximum thickness in that area around 8.5 mm [3]. Also, we quantified the angulation of the parietal bone at the temporal line showing RV'23-005 an angle of 131,2º, whereas other European groups such as the Sima de los Huesos and Neanderthals presented, on average, a much lower angulation (123.26º and 114.69º). The results of the partial parietal bone RV'23-005 add valuable data on the variability of the scarce remains from Middle Pleistocene human anatomy of the Iberian Peninsula, where some fossil sites such as Atapuerca Sima de los Huesos and Galería (Burgos, Spain), and Gruta de Aroeira (Portugal) stand out. Because of the complicated taxonomical interpretation of the Middle Pleistocene in Europe [4], the potential attribution of a precise species of the genus Homo for RV'23-005 is an ongoing investigation. Its morphology and features could represent an H. heidelbergensis deme [sensu 5] from the southern Iberian Plateau that could shed light on the peopling of Western Europe and the evolution of the human lineages.