Hominin cranial fragments from Milner Hall, Sterkfontein, South Africa

The Sterkfontein Caves site is one of the richest early hominin localities in Africa. In addition to significant fossil assemblages from Members 2 and 4 of the Sterkfontein Formation, recent excavations have revealed hominin-bearing sedimentary deposits in the lesser-known Milner Hall. We describe two hominin cranial fragments excavated from the Milner Hall in 2015 and present the results of a highresolution microtomographic-based approach to diagnosing the anatomical and taxonomical origin of these specimens. Based on external morphology, StW 671 and StW 672 are identified as frontal and occipital fragments, respectively. Our non-invasive bi-dimensional quantitative investigation of the two cranial fragments reveals a mean cranial thickness of 8.8 mm for StW 671 and of 5.6 mm for StW 672, and a contribution of the diploic layer to the cumulative cranial thickness that is less than 50%. While the mean cranial thickness of StW 671 falls within the range for Homo, the relative proportion of the diploë in both StW 671 and StW 672 is lower than that found in Australopithecus (>60%) and extant humans (>50%). Accordingly, in terms of both cranial thickness and inner structural organisation, the Milner Hall hominins combine derived and unique traits, consistent with the condition of other postcranial and dental material already described from the deposit. Moreover, our study opens interesting perspectives in terms of analysis of isolated cranial fragments, which are abundant in the hominin fossil record. SIGNIFICANCE : • The Sterkfontein Caves have widely contributed to our understanding of human evolution. • Besides the well-known Members 4 and 2, where the iconic ‘Mrs Ples’ and ‘Little Foot’ have been found, in this study we suggest that the Milner Hall locality represents an additional, stratigraphically associated source of not only fossil hominins, but also Oldowan stone tools. • In particular, we describe for the first time two cranial fragments, StW 671 and StW 672, identified as frontal and occipital bones, respectively. • Our microtomographic-based analysis of these materials reveals some affinities with Homo combined with unique characters. • In this context, our study suggests an intriguing mosaicism consistent with the description of the two fossil hominins found in the Milner Hall.AESOP+ Programme; Claude Leon Foundation; DST-NRF Centre of Excellence in Palaeosciences; French Institute of South Africa; NRF African Origins Platform; Palaeontological Scientific Trust (PAST)http://www.sajs.co.zaam2019Anatom