Research Letter
Page 1 of 6
AuthorS:
Amélie Beaudet1,2
Hominin cranial fragments from Milner Hall,
Sterkfontein, South Africa
Jason L. Heaton3,4,5
Ericka N. L’Abbé2
Travis R. Pickering4,5,6
Dominic Stratford1
AffiliAtionS:
1
School of Geography,
Archaeology and Environmental
Studies, University of the
Witwatersrand, Johannesburg,
South Africa
2
Department of Anatomy,
University of Pretoria, Pretoria,
South Africa
3
Department of Biology,
Birmingham-Southern College,
Birmingham, Alabama, USA
4
Evolutionary Studies Institute,
University of the Witwatersrand,
Johannesburg, 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.
5
Plio-Pleistocene Palaeontology
Section, Department of
Vertebrates, Ditsong National
Museum of Natural History,
Pretoria, South Africa
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.
6
Department of Anthropology,
University of Wisconsin,
Madison, Wisconsin, USA
CorreSpondenCe to:
Amélie Beaudet
emAil:
beaudet.amelie@gmail.com
dAteS:
received: 01 July 2018
revised: 15 Aug. 2018
Accepted: 15 Aug. 2018
published: 27 Nov. 2018
KeywordS:
Cradle of Humankind; late
Pliocene–early Pleistocene;
cranial thickness; diploë; Homo
how to Cite:
Beaudet A, Heaton JL, L’Abbé
EN, Pickering TR, Stratford D.
Hominin cranial fragments
from Milner Hall, Sterkfontein,
South Africa. S Afr J Sci.
2018;114(11/12), Art.
#5262, 6 pages. https://doi.
org/10.17159/sajs.2018/5262
ArtiCle inCludeS:
× Supplementary material
× Data set
© 2018. The Author(s).
Published under a Creative
Commons Attribution Licence.
South African Journal of Science
http://www.sajs.co.za
introduction
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. Two hominin fossils, a molar and a proximal phalanx, excavated from the
T1 depositional unit of the Central Underground Deposits excavation site in the Milner Hall (STK-MH1) have already
been described.1 The complex stratigraphic context of the Milner Hall fossiliferous depositional sequence, in
which an early distal accumulation of the 3.67-Ma-old Member 2 (T3) and 2.18-Ma-old Oldowan artefact-bearing
sediments from Member 5 (T2) contribute to the formation of T12,3, affords that potentially Australopithecus,
Paranthropus and early Homo might be represented in the unit. Interestingly, the description and metric analyses
of the two first hominin specimens excavated from T1 suggest an enigmatic mix of unique, primitive and derived
morphological traits, with potential morphological affinities with the genus Homo.1
With hominin remains dated potentially to either 3.67 Ma or 2.18 Ma, the Milner Hall fossil assemblage may
contribute to ongoing debates about hominin morphological and taxonomic diversity at Sterkfontein during the
late Pliocene and early Pleistocene. In particular, 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.6-8
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.
funding:
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)
1
Volume 114 | Number 11/12
November/December 2018
Research Letter
Page 2 of 6
Milner Hall hominin cranial fragments
Here, we report on two additional hominin fossils excavated from STKMH1 and discuss efforts to assign them taxonomically.
of the outer table were automatically and separately measured at regular
intervals along the two sections perpendicular to the outer cranial surface
using a custom-written program in MATLAB R2013a18 (Mathworks,
https://www.mathworks.com/products/matlab.html) (Figure 2). In total,
30 measurements were collected on StW 671 and on StW 672. Based
on surface area, we computed bone tissue proportions as the percentage
of the bone area represented by outer/inner tables or diploë.18 For
comparative specimens, with the exception of DH 1/DH 3, we selected
a number of measurements (see Results) performed along parasagittal
sections from Beaudet et al.18 and corresponding to the portions of the
cranium documented in StW 671 and StW 672.
material and methods
As comparative material, we investigated four South African hominin
cranial specimens from Sterkfontein (Sts 5, Sts 71, StW 505) and
Swartkrans (SK 46), attributed, respectively, to Australopithecus africanus
and Paranthropus robustus.10-14 In the absence of well-preserved
South African early Pleistocene human crania, we included the Middle
Pleistocene composite cranium DH 1/DH 3 from Rising Star attributed
to Homo naledi as comparative material for the external morphology.15
Additionally, we selected bone thickness values of nine human and
non-human Pliocene and Pleistocene hominin taxa presented in
the supplementary information of Copes and Kimbel16. Our extant
comparative sample comprises adult humans (Homo sapiens, n=10)
and common chimpanzees (Pan troglodytes, n=10) with equal
proportions of male and female individuals within each taxon from the
collections of the University of Pretoria17 (South Africa) and the Royal
Museum for Central Africa (Belgium) respectively. Ethical clearance for
the use of extant human crania was obtained from the Main Research
Ethics Committee of the Faculty of Health Sciences, University of Pretoria
in February 2016 (35/2016).
a
b
The new fossils, StW 671 and StW 672, were scanned at the microfocus
X-ray tomography facility of the Palaeosciences Centre at the University
of the Witwatersrand, in Johannesburg (South Africa), at a spatial
resolution of 28 µm (isotropic voxel size) (Figures 1 and 2). Fossil
comparative material from Sterkfontein and Swartkrans as well as extant
specimens were scanned at the Palaeosciences Centre, at the South
African Nuclear Energy Corporation in Pelindaba (South Africa), or at the
Centre for X-ray Tomography of the Ghent University in Ghent (Belgium).
Additionally, for external morphology, we included in our study the digital
replica of the composite cranium DH 1/DH 3 from Rising Star available
on MorphoSource (www.morphosource.org).15
c
StW 671 and StW 672 were digitally rendered with Avizo v.9.0 software
(Visualization Sciences Group Inc.). We extracted one section in StW 671
and one section in StW 672, sampling maximum length/width of the
preserved cranial fragments (Figure 2). In terms of measurements, we
followed the protocol detailed in Beaudet et al.18 In view of collecting
consistent data throughout the sample, we tried to avoid oversampling
exocranial reliefs and, thus, the section in StW 671 was positioned
orthogonally to the temporal line. The three layers of the bone were
segmented by combining manual and automatic methods (i.e. watershed
transform). Thickness and surface area of the inner table of the diploë and
figure 2:
Cranial structural organisation along sections extracted from
(a) StW 671 and (b) StW 672. Comparative measurements
come from (c) the frontal and occipital parts of the parasagittal
section extracted from the comparative sample. White lines in (c)
delimitate the frontal and occipital portions of the cranial vault.
results
Description of StW 671 and StW 672
StW 671 is a small rectangular fragment of the frontal bone (maximum
length = 38 mm; maximum width = 32.5 mm; maximum intertabular
thickness = 13.7 mm) (Figure 1a). On the lateral aspect, the temporal
line runs along the ectocranial surface. The posterior border of the
fragment corresponds to the coronal suture. The surface immediately
inferior to the temporal line is relatively flat. The endocranial surface is
convex and does not exhibit any diagnostic anatomical features. In terms
of surface preservation, StW 671 shows a few minor weathering cracks
and heavy manganese dioxide staining.
a
b
figure 1:
StW 672 is an ovoid fragment of the occipital bone (maximum length =
34.3 mm; maximum width = 25.1 mm; maximum thickness = 7.4 mm)
(Figure 1b). The ectocranial surface is relatively smooth. The endocranial
surface preserves two shallow grooves and a faint ridge is perceptible
along the medial border. The fragment is globally convex. Because the
ectocranial and endocranial surfaces lack crests and vascular grooves,
respectively, this fragment is proposed to originate from the portion of
the occipital bone superior to the nuchal crest and transverse sinus. The
specimen’s surface shows minor weathering, including cracks, and
evidence of diagenetic flaking in the form of non-morphological pits on
the ectocranial aspect.
Virtual rendering of (a) StW 671 and (b) StW 672 in ectocranial
(left), posterior/medial (middle) and endocranial (right) views.
South African Journal of Science
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Volume 114 | Number 11/12
November/December 2018
Research Letter
Page 3 of 6
Milner Hall hominin cranial fragments
Comparative description of StW 671 and StW 672
frontal bone of the two comparative specimens. The overall morphology
of the StW 671 fragment closely fits the shape of the frontal bone in the
extant human specimen. However, when superimposed, the temporal
line in StW 671 runs inferiorly compared to the human cranium.
StW 671 and StW 672 were superimposed on the digital replica of
Sts 5 (Australopithecus africanus), SK 46 (Paranthropus robustus), a
composite skull of Homo naledi based on DH 1 and DH 3, and extant
human and chimpanzee crania (Figure 3).
The morphology of StW 672 is compatible with the morphology of the
occipital bone for all comparative crania considered in this study.
The overall morphology of StW 671 does not fit well with the external
aspect of the frontal bone of Sts 5, SK 46, DH 1/DH 3 and of the
chimpanzee specimen. Indeed, when superimposed onto Sts 5 and
DH 1/DH 3, the portion of the frontal bone above the temporal line in
StW 671 is more elevated than in the comparative frontal squama. The
temporal line progressively joins the sagittal crest in Paranthropus and
chimpanzees.19 If the temporal line of StW 671 is superimposed onto
the temporal line of SK 46 and of the chimpanzee specimen, the coronal
suture in the Milner Hall specimen is then positioned in the middle of the
Cranial thickness and composition in StW 671 and StW 672
Thickness values and tabular proportions in StW 671 and StW 672 as
well as in the comparative sample are shown in Tables 1 and 2 and in
Figure 4. From Beaudet et al.18, we specifically selected measurements
1–20 and 40–50, respectively representing the frontal and occipital
bones (Figure 2c). Additionally, we compiled comparative measurements
of the frontal bone of non-human and human hominin taxa from Copes
and Kimbel16 in Table 3.
a
Sts 5
SK 46
DH 1/DH 3
extant human
extant chimpanzee
b
figure 3:
Superimposition of (a) StW 671 (in lateral view) and (b) StW 672 (in posterior view) on the digital replicas of Sts 5 (Australopithecus africanus),
SK 46 (Paranthropus robustus), DH 1/DH 3 (Homo naledi) and extant human and chimpanzee crania.
table 1:
Mean frontal bone thickness (FBT, mm) and relative proportions
of the diploë (%) of StW 671 compared to the estimates from
some Plio-Pleistocene hominin specimens and extant humans
and chimpanzees. Standard deviations are shown in brackets.
fBt
outer
table
inner
table
Stw 671
8.8 (0.9)
2.9 (0.3)
StW 505
10.2 (5.3)
–
Sts 5
6.3 (3.1)
0.2 (0.3)
1.1 (0.6) 5.0 (2.5)
Sts 71
7.8 (1.1)
0.9 (0.2)
0.6 (0.3) 6.3 (1.2)
SK 46
7.4 (4.2)
Extant humans
(n=10)
7.6 (0.7)
Extant
chimpanzees
(n=10)
4.6 (1.0)
1.8 (0.4) 4.1 (0.8)
% diploë
http://www.sajs.co.za
oBt
outer
table
inner
table
diploë
% diploë
Stw 672
5.6 (0.9)
1.4 (0.3)
1.8 (0.3)
2.4 (0.9)
41.3 (10.2)
–
StW 505
9.3 (4.9)
–
–
–
–
77.6 (4.1)
Sts 5
9.2 (4.2)
0.6 (0.6)
0.9 (0.3)
7.7 (3.9)
83.4 (35.6)
78.7 (5.3)
Sts 71
7.0 (2.9)
0.2 (0.3)
0.3 (0.2)
6.4 (2.8)
89.5 (43.6)
0.3 (0.5)
6.4 (8.9)
SK 46
6.7 (1.3)
0.6 (0.7)
11.2 (11.8)
1.7 (0.2) 4.1 (0.8)
51.0 (5.9)
Extant humans
(n=10)
6.9 (0.3)
3.8 (0.5)
52.5 (7.6)
15.6 (4.7)
Extant
chimpanzees
(n=10)
4.6 (0.6)
0.8 (0.2)
14.8 (1.9)
–
3.7 (0.5)*
–
1.0 (0.8)
6.1 (1.8)*
1.9 (0.6)
1.3 (0.2)
3.9 (0.6)*
* The outer and inner tables are indistinct (i.e. cortical) in SK 46 and extant chimpanzees.
* The outer and inner tables are indistinct (i.e. cortical) in SK 46 and extant chimpanzees.
South African Journal of Science
Mean occipital bone thickness (OBT, mm) and relative proportions
of the diploë (%) of StW 672 compared to the estimates from
some Plio-Pleistocene hominin specimens and extant humans
and chimpanzees. Standard deviations are shown in brackets.
45.1 (5.9)
7.1 (4.2)*
1.9 (0.4)
diploë
table 2:
3
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Research Letter
Page 4 of 6
table 3:
Milner Hall hominin cranial fragments
The mean frontal bone thickness in StW 671 is thick compared to Sts 5,
Sts 71, SK 46 and the extant human and chimpanzee specimens with the
exception of StW 505 (Table 1, Figure 4a). When compared to the frontal
bone thickness values reported by Copes and Kimbel16, the closest
matches are Homo erectus and Homo heidelbergensis (Table 3).
StW 671 shares the following pattern of bone tissue proportions with
Sts 71, extant humans and Australopithecus afarensis: the diploë is the
thickest bony layer while the inner table is the thinnest (Tables 1 and 3).
As previously noted20,21, Sts 5 lacks a significant portion of its outer
table. In terms of tissue proportions (Table 1, Figure 4a), the diploic
bone contributes 45.1% to the cumulative cranial thickness in StW 671,
while in Sts 5, Sts 71 and extant humans the diploë represents more
than 50% of the total thickness and less than 20% in SK 46 and extant
chimpanzees (Figure 4b).
Mean frontal bone thickness (FBT, mm) of human and nonhuman hominin species from Copes and Kimbel16. Standard
deviations are shown in brackets.
outer
table
inner
table
diploë
n
fBt
Australopithecus afarensis
2
7.5 (1.8)
Paranthropus boisei
7
6.0 (1.2)
–
–
–
Early Homo
10
6.0 (1.5)
–
–
–
European Homo erectus
2
8.5 (0.7)
–
–
–
African Homo erectus
8
9.2 (1.3)
–
–
–
Asian Homo erectus
29
9.2 (2.1)
–
–
–
Homo heidelbergensis
14
8.5 (1.8)
–
–
–
Homo neanderthalensis
13
6.6 (1.6)
–
–
–
Pleistocene Homo sapiens
17
7.5 (2.1)
–
–
–
1.4 (0.4) 1.3 (0.3) 6.0 (0.3)
The mean occipital bone thickness in StW 672 is thin compared to the
comparative fossil specimens and extant humans (Table 2, Figure 4c).
The pattern of bone tissue proportions in the comparative sample is
variable but StW 672 and Sts 5 and Sts 71 have a similar pattern in that
the diploë is the thickest bony layer while the outer table is the thinnest
(Table 2). The proportion of diploë in StW 672 represents 41.3% of the
cumulative cranial thickness and more than 50% in StW 578, Sts 5,
Sts 71 and extant humans but less than 20% in SK 46 and extant
chimpanzees (Figure 4d).
a
b
c
d
figure 4:
(a and c) Box plots of cranial thickness and (b and d) histograms of proportion of diploë in (a, b) StW 671 and (c, d) StW 672 compared to some
Plio-Pleistocene hominin specimens/taxa and extant human crania.
South African Journal of Science
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Milner Hall hominin cranial fragments
discussion
references
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. Interestingly, despite StW 671 showing a derived humanlike cranial morphology and thickness, the proportions of diploë in StW
671 and StW 672, even if comparatively closer to extant humans, do not
clearly match the fossil hominins included in our comparative study. In
this context, our analysis may suggest a combination of derived humanlike traits and unique features. This mosaicism is compatible with the
description and metric analyses of the manual proximal phalanx StW 668
and the upper right first molar StW 669 from Milner Hall and contributes
to a certain degree of taxonomic ambiguity.1 As the diploic bone acts as
a protective barrier for the brain, contributes to mechanical properties of
the cranium and plays a role in the cranial vascular system, our results
may potentially suggest an intriguing distinct palaeobiology in the Milner
Hall hominins.16,22-25
1.
Stratford D, Heaton JL, Pickering TR, Caruana MV, Shadrach K. First
hominin fossils from Milner Hall, Sterkfontein, South Africa. J Hum Evol.
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2.
Stratford DJ, Grab S, Pickering TR. The stratigraphy and formation history of
fossil-and artefact-bearing sediments in the Milner Hall, Sterkfontein Cave,
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cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and
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4.
Clarke RJ. Australopithecus from Sterkfontein Caves, South Africa. In: Reed
K, Fleagle JG, Leakey RE, editors. The paleobiology of Australopithecus.
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Grine FE. The alpha taxonomy of Australopithecus africanus. In: Reed
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Grine FE, Smith HF, Heesey CP, Smith EJ. Phenetic affinities of PlioPleistocene Homo fossils from South Africa: Molar cusp proportions. In:
Grine FE, Fleagle JG, Leakey RE, editors. The first humans: Origin and early
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Curnoe D. A review of early Homo in southern Africa focusing on cranial,
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Clarke RJ. The cranium of the Swartkrans hominid, SK 847 and its
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Further analyses documenting variation in the frontal anatomy (e.g. postorbital constriction) and cranial thickness in early hominins, and more
particularly in South African and East African early Homo, would be
critical for the interpretations of the Milner Hall hominin palaeobiology
and taxonomic attribution. Moreover, comparisons with the East African
early Homo material preserving frontal and occipital bones (e.g. KNM-ER
1470, KNM-ER 1813) would be particularly relevant for supporting the
presence of human-like traits in the Milner Hall hominins’ anatomy and/or
assessing the potential uniqueness of the MH fossil record. Additionally,
because the position of our sections might vary across specimens, our
bidimensional quantitative investigation of the cranial bone thickness
and composition should be combined in future with 3D approaches16
and/or supported by new findings of more complete cranial fragments
from the Milner Hall deposits. Nevertheless, besides reporting additional
specimens from STK-MH1, this study highlights the relevance of the
analysis of cranial thickness and composition in taxonomical studies
and the potential of the Milner Hall for contributing to our knowledge of
the hominin palaeobiodiversity at Sterkfontein.
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Acknowledgements
11. Broom R, Robinson JT. Further evidence of the structure of the Sterkfontein
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We are indebted to E. Gillisen and W. Wendelen (Tervuren), G. Krüger
(Pretoria), L. Kgasi, H. Fourie, S. Potze and M. Tawane (Pretoria)
and B. Zipfel (Johannesburg) for having granted access to fossil and
comparative material under their care. We also thank L. Bam, F. de
Beer and J. Hoffman (Pelindaba), M. Dierick (Ghent) and K. Jakata
(Johannesburg) for microtomographic acquisitions. We are grateful
to the Ditsong National Museum of Natural History and the University
of the Witwatersrand for loaning hominin crania in their collection. For
technical and/or scientific discussion/collaboration we are grateful
to: M. Carmen Arriaza (Johannesburg), R. Clarke (Johannesburg), J.
Dumoncel (Toulouse), K. Carlson (Los Angeles) and A. Oettlé (Pretoria).
We thank the DST-NRF for sponsoring the Micro-XCT facility at Necsa,
and the DST-NRF and the University of the Witwatersrand for funding
the microfocus X-ray CT facility in the ESI (www.wits.ac.za/microct).
The support of the AESOP+ Programme, the Claude Leon Foundation,
the DST-NRF Centre of Excellence in Palaeosciences, the French
Institute of South Africa and the Palaeontological Scientific Trust (PAST)
towards this research is hereby acknowledged. Opinions expressed and
conclusions arrived at, are those of the authors and are not necessarily
to be attributed to the Centre of Excellence in Palaeosciences.
12. Broom R, Robinson JT. Swartkrans ape-men. Paranthropus crassidens.
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Cranial vault thickness variation and inner structural organization in the StW
578 hominin cranium from Jacovec Cavern, South Africa. J Human Evol.
2018;121:201–220. https://doi.org/10.1016/j.jhevol.2018.04.004
Authors’ contributions
Conceptualisation: A.B., T.R.P., J.L.H., D.S.; methodology: A.B., E.N.L.;
data collection: A.B., D.S., T.R.P., J.L.H.; sample and data analysis: A.B.,
E.N.L., T.R.P., J.L.H.; validation: T.R.P., J.L.H., E.N.L.; writing: A.B., D.S.,
T.R.P., J.L.H., E.N.L.; project leadership: D.S.; project management: A.B.;
funding acquisition: A.B., D.S.
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Milner Hall hominin cranial fragments
21. Thackeray JF. Cranial bone of ‘Mrs Ples’ (Sts 5): Fragments adhering to
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