New hominin cranial remains from the Shungura Formation, Lower Omo Valley (Ethiopia)

The formations of the Lower Omo Valley, south-western Ethiopia, have played a key role in our understanding of Plio-Pleistocene hominin taxic diversity and biogeography. Here, we describe two hominin cranial fragments excavated from Shungura Member L (Unit L-9) in 2016 and dated to ca. 1.15 million years ago (Ma). The two cranial fragments, OMO 342-10248a and OMO 342-10248b, were photographed, scanned, physically and digitally measured and compared to extant humans as well as Plio-Pleistocene hominin specimens. Based on the morphology of the ectocranial and endocranial surfaces, OMO 342-10248a and OMO 342-10248b are identified as a right parietal bone and an occipital bone, respectively. The average cranial thickness measured in OMO 342-10248a falls within the range of Asian Homo erectus and closely approximates individual values measured in some specimens of African Homo erectus, in particular the one of the remain P 996-17a from Unit K-3 of the Shungura Formation. The average cranial thickness measured in OMO 342-10248b falls within the range of Australopithecus africanus, Paranthropus boisei and African Homo erectus. The inner structural arrangement of both specimens is diploë-dominated and medical CT scans reveal the presence of the posterior temporal diploic vessels in OMO 342-10248a. The endocranial surface of OMO 342-10248a preserves imprints of the middle branch of the middle meningeal vessels and reveals substantial anastomoses. Overall, both specimens share similarities with Homo, and more particularly with Homo erectus. Resemblances with P 996-17a further support the possibility of Homo erectus-like hominins being present in the Lower Omo Valley 1.15 Ma.