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 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.
