Abstract
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 high-resolution microtomographic-based approach to diagnosing the anatomical and taxonomical origin of these specimens. Based on external morphology, StW 671 and StW 672 are tentatively 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 and 5.6 mm for StW 671 and StW 672 respectively 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 of Homo, the relative proportion of the diploë in both StW 671 and StW 672 is lower than in Australopithecus (>60%) and extant humans (>50%). Accordingly, in terms of both cranial thickness and inner structural organization, 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.
Highlights
Milner Hall (MH) is a deep underground chamber of the Sterkfontein Caves (South Africa) that extends about 100 m in a roughly east-west direction
The re-attribution of hominin remains previously assigned to early Homo or Australopithecus (e.g. StW 53)[8], and the fragmentary nature of fossil specimens identified as early Homo (e.g. SK 847)[9], complicate our understanding of early human diversity in South Africa
While the overall morphology and cranial thickness of StW 671 approximates the human condition, the proportion of diploë in both StW 671 and StW 672 is lower than that found in human and nonhuman hominin taxa investigated in this study with the exception of Paranthropus
Summary
Milner Hall (MH) is a deep underground chamber of the Sterkfontein Caves (South Africa) that extends about 100 m in a roughly east-west direction. Because of the poorly known degree of intraspecific variation in the Australopithecus hypodigm, the Sterkfontein hominin fossil assemblages have been the focus of longstanding discussions regarding the presence of one or two Australopithecus species.[4,5] In addition, the specieslevel diagnosis of purported early Homo remains from Member 5 of the Sterkfontein Formation is contentious.[6,7,8] In particular, the re-attribution of hominin remains previously assigned to early Homo or Australopithecus (e.g. StW 53)[8], and the fragmentary nature of fossil specimens identified as early Homo (e.g. SK 847)[9], complicate our understanding of early human diversity in South Africa In this context, the Milner Hall deposits have the potential to provide further evidence to critically assess hominin palaeobiodiversity and the taxonomic context of the Sterkfontein hominin-bearing deposits
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