# Taxonomic revision of the SK 15 mandible based on bone and tooth structural organization

Clément Zanolli, Jean-Jacques Hublin, Ottmar Kullmer, Friedemann Schrenk, Lazarus Kgasi, Mirriam Tawane, Song Xing

<figure><img src="image_1.png" /></figure>

## To cite this version:

Clément Zanolli, Jean-Jacques Hublin, Ottmar Kullmer, Friedemann Schrenk, Lazarus Kgasi, et al. Taxonomic revision of the SK 15 mandible based on bone and tooth structural organization. Journal of Human Evolution, 2025, 200, pp.103634. 10.1016/j.jhevol.2024.103634. hal-04977650

HAL Id: hal-04977650

https://hal.science/hal-04977650v1

Submitted on 21 May 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# Taxonomic revision of the SK 15 mandible based on bone and tooth structural organization

<figure><img src="image_4.png" /></figure>

Check for
updates

Clément Zanolli<sup>a, b, *</sup>, Jean-Jacques Hublin<sup>c</sup>, Ottmar Kullmer<sup>d, e</sup>, Friedemann Schrenk<sup>d, e</sup>, Lazarus Kgasi<sup>f, g</sup>, Mirriam Tawane<sup>f</sup>, Song Xing<sup>h, i</sup>

## ARTICLE INFO

**Article history:**
Received 8 December 2023
Accepted 11 December 2024
Available online 2 January 2025

Handling Editor: A Taylor

**Keywords:**
*   *Homo ergaster*
*   *Paranthropus robustus*
*   *Telanthropus capensis*
*   *Paranthropus capensis*
*   Southern Africa
*   Swartkrans

## ABSTRACT

The hominin mandible SK 15 was discovered in April 1949 in Swartkrans Member 2, dated to ~1.4 Ma. Albeit distorted on the right side, the left and right corpus of SK 15 are relatively low and thick, even compared to most Early to Middle Pleistocene *Homo* specimens. It preserves the left molar row and the right M₂ and M₃ that show a distalward increase in mesiodistal diameter. SK 15 was originally attributed to *Telanthropus capensis* but is now generally attributed to *Homo erectus/Homo ergaster*, even if it was previously suggested to possibly belong to *Australopithecus*. Similarities between SK 15 and *Homo naledi* mandible and tooth morphology were also claimed. To clarify the taxonomy of SK 15, we used X-ray microtomography to investigate aspects of bone and tooth structural organization. Geometric morphometric analyses of the dental arcade shape, mandible symphysis outline, and the M₂ and M₃ enamel-dentine junction shape were conducted. For mandibular symphysis shape, SK 15 exhibits an australopith signal, whereas for both the dental arcade and enamel-dentine junction analyses, the specimen is statistically classified as *Paranthropus*. Altogether, the results show that SK 15 unambiguously falls outside the variation of *H. erectus/H. ergaster* and that it is most compatible with the morphology of *Paranthropus*, albeit showing smaller dimensions and an absence of some dental morphological features (e.g., developed protostylid, distally tapering M₃, short molar roots) typically found in specimens of *Paranthropus aethiopicus*, *Paranthropus boisei*, and *Paranthropus robustus*. In particular, SK 15 differs markedly in size and morphology from mandibular remains of *P. robustus* from Swartkrans Member 2. We thus tentatively attribute SK 15 to *Paranthropus capensis*, a more gracile species of *Paranthropus* than the other three currently recognized species of this genus and discuss the implications for the existence of another species of *Paranthropus* in southern Africa during the Early Pleistocene.

© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

The hominin mandible SK 15 was discovered in April 1949 in Swartkrans Member 2 (Broom and Robinson, 1949) and dated to ~1.4 Ma (Balter et al., 2008). Albeit distorted on the right side, SK 15

is one of the best preserved and most complete hominin mandibles of the African paleoanthropological record (Fig. 1; Schwartz and Tattersall, 2003). The left and right corpora of SK 15 are low and thick, comparable to some Early to Middle Pleistocene *Homo* specimens, but smaller in all dimensions than the material generally attributed to *Paranthropus robustus* (Robinson, 1961; Grine, 2001). The SK 15 mandible preserves the left molar row and the right M₂ and M₃, and the molars show a distalward increase in

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, MCC, PACEA, UMR 5199, Pessac F-33600, France</footnote>

<footnote><sup>b</sup> Evolutionary Studies Institute, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, Johannesburg 2000, South Africa</footnote>

<footnote><sup>c</sup> Chaire de Paléoanthropologie, CIRB (UMR 7241–U1050), College de France, 11, Place Marcelin-Berthelot, Cedex 05, Paris 75231, France</footnote>

<footnote><sup>d</sup> Division of Palaeoanthropology, Senckenberg Research Institute and Natural History Museum Frankfurt, Frankfurt, Germany</footnote>

<footnote><sup>e</sup> Department of Palaeobiology and Environment, Institute of Ecology, Evolution, and Diversity, Goethe University Frankfurt, Frankfurt, Germany</footnote>

<footnote><sup>f</sup> Ditsong National Museum of Natural History, Pretoria 0001, South Africa</footnote>

<footnote><sup>g</sup> Palaeo-Research Institute, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa</footnote>

<footnote><sup>h</sup> Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China</footnote>

<footnote><sup>i</sup> CAS Center for Excellence in Life and Paleoenvironment, Beijing 100044, China</footnote>

<footnote>* Corresponding author.
E-mail address: clement.zanolli@gmail.com (C. Zanolli).</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Photographs of the specimen SK 15 in occlusal (A) and lateral (B) views. (For interpretation of the references to color in this figure, the reader is referred to the Web version of this article).</figcaption></figure>

mesiodistal diameter, as is generally the case in the australopiths and in some early *Homo* specimens but which rarely occurs in *Homo erectus* s.l. (Wood, 1991; Curnoe, 2006). SK 15 was originally attributed to *Telanthropus capensis* and represents the holotype of this taxon (Broom and Robinson, 1949). However, it was successively transferred to early *Homo*, and more specifically to *H. erectus/Homo ergaster* (Robinson, 1961), together with other fossils from the same site but from the older Member 1 (dating to 2.22 ± 0.05 [0.09] Ma; Kuman et al., 2021): SK 45 and SK 847 (Broom and Robinson, 1950; Clarke, 1985). However, SK 847 is a fragmentary cranium with partially edentulous maxilla, only preserving some root fragments, the worn left I² and the left M³ (Clarke, 1977), and SK 45 is a right portion of mandible from the M₁ to the base of the ascending ramus, preserving the highly worn M₁ and M₂ (Broom and Robinson, 1950), making taxonomic associations with SK 15 challenging. SK 15 has been attributed to *H. erectus/H. ergaster* by most since then (Tobias and von Koenigswald, 1964; Kurth, 1965; Campbell, 1965; Day, 1977; Howell, 1978; Rightmire, 1990; Tobias, 1991; Bilsborough, 1992; Antón, 2003; Grine, 2005; Ungar et al., 2006; Hawks et al., 2017), even though it has also been occasionally suggested to represent *Australopithecus* (Clarke, 2017) or *Paranthropus* (Wolpoff, 1970). Based on molar crown size and cusp proportions, SK 15 was also attributed to *Homo habilis* (Curnoe, 2006), then tentatively to the basket taxon *Homo gautengensis* (Curnoe, 2010), and similarities in mandible and tooth morphology with the Middle Pleistocene hominin, *Homo naledi*, from Rising Star Cave were also noted (Hawks et al., 2017; Kupczik et al., 2019). A recent analysis of the enamel-dentine junction (EDJ) of the M₂ and M₃ of SK 15 most closely resembles that of *Paranthropus* (Zanolli et al., 2022a). The taxonomy of many fossil hominin remains is

debated (for a review of the Early to Middle Pleistocene *Homo* record, see Antón and Middleton, 2023), but it is unusual for the same specimen to be attributed consecutively to three hominin genera (*Homo*, *Australopithecus*, *Paranthropus*, and excluding *Telanthropus*).

Morphological size has played a key role in the taxonomic attribution of SK 15, with a majority of authors attributing it to *Homo* because it shows smaller dimensions of the mandible and teeth than those of *Australopithecus* and *Paranthropus* (e.g., Robinson, 1961; Howell, 1978; Wood, 1991; Schwartz and Tattersall, 2003; Grine, 2005; Antón, 2012; Hawks et al., 2017). However, a number of aspects of SK 15 morphology differentiate it from penecontemporaneous *Homo* specimens. These include a wide extramolar sulcus (to an extent more generally found in australopiths), molar cusp proportions that generally differ from those of *H. habilis*, *H. erectus* s.l. and later *Homo*, a larger M₃ than the M₂, and a buccalward deflection of the M₃ distal root as seen in *P. robustus* (Wood, 1991; Grine, 2005; Curnoe, 2006; Grine et al., 2009).

We aim here to test competing hypotheses as to whether SK 15 belongs to *Homo*, *Australopithecus*, or *Paranthropus* through the application of microtomography and geometric morphometrics to examine three key and independent aspects of the morphology of the mandible and teeth. First, the shape of the dental arcade, which reflects the relative length and shape of the anterior and postcanine teeth and has been shown to distinguish between hominin taxa (Stelzer et al., 2018a, 2018b). Second, the shape of the mandibular symphysis, and in particular of the inferior part, which is regarded as a diagnostic trait for hominin taxonomy (Guy et al., 2008; Brown and Maeda, 2009). Third, the EDJ shape of the postcanine teeth, which is recognized as a reliable taxonomic proxy to discriminate between genera, species, and subspecies of hominids (e.g., Skinner et al., 2008, 2009a, 2016; Pan et al., 2017, 2020; Zanolli et al., 2018, 2022a, 2022b; Davies et al., 2020). Collectively, these three aspects of mandibular and dental morphology constitute a robust and novel assessment of the taxonomy of this important South African hominin specimen.

## 2. Material and methods

### 2.1. Material

The specimen SK 15 was scanned using the X-Tek (Metris) XT H225L industrial µCT system at the South African Nuclear Energy Corporation (Pelindaba, South Africa) according to the following parameters: 125-kV voltage, 60-µA current, an angular step of 0.36° over a scan angle of 360°. The final volume was reconstructed with an isotropic voxel size of 63.4 µm.

Using the commercial software Avizo v. 7.1 (FEI Visualization Sciences Group, Hillsboro) and the freeware ImageJ v. 1.52a (Schneider et al., 2012), a semiautomatic, threshold-based segmentation was carried out following the half-maximum height method (Spoor et al., 1993) and the region of interest thresholding protocol (Fajardo et al., 2002; Coleman and Colbert, 2007).

We analyzed here the shape of the dental arcade, mandibular symphysis and EDJ of the right M₂ (the left M₂ being damaged) and both M₃s of SK 15 in comparison with specimens representing *Australopithecus*, *Paranthropus*, and Early to Middle Pleistocene *Homo* (Supplementary Online Material [SOM] Table S1). The *Australopithecus* sample includes the species *Australopithecus anamensis*, *Australopithecus afarensis*, *Australopithecus deyiremeda*, *Australopithecus bahrelghazali*, *Australopithecus prometheus*, and *Australopithecus africanus*. The *Paranthropus* sample includes *Paranthropus aethiopicus*, *Paranthropus boisei*, and *P. robustus*. The Early to Middle Pleistocene *Homo* sample includes the species *H. habilis*, *H. ergaster*, *H. erectus* s.s., *Homo antecessor*, and *Homo heidelbergensis*.
SK 18a is an isolated mandibular premolar that Broom and Robinson (1952) consider to be a left P₃. It has been associated with SK 15 (Broom and Robinson, 1949; Robinson, 1953) and is generally attributed to *Homo* (Grine, 2005); however, Davies et al. (2020) and Zanolli et al. (2022a) found similarities in the morphology of the EDJ with *Paranthropus* and australopiths, respectively. As its association is not certain, we have chosen not to include it in this assessment of the taxonomic affiliation of SK 15. SK 43 is a worn and fragmented tooth crown that Tobias et al. (1977) and Brain (1981) listed as a right P₄ and indicate that it is associated with SK 15 and SK 18a. Our observations of the preserved morphology suggest it could also be a maxillary premolar, but its poor preservation prevents any comprehensive assessment of its taxonomic affiliation, and we have not included it in this study.

## 2.2. Generalized Procrustes analyses of mandible shape

Dental arcade shape was analyzed with bidimensional geometric morphometrics based on digital images of the mandibles in the occlusal view (with the occlusal surface of the teeth parallel to the observer). Due to some inferosuperior taphonomic deformations of the SK 15 mandible (SOM Fig. S1A), we made three reconstructions of the mandible to analyze the dental arcade shape. The first one (v1) was made by virtually cutting the mandible at the symphysis level and realigning the left dental arcade at the same level as the right one (SOM Fig. S1B). To evaluate the possible influence of displacement of some parts of the corpus due to breakages, we also generated two three-dimensional (3D) reconstructions of the mandible by segmenting individually the best preserved portions, realigning them, and mirroring them. The right anterior portion from the I₁ to P₃ was thus mirrored and aligned with the left portion of the corpus between the C₁ and P₄; then two parts of the left corpus between the M₁ and M₂, and between the M₃ and the ascending ramus were rotated and replaced in anatomical position (SOM Fig. S1C). Small gaps were filled, and the root sockets of the missing teeth were cleaned as much as possible (due to taphonomic damage, the internal bone structure is poorly preserved and the root sockets are not preserved entirely). The three reconstructions of the mandible are illustrated in the occlusal view in SOM Figure S1, and the 3D models of the reconstructions v2 and v3 are provided as SOM Files S1 and S2. Seventeen landmarks were placed either on the mesial and distal aspects of the preserved teeth or on the mesial and distal midpoints of the alveolar root sockets (SOM Fig. S1) using TPSDig v.2.0 (Rohlf, 2003). Before the generalized Procrustes analysis, the landmarks were symmetrized using reflected relabeling to discard asymmetric measuring error (Stelzer et al., 2018a, 2018b). To test the potential implication of tilting of the mandibles in the pictures, five views of the specimen Tighenif 1 were recorded, without and with marked tilting (SOM Fig. S2). The degree of tilting included in the test is visibly severe and is considered to be far greater than the potential tilting that could exist in the pictures collected in the literature for the comparative material. The seventeen landmarks were placed on the five views of the Tighenif 1 mandible, and after Procrustes superimposition with the 'Morpho' library v. 2.11 (Schlager, 2023) in R v. 4.2.3 (R Core Team, 2023), principal component analyses (PCAs) were conducted, showing that the distance between the nontilted and markedly tilted versions of Tighenif 1 are less distant to each other than the two most extreme specimens of the taxon (i.e., the distances between the Tighenif 1 versions are less than the intra-generic variation of *Homo*; SOM Fig. S3), suggesting that a slight tilting of the pictures collected in the literature would have a negligible impact on the analysis.

The shape of the symphysis was also investigated using a bidimensional geometric morphometric approach (the symphysis of SK

15 is not sufficiently well preserved to conduct 3D analyses of this structure like those of Zimmer et al., 2022). One hundred semilandmarks were placed in TPSDig along the outline of the symphysis of SK 15 (SOM Fig. S4) and comparative specimens beginning on the anterior superior margin of the cortical bone and moving toward the posterior superior margin. Only well-preserved symphyses were included in the analyses, and in case of minimal bone loss (local cracks and cortical bone chipping), missing regions were estimated by following the curve of the outline around the damaged part. The semilandmarks were allowed to slide along the curve with the R package 'Morpho,' and a generalized Procrustes analysis was then conducted.

## 2.3. Diffeomorphic surface matching of the enamel-dentine junction

We used a diffeomorphic surface matching (DSM) approach to analyze the EDJ conformation of the right M₂ (the left one being damaged) and left and right M₃ of SK 15. Diffeomorphic surface matching analyses capture the taxonomically relevant aspects of the EDJ morphology, including both prominent features (such as the dentine horns and marginal crests) and more subtle traits (such as the protostyloid and occlusal basin morphology) and thus improves on traditional (semi)landmark-based geometric morphometric analyses (Beaudet et al., 2016; Zanolli et al., 2018, 2022a, 2022b, 2023; Braga et al., 2019). This landmark-free, mesh-based approach relies on the construction of average surface models, and the difference between surfaces is interpreted as the amount of deformation needed to align them by using DSM (Glaunès and Joshi, 2006; Durrleman et al., 2014). The deformations between surfaces are mathematically modeled as smooth and invertible functions (i.e., diffeomorphisms).

For each dental position, the EDJ surfaces were decimated to 50,000 polygons, manually oriented, then superimposed using the rigid and uniform scale option of the 'Align Surfaces' module in Avizo. This was done by minimizing the root-mean-square distance between the points of each specimen to corresponding points on the reference surface using an iterative closest-point algorithm. We used the Deformetrica v. 4.3 software (Böne et al., 2018) to generate an atlas of surfaces. The method estimates an average object configuration or the mean shape from a collection of object sets (here, the EDJ surfaces) and computes the deformations from the mean shape to each specimen. In addition, a set of initial control points located near its most variable parts and a set of momenta parameterizing the deformations of the mean shape to each individual were estimated (Durrleman et al., 2012). For the DSM analysis of the M₂ EDJ, 2550 control points were generated, whereas 1440 were computed for that of the M₃ EDJ. The control points and momenta were then used to perform the statistical analyses to explore the EDJ shape variation and to classify the data. The output data were imported in R with the package 'RToolsForDeformetrica' v. 0.1 (Dumoncel, 2021). In addition to the southern African sample that was used by Zanolli et al. (2022a), the analyses of the EDJ of this study include material attributed to *A. afarensis* and *P. boisei* to have samples that are more comparable to the analyses of dental arcade and symphysis shape in terms of hominin diversity.

To test the potential influence of sexual dimorphism on the EDJ of the M₂s and M₃s, we ran geometric morphometric analyses comparing samples of male and females of *Gorilla gorilla* and *Homo sapiens* to *P. robustus* specimens (SOM Table S2). We then computed the Euclidian distances in the between-group PCA (bgPCA) and canonical variates analysis (CVA) morphospaces between all individuals within each genus. The expectation is that, if the EDJ shape of these molars is influenced by sexual dimorphism, gorillas
should be more variable than modern humans and that the degree of sexual dimorphism in *P. robustus* EDJs could be assessed within this frame.

Regressions of size (using the common logarithm of the EDJ area as a proxy for size) on shape were conducted.

## 2.4. Statistical analyses

Using the packages 'ade4' v. 1.7-6 (Dray and Dufour, 2007) and 'Morpho' for R, we first computed PCAs using the Procrustes tangent shape coordinates (for the dental arcade and mandible symphysis) and deformation moments (for the EDJs), followed by bgPCAs based first on the three groups assigned with equal prior probabilities (*Australopithecus*, *Paranthropus*, and Early to Middle Pleistocene *Homo*) and then based on two groups to distinguish between *Australopithecus* and *Paranthropus* (for the M₃ EDJ shape only, see Results section) and then between the species *P. boisei* and *P. robustus* (for the M₂ and M₃ EDJ shape, the only structures for which the comparative samples are large enough to conduct statistical analyses). The SK 15 specimen and the two *H. habilis* fossils (OH 7 and KNM-ER, 1802) were then projected a posteriori onto the bgPCA morphospace. We followed the recommendations of Cardini and Polly (2020) by computing the cross-validated bgPCA (cv-bgPCA) to assess the validity of the standard bgPCA group discrimination using the package 'Morpho.' We also performed Jackknife cross-validated CVAs using the same groups as for the bgPCA using the R package 'Morpho.' Since CVA computation requires the number of variables to be much smaller than the number of specimens, we computed the CVA based on a subset of the first principal component (PC) scores (between four and seven) showing the highest degree of correct classification (screening the correct classification results and selecting the minimum number of PC scores enabling to reach the optimum of correct classification; Hastie et al., 2009). This choice of the subset of PC scores is a compromise between including a sufficient proportion of overall shape variation and limiting the number of variables to avoid unrealistic and unstable levels of discrimination (Skinner et al., 2009a; Mitteroecker and Bookstein, 2011). We also assessed the structure of the data and results of the multivariate analyses to test that group separation is not spurious (Cardini and Polly, 2020). The amount of variance (r²) explained by group differences in the raw shape data, in both bgPCA and cv-bgPCA scores, and in the CVA scores was estimated by computing a permutational multivariate analysis of variance (1000 permutations) based on the Euclidean distance between the means and implementing a Holm correction using the R packages 'vegan' v. 2.6-4 (Oksanen et al., 2022) and 'pairwiseAdonis' v. 0.3 (Martinez Arbizu, 2020). Using 'Morpho,' for both bgPCA and CVA, we computed the typicality probabilities that SK 15 teeth belong to one of the comparative groups or if they are outliers (or potentially belong to a taxon not represented in the comparative sample).

# 3. Results

## 3.1. Dental arcade shape

The geometric morphometric analyses of dental arcade shape statistically discriminate between *Australopithecus*, *Paranthropus*, and Early-Middle Pleistocene *Homo* (Fig. 2A, B), and both bgPCA and CVA show no spurious group separation (SOM Fig. S5A and B; SOM Tables S3 and S4). As illustrated on the bgPCA and CVA plots, the three reconstructions of the dental arcade of SK 15 display a similar shape to that of *Paranthropus*, with a reduced anterior dentition and large postcanine teeth, that is distinguished from the U-shaped arcade with mesiodistally elongated teeth typical of

*Australopithecus* and from the more parabolic tooth row with reduced dental crowns seen in Early to Middle Pleistocene *Homo* (Fig. 2A, B). SK 15 reconstructions also differ from the *Australopithecus*-like elongated and narrow mandible of *H. habilis*. The three reconstructions of SK 15 dental arcade show proportionally small anterior teeth with larger postcanine dentition as in *Paranthropus* and are statistically classified as belonging to the latter genus (Table 1).

## 3.2. Symphysis shape

The bgPCA and CVA analyses of symphysis shape distinguish well between Early-Middle Pleistocene *Homo* and the australopiths (Fig. 2C, D), but *Australopithecus* and *Paranthropus* substantially overlap (SOM Fig. S5C and D; SOM Tables S3 and S4). In fact, *Australopithecus* and *Paranthropus* are statistically indistinguishable in symphysis shape (SOM Table S3), precluding any reliable distinction between the latter two genera and suggesting that the result for SK 15 should be interpreted with caution. The SK 15 symphysis is proportionally thicker than in *Homo*, similar to that seen in the australopiths and in *H. habilis* (Fig. 2C, D) and is statistically attributed to *Australopithecus* (Table 1). It does not exhibit the marked superior torus that is typical of some *Paranthropus* and *Australopithecus* specimens, but this trait is also highly variable among specimens of each genus.

## 3.3. Molar enamel-dentine junction shape

Results of the geometric morphometric analyses of the EDJ of SK 15 M₂ and M₃s compared with *Australopithecus*, *Paranthropus*, and Early to Middle Pleistocene *Homo* are illustrated in Figure 3. The three genera are quite well distinguished, even if one *Australopithecus* M₂ (StW 534) falls at the edge of the Early-Middle Pleistocene *Homo* molar sample in the cross-validated analyses, and a few *Paranthropus* and *Australopithecus* M₂s and M₃s overlap in the cross-validated results (SOM Fig. S6; SOM Tables S3 and S4). The EDJs of SK 15 molars fall far from *H. erectus* s.l. and Middle Pleistocene *Homo*, which is characterized by a relatively taller EDJ with a rounder occlusal outline and proportionally wider occlusal basin, and from *Australopithecus* and *H. habilis*, which exhibit a more laterally developed protostylid. SK 15 is closest to the waisted EDJ with a V-shaped lingual marginal crest in the occlusal view and more mesially placed entoconid typical of *Paranthropus* (Fig. 3). However, while the right M₂ and left M₃ are indeed statistically classified into the latter genus in both bgPCA and CVA, the right M₃ is attributed to *Australopithecus* in the bgPCA and to *Paranthropus* in the CVA (Table 1). Considering the ambiguous signal provided by the M₃s of SK 15, the same analysis was repeated but without *Homo*. Results of the analysis comparing SK 15 M₃s to *Australopithecus* and *Paranthropus* show that the two genera are discriminated (SOM Tables S5 and S6) and that the two M₃s of the Swartkrans specimens display highest typicality probabilities for an attribution to *Paranthropus* (Table 2). Finally, the geometric morphometric analyses of the EDJ shape comparing SK 15 with *P. boisei* and *P. robustus* separate well the two species (SOM Tables S7 and S8), and SK 15 molars show typicality probabilities lower than 0.05 for most analyses (except for the right M₃ that is at the limit of the range of variation of *P. robustus*; Table 3), indicating that the specimen is significantly different from the two *Paranthropus* species.

Euclidian distances between the EDJ shape of the M₂s and M₃s of *P. robustus* and of males and females of *G. gorilla* and *H. sapiens* show comparable ranges of distribution between taxa (SOM Fig. S7).
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Bivariate plots of the between-group principal component 2 (bgPC2) against the between-group principal component 1 (bgPC1; (A) and of canonical variate 2 (CV2) against canonical variate 1 (CV1; (B) based on the landmark analysis of the dental arcade shape, and bivariate plots of bgPC2 against bgPC1 (C) and CV2 against CV1 (D) of the symphysis shape analysis. The wireframes at the end of each axis illustrate the extreme shapes along each component. Middle Pleistocene *Homo* specimens are highlighted as filled triangles. (For interpretation of the references to color in this figure, the reader is referred to the Web version of this article).</figcaption></figure>

**Table 1**

Typicality probabilities based on the between-group principal component analyses (bgPCAs) and canonical variate analyses (CVAs) of the geometric morphometric analyses of the dental arcade, mandible symphysis, and enamel-dentine junction (EDJ) shape of SK 15 compared with *Australopithecus*, *Paranthropus*, and Early to Middle Pleistocene *Homo*.

<table><thead><tr><th rowspan="2">Structure</th><th colspan="2">Australopithecus</th><th colspan="2">Paranthropus</th><th colspan="2">Homo</th></tr><tr><th>bgPCA</th><th>CVA</th><th>bgPCA</th><th>CVA</th><th>bgPCA</th><th>CVA</th></tr></thead><tbody><tr><td>Dental arcade (v1)</td><td>0.028</td><td>0.013</td><td><strong>0.189</strong></td><td><strong>0.521</strong></td><td>&lt;0.001</td><td>&lt;0.001</td></tr><tr><td>Dental arcade (v2)</td><td>0.019</td><td>0.005</td><td><strong>0.078</strong></td><td><strong>0.173</strong></td><td>0.010</td><td>0.003</td></tr><tr><td>Dental arcade (v3)</td><td>0.011</td><td>0.002</td><td><strong>0.239</strong></td><td><strong>0.381</strong></td><td>0.002</td><td>&lt;0.001</td></tr><tr><td>Mandible symphysis</td><td><strong>0.378</strong></td><td><strong>0.252</strong></td><td>0.004</td><td>0.003</td><td>0.003</td><td>0.007</td></tr><tr><td>Right M<sub>2</sub> EDJ</td><td>0.077</td><td>&lt;0.001</td><td><strong>0.154</strong></td><td><strong>0.840</strong></td><td>&lt;0.001</td><td>&lt;0.001</td></tr><tr><td>Left M<sub>3</sub> EDJ</td><td>0.069</td><td>0.098</td><td><strong>0.758</strong></td><td><strong>0.903</strong></td><td>&lt;0.001</td><td>0.001</td></tr><tr><td>Right M<sub>3</sub> EDJ</td><td><strong>0.300</strong></td><td>0.375</td><td><strong>0.299</strong></td><td><strong>0.488</strong></td><td>&lt;0.001</td><td>&lt;0.001</td></tr></tbody></table>

# 4. Discussion

## 4.1. Considerations on the results of the present study

The results of this study including analyses of the shape of the dental arcade, mandibular symphysis and EDJ of the molars confirm previous results (Zanolli et al., 2022a) and demonstrate that SK 15 resembles the australopith condition more so than that of *H. erectus/H. ergaster* and/or Middle Pleistocene *Homo*. Across the various analyses conducted of SK 15, *Australopithecus*, *Paranthropus*, and Early to Middle Pleistocene *Homo*, SK 15 is most consistently classified (i.e., based on 11 of 14 of the geometric morphometric analyses) as a *Paranthropus* specimen (Table 1).

The results of the geometric morphometric analysis show that symphysis shape reliably discriminates only between *H. erectus* s.l. and the australopiths (including SK 15) but not between

<footnote>ª The highest probabilities (above statistical threshold of 0.05) are highlighted in bold for each analysis.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Bivariate plots of the between-group principal component 2 (bgPC2) against the between-group principal component 1 (bgPC1; (A) and of canonical variate 2 (CV2) against canonical variate 1 (CV1; (B) based on the diffeomorphic surface matching analyses of the enamel-dentine junction (EDJ) of the M<sub>2</sub>s, and bivariate plots of bgPC2 against bgPC1 (C) and CV2 against CV1 (D) of the M<sub>3</sub> EDJ shape analysis. The surfaces at the end of each axis illustrate the extreme shapes along each component. Middle Pleistocene *Homo* specimens are highlighted as filled triangles. (For interpretation of the references to color in this figure, the reader is referred to the Web version of this article).</figcaption></figure>

**Table 2**

Typicality probabilities based on the between-group principal component analyses (bgPCAs) and canonical variate analyses (CVAs) of the geometric morphometric analyses of the enamel-dentine junction (EDJ) shape of SK 15 M<sub>3</sub>s compared with *Australopithecus* and *Paranthropus*.<sup>a</sup>

<table><thead><tr><th rowspan="2">Tooth</th><th colspan="2">Australopithecus</th><th colspan="2">Paranthropus</th></tr><tr><th>bgPCA</th><th>CVA</th><th>bgPCA</th><th>CVA</th></tr></thead><tbody><tr><td>Left M<sub>3</sub></td><td>0.032</td><td>0.058</td><td><strong>0.785</strong></td><td><strong>0.610</strong></td></tr><tr><td>Right M<sub>3</sub></td><td>0.110</td><td>0.177</td><td><strong>0.411</strong></td><td><strong>0.293</strong></td></tr></tbody></table>

*Australopithecus* and *Paranthropus* (see also Zimmer et al., 2022). As indicated by the cross-validated results of both bgPCA and CVA, there is substantial overlap and no statistical difference between *Australopithecus* and *Paranthropus* (SOM Fig. S5; SOM Table S3), suggesting that SK 15 is compatible with an australopith morphology rather than that of *Homo*. Indeed, the symphysis of SK 15 is thick, with a marked

**Table 3**

Typicality probabilities based on the between-group principal component analyses (bgPCAs) and canonical variate analyses (CVAs) of the geometric morphometric analyses of the enamel-dentine junction (EDJ) shape of SK 15 molars compared with *Paranthropus boisei* and *Paranthropus robustus*.<sup>a</sup>

<table><thead><tr><th rowspan="2">Tooth</th><th colspan="2">Paranthropus boisei</th><th colspan="2">Paranthropus robustus</th></tr><tr><th>bgPCA</th><th>CVA</th><th>bgPCA</th><th>CVA</th></tr></thead><tbody><tr><td>Right M<sub>2</sub></td><td>&lt;0.001</td><td>0.024</td><td>0.039</td><td>0.039</td></tr><tr><td>Left M<sub>3</sub></td><td>&lt;0.001</td><td>&lt;0.001</td><td>&lt;0.027</td><td>0.026</td></tr><tr><td>Right M<sub>3</sub></td><td>&lt;0.001</td><td>&lt;0.001</td><td><strong>&lt;0.080</strong></td><td><strong>0.061</strong></td></tr></tbody></table>

medial buttressing and a rounded base similar to some *Australopithecus* specimens but lacking the marked inferior and superior tori generally seen in *P. boisei* and *P. robustus* (SOM Fig. S4).

In the bgPCA analysis of the M<sub>3</sub> EDJ including the three hominin genera, the right antimere of SK 15 shows an intermediate signal between *Australopithecus* and *Paranthropus*, whereas the left

<footnote><sup>a</sup> The highest probabilities (above statistical threshold of 0.05) are highlighted in bold for each analysis.</footnote>

<footnote><sup>a</sup> The highest probabilities (above statistical threshold of 0.05) are highlighted in bold for each analysis.</footnote>
antimere is clearly within the range of *Paranthropus* (Fig. 3C, D; Table 1). However, in the analysis focusing on the comparison between *Australopithecus* and *Paranthropus*, the EDJ of both M2s is statistically attributed to *Paranthropus* (Table 2). The slight difference in signal with the analyses published by Zanolli et al. (2022a)—in which only one species of *Paranthropus* was included (*P. robustus*) and the EDJs of SK 15 molars were attributed to *Paranthropus*—and the present study stems from the addition of more species of *Australopithecus* (*A. afarensis*) and *Paranthropus* (*P. boisei*), which increased the variation of the morphospace and reduced intergeneric differences. In any case, SK 15 molars fall far from *H. erectus* s.l. both in Zanolli et al. (2022a) analyses and in the current study (Fig. 3; Table 1).

A few potential limitations should be acknowledged. Due to taphonomic breaks and displacements of several portions of the mandible corpora (but without noticeable plastic deformation), the analysis of dental arcade shape relies on three different reconstructions. All three reconstructions, with different degrees of realignment of the mandible fragments (SOM Fig. S1), fall close to each other in the geometric morphometric analyses (Fig. 2A and B). While the original morphology of the specimen cannot be known with certainty, the different reconstructions represent a possible range of variation of SK 15. Additionally, the sample of early *Homo* specimens currently available to the authors of this work is limited, but 1) it includes the type specimens of *H. ergaster* (KNM-ER 992) and *H. habilis* (OH 7) and 2) SK 15 still substantially differs in dental arcade and EDJ shape from representative specimens of *H. habilis*, *H. ergaster*, *H. erectus* s.s., *H. antecessor*, and *H. heidelbergensis*. The future inclusion of additional early *Homo* samples should help better characterizing the range of variation for *H. habilis* and the position of this species with respect to *Australopithecus* (cf. Davies et al., 2024), but it is unlikely to change the taxonomic interpretations regarding SK 15.

## 4.2. Taxonomic revision of SK 15

Compared with *H. habilis*, the dental arcade of SK 15 is not U-shaped but is more rounded, and the mandible symphysis does not show any buttressing (Spoor et al., 2015; Table 4). Even if compatible in overall size with *Homo* (Robinson, 1961; Grine, 2001), the quantitative analyses of mandible and dental shape show that SK 15 is significantly distinguished from *H. erectus/H. ergaster* and Middle Pleistocene *Homo* (Figs. 2 and 3; Tables 1 and 4). In addition, the EDJs of the M2s and M3s of SK 15 differ from those of early *Homo* by the lack of lingual accessory cusp (that is present in all of the *H. habilis* molars and most of those of *H. erectus* s.l. investigated so far; Davies et al., 2021), a wider occlusal basin with respect to crown base, and a more internally positioned metaconid than the entoconid (Davies et al., 2021: Supplementary File). At the EDJ level, the protostylid forms a short mesiobuccal shelf, a morphology that is not recorded in early *Homo* molars (Davies et al., 2021; Zanolli et al., 2022a; Table 4). The roots of SK 15 molars show smaller dimensions and different proportions than those of *H. habilis* and *H. erectus* s.l. (Kupczik et al., 2019), with one exception: SK 45, was found to show root size and proportions similar to those of SK 15 (Kupczik et al., 2019), but the former specimen is in fact much more hypotaurodont than the latter (SOM Fig. S8; Table 4).

Previously published data show that SK 15 substantially differs from *Australopithecus* in corpus shape (corpus shape index at M₁ is 68.9 for SK 15, whereas it is of 60.8 on average for 16 *Australopithecus* specimens from eastern and southern Africa; Wood, 1991; Grine, 2005) and resembles the proportionally broader and lower corpus of *Paranthropus* (corpus shape index at M₁ is 69.1 on average for 39 *Paranthropus* specimens from eastern and southern Africa; Wood, 1991; Table 4). However, despite this similarity in

corpus proportions, SK 15 was showed to differ from *P. robustus* in mandible bone biomechanics (including general bending rigidity, bending and torsional strength, corpus shape; Daegling et al., 2016). The EDJ of the molars of SK 15 exhibits interconulid type (on the M₁ and M₃) and hypoconulid type (on the M₂) distal accessory cusps, a combination of features that is highly frequently found in *Paranthropus*, and less common in *Australopithecus* (and rare in African early *Homo*; Davies et al., 2021; Table 4). The absence of lingual accessory cusp in SK 15 molars is most similar to the condition of *Paranthropus* (Table 4), whereas *Australopithecus* more commonly shows expression of this trait (Davies et al., 2021). The M₁ and M₂ of SK 15 show hypotaurodont roots (i.e., root bifurcation is close to the crown, as quantified by a volumetric bifurcation index between 25.0 and 49.9%; Fig. 4), as in *Paranthropus*, whereas *Australopithecus* mostly exhibits mesotaurodont M₁s and M₂s (i.e., root bifurcation is lower below the crown than in hypotaurodont, with volumetric bifurcation index values of 50.0 and 74.9%; Kupczik et al., 2019). However, the SK 15 M₃ is mesotaurodont, as in *Australopithecus* (Kupczik et al., 2019), a condition that likely results from its short roots (Table 4). SK 15 also exhibits a buccally deflected distal M₃ root relative to the mesial root, a feature that is also expressed in *P. robustus* but absent in *P. boisei* and generally in *A. africanus* (also absent in *Homo*, except in *H. naledi*; Kupczik et al., 2019; Table 4).

Based on the present study, the most parsimonious hypothesis is that SK 15 belongs to *Paranthropus*, and we thus reassign the specimen to the latter genus. From a biological point of view, the presence of caries in SK 15 was interpreted as evidence of sugar consumption in early *Homo* (Towle et al., 2021), but the reallocation of this specimen to *Paranthropus* is also more in line with the frequency of this pathology in the latter genus.

Consequently, the question of its specific attribution needs to be investigated. Considering that SK 15 exhibits smaller dimensions and unusual morphological features compared with specimens usually attributed to *P. robustus* and *P. boisei*, two alternative hypotheses can be formulated regarding species attribution: 1) the specimen can be attributed to one of the currently recognized species of *Paranthropus* (*P. aethiopicus*, *P. boisei*, or *P. robustus*); 2) it represents another, previously unrecognized, species of *Paranthropus*. In most analyses of the dental arcade, symphysis, and EDJ shape, SK 15 falls at the limit of the *Paranthropus* variation (represented by the species *P. aethiopicus*, *P. boisei* and *P. robustus* for dental arcade and symphysis shape, and by the latter two species for EDJ shape; Figs. 2 and 3). Corpus height and breadth of SK 15 at the M₁ level are statistically lower than in *P. boisei* and *P. robustus*, whereas corpus shape index is statistically within the range of both species of *Paranthropus* (SOM Table S9). In addition, SK 15 molars are three standard deviations smaller than those of *P. boisei* and *P. robustus*, the crown diameters being statistically smaller than in the latter two species (Wood, 1991; Curnoe, 2006; SOM Table S9). The EDJ area of SK 15 M₂ (257.6 mm²) is in the low end of the range of values of *P. robustus* (n = 22; range = 252.6–445.5 mm²) and well below that of *P. boisei* (n = 6; range = 290.0–518.3). The EDJ areas of the M₃s (left = 230.1 mm²; right = 233.1 mm²) are also well below the ranges of both *P. robustus* (n = 25; range = 265.1–435.8 mm²) and *P. boisei* (n = 5; range = 415.0–624.1 mm²). When the EDJ shape of SK 15 molars is compared with those of *P. boisei* and *P. robustus* specimens, it is statistically outside the range of the latter two species, except for the left M₃ in the CVA that shows a low typicality probability value compatible with *P. robustus* (Table 3). Regressions of size on shape of the EDJ analyses also show that SK 15 molars are outside the variation of *P. robustus* and *P. boisei*, the EDJ of the M₃s being substantially smaller than in both species (SOM Fig. S9). The M₃s of SK 15 do not exhibit the extreme degree of distal tapering seen in most *P. robustus* M₃s from
<caption>**Table 4**</caption>

List of the main diagnostic traits of SK 15 compared with *Paranthropus robustus*, *Paranthropus boisei*, *Homo habilis*, and *Homo erectus* s.l.

<table><thead><tr><th>Feature</th><th>SK 15</th><th>Paranthropus robustus</th><th>Paranthropus boisei</th><th>Homo habilis</th><th>Homo erectus s.l.</th></tr></thead><tbody><tr><td>Dental arcade shape</td><td>Rounded and elongated</td><td>Rounded and elongated</td><td>Rounded and elongated</td><td>U-shaped and elongated</td><td>Rounded and short</td></tr><tr><td>Symphysis shape</td><td>Low and thick, no torus</td><td>Low and thick, with frequent presence of superior and inferior tori</td><td>Low and thick, tori variably present</td><td>Low and thick, superior torus rarely present, inferior torus absent</td><td>Tall and thin, tori occasionally present</td></tr><tr><td>Corpus proportions<sup>a</sup></td><td>Low and thick</td><td>Low and thick</td><td>Low and thick</td><td>Generally tall and moderately thick</td><td>Tall and thin</td></tr><tr><td>Molar crown dimensions<sup>b</sup></td><td>Moderate</td><td>Very large</td><td>Very large</td><td>Moderate</td><td>Small to moderate</td></tr><tr><td>Protostylid at OES<sup>c</sup></td><td>Absent in the M<sub>1</sub> and M<sub>2</sub>, and short shelf in the M<sub>3</sub></td><td>Forming a pit or a shelf</td><td>Variably expressed (absent to extended)</td><td>Variably expressed (absent to moderately developed)</td><td>Variably expressed (absent to moderately developed)</td></tr><tr><td>Protostylid at the EDJ<sup>c</sup></td><td>Short shelf</td><td>Shelf with extended fovea(e) enclosed by an elongated buccal crest</td><td>Fovea or pit (rarely forming a shelf)</td><td>Fovea or pit</td><td>Generally absent, but sometimes forming a pit and/or digitations</td></tr><tr><td>Distal accessory cusp on the M<sub>1</sub> EDJ<sup>c</sup></td><td>Present</td><td>Frequently present</td><td>Present</td><td>Absent</td><td>Absent in African specimens, but frequently present in Asian M<sub>1</sub>s</td></tr><tr><td>Lingual accessory cusp on the M<sub>1</sub> EDJ<sup>c</sup></td><td>Absent</td><td>Absent</td><td>Absent</td><td>Systematically present</td><td>Frequently present</td></tr><tr><td>Root proportions<sup>d</sup></td><td>Short and hypotaurodont in the M<sub>1</sub>s and M<sub>2</sub>s, short and mesotaurodont in the M<sub>3</sub>s</td><td>Long and hypotaurodont in all molars</td><td>Long in the M<sub>1</sub>s and M<sub>2</sub>s, short in the M<sub>3</sub>s, hypotaurodont in all molars</td><td>Short and hypotaurodont in all molars</td><td>Long and hypotaurodont to mesotaurodont</td></tr><tr><td>Buccally deflected M<sub>3</sub> distal root<sup>d</sup></td><td>Present</td><td>Present</td><td>Absent</td><td>Absent</td><td>Absent</td></tr></tbody></table>

EDJ = enamel-dentine junction; OES = outer enamel surface.

Swartkrans (e.g., Zanolli et al., 2022a: Fig. S8), including in specimens from the same chronostratigraphic Member 2 such as SKX 5014 (Fig. 4). In fact, the molars of SK 15 differ morphologically (both at the outer enamel surface and EDJ) from specimens attributed to *P. robustus* by the relative position of the cusps, the weakly expressed and mesiobuccally restricted protostylid, the absence of accessory crests, the absence of buccal wrinkling (especially visible in *P. robustus* M3s), and the proportionally shorter pulp chamber, especially in M1s and M2s (Fig. 4). Finally, these differences cannot simply be explained by the smaller dimensions of SK 15 molars, especially at the EDJ level as all of the *P. robustus* molars investigated in this study, from the smallest to the largest (SOM Fig. S10), have an extended protostylid on the buccal side (most of the time between the protoconid and hypoconid dentine horns but also sometimes on the buccal aspect of the hypoconid and hypoconulid, with a variable degree of buccal ridge development), a generally more complex occlusal crest pattern and a more distally positioned buccolingual waisting. In particular, the specimens DNH 60 and DNH 21, which approximate SK 15 in crown dimensions (Moggi-Cecchi et al., 2010), show the same morphological traits as larger specimens from Swartkrans and Drimolen (SOM Fig. S10), with an extended protostylid, which is typical of the species *P. robustus* (Skinner et al., 2009b).

A recent analysis of the EDJ of the premolar SK 18a showed that it displays a molarized morphology that is more compatible with that of the australopitons (Zanolli et al., 2022a). While there is no evidence that SK 15 and SK 18a belong to the same individual (the root sockets of the premolars are too damaged in SK 15 to ascertain it and the stratigraphic association is unclear), considering the signal provided by the analyses of the EDJ shape, the two specimens could belong to the same taxon.

For SK 15 to belong to *P. robustus* (or even less likely *P. aethiopicus* or *P. boisei*), it would imply that it is an extreme outlier of the species, differing both in size and morphology from the latter. Such differences (especially in shape of the molars) have not been reported so far within a single taxon. In fact, even extant *Gorilla*, which shows the largest degree of sexual dimorphism among extant hominoids (both in size and morphology of the canines and premolars), does not show significant differences in molar shape between males and females (Uchida, 1998). The EDJ of male and female *Gorilla* M2 and M3 is highly similar in terms of overall shape, cingulid development, crest pattern, and even in size (SOM Fig. S11), and our quantitative analyses show that the EDJ shape of male and female gorillas and modern humans vary similarly (SOM Fig. S7). Considering that *G. gorilla* is the most sexually dimorphic of the extant hominids (Taylor, 1997), that it is the opposite for *H. sapiens* (Plavcan, 2001), and that *P. robustus* exhibits a comparable range of variation as the two extant hominid species, just as for the external crown morphology (Uchida, 1998), sexual dimorphism does not appear to be a factor increasing the variation of the EDJ shape in hominid molars. Sexual dimorphism is thus unlikely to explain the marked differences in shape and size between SK 15 and specimens of *P. robustus* (even with those from Member 2 where SK 15 was found; Fig. 4). In addition, the EDJ of SK 15 molars differs more in size with *P. robustus* than between *P. robustus* and *P. boisei*; it shows an absence of extended protostylid as in *P. boisei* but an overall EDJ shape that resembles more that of *P. robustus* (SOM Fig. S12). Considering that the differences between the EDJ of SK 15 molars and those of the two *Paranthropus* species exhibit a similar degree as between those of *H. habilis* and *H. erectus*, and even exceed those seen between *A. afarensis* and *A. africanus* (SOM Fig. 11), and taking into account the other

<footnote>a Based on the corpus shape index (cf. Wood, 1991; Grine, 2005).</footnote>

<footnote>b Based on mesiodistal and buccolingual diameters from Wood (1991).</footnote>

<footnote>c Based on Davies et al. (2021: Supplementary Information).</footnote>

<footnote>d Based on Kupczik et al. (2019).</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 4.** The crown and roots of SK 15 molars and of *Paranthropus robustus* specimens (SKX 4446 and SKX 5014) from Swartkrans Member 2 are shown in the buccal view, and their enamel-dentine junction (EDJ) and pulp cavity are shown in occlusal (occ) and buccal (bucc) views. The root apex of SKX 4446 M₂ is not completely formed, and the estimated missing portion is reconstructed in semitransparent yellow. (For interpretation of the references to color in this figure, the reader is referred to the Web version of this article).</figcaption></figure>

differences with *P. robustus* measured in mandible size (in particular with the corpus shape index) and tooth size (SOM Table S9), dental arcade and symphysis shape (Fig. 3), and molar morphology and EDJ shape (even if the left M₃ is at the limit of the range of variation of *P. robustus*; Table 3; Fig. 4), it is highly unlikely that SK 15 represents an outlier individual of *P. robustus*. Therefore, it is reasonable to assign SK 15 to another species of *Paranthropus*, likely closely related to *P. robustus* but showing a more gracile morphology. The species *Paranthropus crassidens*, now synonymized with *P. robustus*, could be considered, but the type specimen SK 6 is included in the present study, and it markedly differs from SK 15 by having much larger molar crowns, an extended protostylid, and different dentine horn positions (SOM Fig. S10). The recently erected subspecies *Paranthropus robustus ukusa* from Drimolen, while showing slightly smaller postcanine teeth than those of *Paranthropus robustus robustus* (Moggi-Cecchi et al., 2010; Martin et al., 2024), shares a number of features with the latter and in particular an extensive protostylid that is absent in SK 15 molars (SOM Fig. S10). Statistical analysis of the EDJ shape show that SK 15 differs from *P. robustus* (including specimens from Drimolen referred to *P. robustus ukusa* and from Kromdraai and Swartkrans

allocated to *P. robustus robustus* by Martin et al., 2024; Table 3). Considering that SK 15 was originally attributed to *Telanthropus capensis* (Broom and Robinson, 1949), and that the species is potentially valid and available (Reed et al., 2023), the specific epithet *capensis* has priority, and SK 15 should thus be allocated to *Paranthropus capensis*, being the holotype of the species. The following features can be listed as diagnostic features of *P. capensis* based on SK 15: jaw and teeth of smaller size than those of *P. robustus*, *P. boisei*, and *P. aethiopicus*; mandibular corpus proportionally lower and thicker than in *Homo* and *Australopithecus* but in the range of *Paranthropus*; molar crowns with a relatively simple occlusal morphology (without an extended protostylid, lateral wrinkling, or accessory crests at the outer enamel surface and EDJ levels); a less tapering M₃ than in *P. robustus* but more than in *P. boisei*; relatively shorter and stouter molar roots than in *P. robustus* and resembling the rounded root apex of *P. boisei*; and a buccally deflected M₃ distal root.

These traits could be used to identify other remains belonging to *P. capensis*, but a revision of the southern African paleoanthropological assemblages would be needed beforehand as it is too premature to formally assign other specimens to the hypodigm for
now. However, a recent paleoproteomic analysis revealed a high degree of morphological and proteomic variation among four *Paranthropus* dental specimens from the 2-Ma Member 1 of Swartkrans, suggesting based on the absence of AMELY proteins that some of the largest teeth likely belong to female individuals, whereas smaller ones (like SK 835) are males and hinting at the possible presence of two species within the sample (Madupe et al., 2023). The M³ SK 835 shows smaller dimensions (Dean et al., 2020) and a different shape of the EDJ than most specimens from Swartkrans and TM 1517, the holotype of *P. robustus*. SK 835 also displays a different proteomic signature than three *P. robustus* teeth (Madupe et al., 2023), and could potentially belong to *P. capensis*.

The previously noted morphological similarities between SK 15 and the dentognathic material attributed to *H. naledi* are also intriguing (Hawks et al., 2017; Kupczik et al., 2019; Delezene et al., 2023). In addition, the EDJ of *H. naledi* premolars was shown to resemble the shape of *P. robustus* (Davies et al., 2020). Further analyses of the Rising Star Cave hominins, and in particular of the EDJ of the molars, will be needed to further test possible relationships with SK 15 and with *Paranthropus*.

## 5. Conclusions

Southern Africa hosted a large diversity of hominins during the Late Pliocene to the Early Pleistocene, including three species of *Australopithecus* (*A. prometheus*, *A. africanus*, *A. sediba*; Berger et al., 2010; Clarke and Kuman, 2019), at least two species of *Paranthropus* (*P. robustus* and the Gondolin molar GDA 2 identified as belonging to *P. boisei*; Skinner et al., 2018, in press), and possibly two species of early *Homo* (the presence of *H. habilis* being unclear, and *H. erectus* s.l.; Clarke, 2017; Zanolli et al., 2022a). In addition, the taxonomic diversity within southern African *Paranthropus* is debated (Braga et al., 2022; Zanolli et al., 2022a; Leece et al., 2023; Madupe et al., 2023; Martin et al., 2024). Martin et al. (2024) proposed that the Drimolen fossils represent a subspecies of *P. robustus*, *P. robustus ukusa*, which differs from the material from other sites that would belong to *P. robustus robustus*. The presence of two subspecies of *P. robustus* in contemporaneous sites of the Cradle is intriguing, and the (sub)species diversity of *Paranthropus* in southern Africa deserves to be investigated further.

The results of the present study, combined with previously reported evidence on the morphology of SK 15, point to an attribution of the specimen to the genus *Paranthropus*. Considering that the mandible and teeth are substantially smaller and that some nonmetric features differ from *P. boisei* (and *P. aethiopicus*) and *P. robustus* (Wood, 1991; Curnoe, 2006; Grine, 2005; Table 4), which also explains why the specimen was lumped into *Homo* for decades, it appears evident that SK 15 represents another species of *Paranthropus* that was previously unrecognized, *P. capensis*. The species very likely split from *P. robustus* before 2 Ma at a time when this taxon showed a variable degree of size and robustness (Martin et al., 2020). The presence of other *P. capensis* specimens mixed among the current hypodigm of *P. robustus* should be tested further, in particular by revising the fossil assemblages from Swartkrans, Drimolen, and Kromdraai that include most of the *Paranthropus* specimens from southern Africa.

## Acknowledgments

For their contribution with scanning, granting access to, and/or sharing data of some of the specimens included in this study, we acknowledge C. Argot, F. Bernardini, J. Braga, G. Chinamatira, F. de Beer; D. Grimaud-Hervé, J. Hoffman, K. Jakata, J. Kibii, E. L'Abbé, H. Lelièvre, R. Macchiarelli, L. Mancini, A. Mazurier, E. Mbua, A.C. Oettlé, L. Pan, S. Potze, F. Sémah, C. Tuniz, H. Widianto, B. Zipfel, as

well as the online platform European Synchrotron Radiation Facility heritage database for paleontology, evolutionary biology, and archaeology (paleo.esrf.eu). We also deeply thank M.M. Skinner and T.W. Davies for providing access to comparative enamel-dentine junction models of hominin molars. For technical support and access to tomographic scans and associated digital data, we thank the following institutions: University of the Witwatersrand (Johannesburg), Ditsong National Museum of Natural History (Pretoria), National Museum of Kenya (Nairobi), the Ethiopian Heritage Authority (Addis Ababa), National Museum of Tanzania (Dar es Salaam), Max Planck Institute for Evolutionary Anthropology (Leipzig), Centre de Microtomographie Université de Poitiers, South African Nuclear Energy Corporation SOC Ltd. (Pelindaba), University of Pretoria (Pretoria) and International Centre for Theoretical Physics (Trieste). We thank the South African Heritage Resources Agency for authorizing the study of South African fossil material. For scientific discussion and comments, we thank J. Braga, L. Bruxelles, R.J. Clarke, T.W. Davies, F. Détroit, J. Dumoncel, J.-B. Fourvel, F.E. Grine, R. Macchiarelli, L. Pan, S. Schlager, M.M. Skinner, D. Stratford, D. Strait, F. Thackeray, A. Urciuoli, B. Zipfel and B.A. Wood. We acknowledge support from the National Center for Scientific Research (CNRS)/IN2P3 Computing Center (Lyon, France) for providing computing and data processing resources needed for this work. This research was funded by Erasmus+ Programme of the European Union Bakeng se Afrika, French CNRS, AFRIQUE team of the PACEA Laboratory (UMR 5199), University of Bordeaux, AESOP + program, National Research Foundation, Max Planck Society. This study received financial support from the French government in the framework of the University of Bordeaux's IdEx "Investments for the Future" program/GPR "Human Past" and from the Bakeng se Afrika project. Finally, we sincerely thank the Editor-in-Chief (A. Taylor), the Associate Editor, and four anonymous reviewers for their comments and suggestions that helped us improve a previous version of this paper.

## Author contributions

**Clément Zanolli:** Writing – original draft, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Jean-Jacques Hublin:** Writing – review & editing, Funding acquisition. **Ottmar Kullmer:** Writing – review & editing, Resources, Methodology, Investigation. **Friedemann Schrenk:** Writing – review & editing, Resources, Investigation. **Lazarus Kgasi:** Writing – review & editing, Resources, Data curation. **Miriam Tawane:** Writing – review & editing, Resources, Data curation. **Song Xing:** Writing – review & editing, Resources, Investigation, Data curation.

## Supplementary Online Material

Supplementary Online Material to this article can be found online at https://doi.org/10.1016/j.jhevol.2024.103634.

## References

* Antón, S.C., 2003. Natural history of *Homo erectus*. Yearb. Phys. Anthropol. 46, 126–170.

* Antón, S.C., 2012. Early *Homo*: Who, when, and where. Curr. Anthropol. 53, S278–S298.

* Antón, S.C., Middleton, E.R., 2023. Making meaning from fragmentary fossils: Early *Homo* in the Early to early Middle Pleistocene. J. Hum. Evol. 179, 103307.

* Balter, V., Blichert-Toft, J., Braga, J., Telouk, P., Thackeray, F., Albarède, F., 2008. U–Pb dating of fossil enamel from the Swartkrans Pleistocene hominid site, South Africa. Earth Planet. Sci. Lett. 267, 236–246.

* Beaudet, A., Dumoncel, J., Thackeray, J.F., Bruxelles, L., Duployer, B., Tenailleau, C., Bam, L., Hoffman, J., de Beer, F., Braga, J., 2016. Upper third molar internal structural organization and semicircular canal morphology in Plio-Pleistocene South African cercopithecoids. J. Hum. Evol. 95, 104–120.
Berger, L.R., de Ruiter, D.J., Churchill, S.E., Schmid, P., Carlson, K.J., Dirks, P.H.G.M., Kibii, J.M., 2010. *Australopithecus sediba*: A new species of *Homo*-like australopith from South Africa. *Science* 328, 195–204.

Bilsborough, A., 1992. Human Evolution. Blackie Academic & Professional, London.

Böne, A., Louis, M., Martin, B., Durrleman, S., 2018. Deformetrica 4: An open-source software for statistical shape analysis. In: Reuter, M., Wachinger, C., Lombaert, H., Paniagua, B., Lüthi, M., Egger, B. (Eds.), Shape in Medical Imaging, International Workshop, ShapeMI 2018 Held in Conjunction with MICCAI 2018 Granada, Spain, September 20, 2018 Proceedings. LCNS Springer, Cham, pp. 3–13.

Brain, C.K., 1981. The Hunters or the Hunted? An Introduction to African Cave Taphonomy. University of Chicago Press, Chicago.

Braga, J., Zimmer, V., Dumoncel, J., Samir, C., de Beer, F., Zanolli, C., Pinto, D., Rohlf, F.J., Grine, F.E., 2019. Efficacy of diffeomorphic surface matching and 3D geometric morphometrics for taxonomic discrimination of Early Pleistocene hominin mandibular molars. *J. Hum. Evol.* 130, 21–35.

Braga, J., Chinamatira, G., Zipfel, B., Zimmer, V., 2022. New fossils from Kromdraai and Drimolen, South Africa, and their distinctiveness among *Paranthropus robustus*. *Sci. Rep.* 12, 13956.

Broom, R., Robinson, J.T., 1949. A new type of fossil man. *Nature* 164, 322–323.

Broom, R., Robinson, J.T., 1950. Man contemporaneous with the Swartkrans ape-man. *Am. J. Phys. Anthropol.* 8, 151–156.

Broom, R., Robinson, J.T., 1952. Swartkrans ape-man. *Paranthropus crassidens*. Transvaal Mus. Mem. 6, 1–124.

Brown, P., Maeda, T., 2009. Liang Bua *Homo floresiensis* mandibles and mandibular teeth: A contribution to the comparative morphology of a new hominin species. *J. Hum. Evol.* 57, 571–596.

Campbell, B.G., 1965. The nomenclature of the Hominidae including a definitive list of hominid taxa. *Roy. Antropol. Inst. Occas.* Pap. 22, 1–34.

Cardini, A., Polly, P.D., 2020. Cross-validated between group PCA scatterplots: A solution to spurious group separation? *Evol. Biol.* 47, 85–95.

Clarke, R.J., 1977. The cranium of the Swartkrans hominin, SK 847, and its relevance to human origins. Ph.D. Dissertation, University of the Witwatersrand.

Clarke, R.J., 1985. *Australopithecus* and early *Homo* in southern Africa. In: Delson, E. (Ed.), *Ancestors: The Hard Evidence*. Roy. R. Liss, New York, pp. 171–177.

Clarke, R.J., 2017. *Homo habilis*: The inside story. In: Sahnouni, M., Semaw, S., Garaizar, J.R. (Eds.), *Proceedings of the II Meeting of African Prehistory. Centro Nacional de Investigación Sobre la Evolución Humana, Burgos, pp. 25–51*.

Clarke, R.J., Kuman, K., 2019. The skull of StW 573, a 3.67 Ma *Australopithecus habilis* skeleton from Sterkfontein Caves, South Africa. *J. Hum. Evol.* 134, 102634.

Coleman, M.N., Colbert, M.W., 2007. Technical note: CT thresholding protocols for taking measurements on three-dimensional models. *Am. J. Phys. Anthropol.* 133, 723–725.

Curnoe, D., 2006. Odontometric systematic assessment of the Swartkrans SK 15 mandible. *HOMO* 57, 263–294.

Curnoe, D., 2010. A review of early *Homo* in southern Africa focusing on cranial, mandibular and dental remains, with the description of a new species (*Homo gautengensis* sp. nov.). *HOMO* 61, 151–177.

Daegling, D.J., Carlson, K.J., Tafforeau, P., de Ruiter, D.J., Berger, L.R., 2016. Comparative biomechanics of *Australopithecus sediba* mandibles. *J. Hum. Evol.* 100, 73–86.

Davies, T.W., Delezene, L.K., Gunz, P., Hublin, J.-J., Berger, L.R., Gidna, A., Skinner, M.M., 2020. Distinct mandibular premolar morphology in *Homo naledi* and its implications for the evolution of *Homo* species in southern Africa. *Sci. Rep.* 10, 13196.

Davies, T.W., Alemseged, Z., Gidna, A., Hublin, J.-J., Kimbel, W.H., Kullmer, O., Spoor, F., Zanolli, C., Skinner, M.M., 2021. Accessory cusp expression at the enamel-dentine junction of hominin mandibular molars. *PeerJ* 9, e11415.

Davies, T.W., Gunz, P., Spoor, F., Alemseged, Z., Gidna, A., Hublin, J.-J., Kimbel, W.H., Kullmer, O., Plummer, W.P., Zanolli, C., Skinner, M.M., 2024. Dental morphology in *Homo habilis* and its implications for the evolution of early *Homo*. *Nat. Commun.* 15, 286.

Day, M.H., 1977. Guide to Fossil Man, second ed. Cassell, London.

Dean, M.C., Zanolli, C., Le Cabec, A., Tawane, M., Garrevoet, J., Mazurier, A., Macchiarelli, R., 2020. Growth and development of the third permanent molar in *Paranthropus robustus* from Swartkrans, South Africa. *Sci. Rep.* 10, 19053.

Delezene, L.K., Skinner, M.M., Bailey, S.E., Brophy, J.K., Elliott, M.C., Gurtov, A., Irish, J.D., Moggi-Cecchi, J., de Ruiter, D.J., Hawks, J., Berger, L.R., 2023. Descriptive catalog of *Homo naledi* dental remains from the 2013 to 2015 excavations of the Dinaledi Chamber, site U.W. 101, within the Rising Star cave system, South Africa. *J. Hum. Evol.* 180, 103372.

Dray, S., Dufour, A., 2007. The ade4 package: Implementing the duality diagram for ecologists. *J. Stat. Software* 22, 1–20.

Dumoncel, J., 2021. RToolsForDeformetrica. R package version 0.1. https://gitlab.com/jeandumoncel/tools-for-deformetrica/-/tree/master/src/R_script/RToolsForDeformetrica.

Durrleman, S., Pennec, X., Trouvé, A., Ayache, N., Braga, J., 2012. Comparison of the endocranial ontogenies between chimpanzees and bonobos via temporal regression and spatiotemporal registration. *J. Hum. Evol.* 62, 74–88.

Durrleman, S., Prastawa, M., Charon, N., Korenberg, J.R., Joshi, S., Gerig, G., Trouvé, A., 2014. Morphometry of anatomical shape complexes with dense deformations and sparse parameters. *Neuroimage* 101, 35–49.

Fajardo, R.J., Ryan, T.M., Kappelman, J., 2002. Assessing the accuracy of high-resolution X-ray computed tomography of primate trabecular bone by comparisons with histological sections. *Am. J. Phys. Anthropol.* 118, 1–10.

Glaunès, J., Joshi, S., 2006. Template estimation from unlabeled point set data and surfaces for computational anatomy. In: Pennek, X., Joshi, S. (Eds.), *Proceedings of the International Workshop on the Mathematical Foundations of Computational Anatomy*. LNCS Springer, Copenhagen, pp. 29–39.

Grine, F.E., 2001. Implications of morphological diversity in early *Homo* crania from eastern and southern Africa. In: Tobias, P.V., Raath, M., Moggi-Cecchi, J., Doyle, G. (Eds.), Humanity from the African Naissance to the Coming Millennia. Firenze University Press, Firenze, pp. 107–115.

Grine, F.E., 2005. Early *Homo* from Swartkrans, South Africa: A review of the evidence and an evaluation of recently proposed morphs. *S. Afr. J. Sci.* 101, 43–52.

Grine, F.E., Smith, H.F., Heesy, C.P., Smith, E.J., 2009. Phenetic affinities of Plio-Pleistocene *Homo* fossils from South Africa: Molar cusp proportions. In: Grine, F.E., Fleagle, J.G., Leakey, R.E. (Eds.), The First Humans – Origin and Early Evolution of the Genus *Homo*. Springer, New York, pp. 49–62.

Guy, F., Mackaye, H.-T., Likius, A., Vignaud, P., Schmittbuhl, M., Brunet, M., 2008. Symphyseal shape variation in extant and fossil hominoids, and the symphysis of *Australopithecus bahrelghazali*. *J. Hum. Evol.* 55, 37–47.

Hastie, T., Tibshirani, R., Friedman, J., 2009. The Elements of Statistical Learning, second ed. Springer, New York.

Hawks, J., Elliott, M., Schmid, P., Churchill, S.E., Ruiter, D.J.d., Roberts, E.M., Hilbert-Wolf, H., Garvin, H.M., Williams, S.A., Delewane, L.K., Feuerriegel, E.M., Rando-dQuinney, P., Kivell, T.L., Laird, M.F., Tawane, G., DeSilva, J.M., Bailey, S.E., Brophy, J.K., Meyer, M.R., Skinner, M.M., Tocheri, M.W., VanSickle, C., Walker, C.S., Campbell, T.L., Kuhn, B., Kruger, A., Tucker, S., Gurtov, A., Hlophe, N., Hunter, R., Morris, H., Peixotto, B., Ramalepa, M., Rooyen, D.v., Tsikoane, M., Boshoff, P., Dirks, P.H.G.M., Berger, L.R., 2017. New fossil remains of *Homo naledi* from the Lesedi Chamber, South Africa. *eLife* 6, e24232.

Howell, F.C., 1978. Hominidae. In: Maglio, V.J., Cooke, H.B.S. (Eds.), Evolution of African Mammals. Harvard University Press, Cambridge, pp. 154–248.

Kuman, K., Granger, D.E., Gibbon, R.J., Pickering, T.R., Caruana, M.V., Bruxelles, L., Clarke, R.J., Heaton, J.L., Stratford, D., Brain, C.K., 2021. A new absolute date from Swartkrans Cave for the oldest occurrences of *Paranthropus robustus* and Oldowan stone tools in South Africa. *J. Hum. Evol.* 156, 103000.

Kupczik, K., Delezene, L.K., Skinner, M.M., 2019. Mandibular molar robust and pulp cavity morphology in *Homo naledi* and other Plio-Pleistocene hominins. *J. Hum. Evol.* 130, 83–95.

Kurth, G., 1965. *Die (Eu)homininen: ein Jewelsbild nach dem Kenntnisstand von 1974*. In: Herberer, G. (Ed.), Meschliche Abstammungslehre. Gustav Fischer, Stuttgart, pp. 357–425.

Leece, A.B., Martin, J.M., Baker, S., Wilson, C., Strait, D.S., Schwartz, G.T., Herries, A.I.R., 2023. New hominin dental remains from the ~2.04–1.95 Ma Drimolen Main Quarry, South Africa. *Ann. Hum. Biol.* 50, 407–427.

Madupe, P., Koenig, C., Patramanis, I., Rüther, P.L., Hlazo, N., Mackie, M., Tawane, M., Krueger, J., Taurozzi, A.J., Troché, G., Kibiï, J., Pickering, R., Dickinson, M., Sahle, Y., Kgotleng, D., Musiba, C., Manthi, F., Bell, L., DuPlessis, M., Gilbert, C., Zipfel, B., Kuderna, L.F.K., Lizano, E., Welker, F., Kyriakidou, P., Cox, J., Mollereau, C., Tokarski, C., Blackburn, J., Ramos-Madrigal, J., Marques-Bonet, T., Penkman, K., Zanolli, C., Shroeder, L., Racimo, F., Olsen, J.V., Ackermann, R.R., Cappellini, E., 2023. Enamel proteins reveal biological sex and genetic variability within southern African *Paranthropus*. *Preprint*. https://doi.org/10.1101/2023.07.03.547326 available on the bioRxiv server at.

Martin, J.M., Leece, A.B., Neubauer, S., Baker, S.E., Mongle, C.S., Boschian, G., Schwartz, G.T., Smith, A.L., Ledogar, J.A., Strait, D.S., Herries, A.I.R., 2020. Drimolen chromium DNH 155 documents microevolution in an early hominin species. *Nat. Ecol. Evol.* 5, 38–45.

Martin, J.M., Leece, A.B., Baker, S.E., Herries, A.I.R., Strait, D.S., 2024. A lineage perspective on hominin taxonomy and evolution. *Evol. Anthropol.* 33, e22018.

Martinez Arizu, P., 2020. pairwiseAdonis: Pairwise multilevel comparison using Adonis. R package version 0.4. https://github.com/pmartinezarizu/pairwiseAdonis.

Mitteroecker, P., Bookstein, F., 2011. Linear discrimination, ordination, and the visualization of selection gradients in modern morphometrics. *Evol. Biol.* 38, 100–114.

Moggi-Cecchi, J., Menter, C., Boccone, S., Kayser, A., 2010. Early hominin dental remains from the Plio-Pleistocene site of Drimolen, South Africa. *J. Hum. Evol.* 58, 374–405.

Oksanen, J., Blanchet, F.G., Friendly, M., Kindt, R., Legendre, P., McGlinn, D., Minchin, P.R., O'Hara, R.B., Simpson, G.L., Solymos, P., Stevens, M.H.H., Szoecs, E., Wagner, H., 2020. vegan: Community ecology package. R package version 2.5-7. https://CRAN.R-project.org/package=vegan.

Pan, L., Thackeray, J.F., Dumoncel, J., Zanolli, C., Oettlé, A., de Beer, F., Hoffman, J., Duployer, B., Tenailleau, C., Braga, J., 2017. Intra-individual multicameral variation expressed at the enamel-dentine junction of lower post-canine dentition of South African fossil hominins and modern humans. *Am. J. Phys. Anthropol.* 163, 806–815.

Pan, L., Dumoncel, J., Mazurier, A., Zanolli, C., 2020. Hominin diversity in East Asia during the Middle Pleistocene: A premolar endostructural perspective. *J. Hum. Evol.* 148, 102888.

Plavcan, J.M., 2001. Sexual dimorphism in primate evolution. *Yearb. Phys. Anthropol.* 44, 25–53.

R Core Team, 2023. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna.

Reed, D., Raney, E., Johnson, J., Jackson, H., Virabalin, N., Mbonu, N., 2023. Hominin nomenclature and the importance of information systems for managing complexity in paleoanthropology. *J. Hum. Evol.* 175, 103308.
Rightmire, G.P., 1990. The Evolution of *Homo erectus*. Cambridge University Press, Cambridge.

Robinson, J.T., 1953. *Telanthropus* and its phylogenetic significance. Am. J. Phys. Anthropol. 11, 1–38.

Robinson, J.T., 1961. The australopithecines and their bearing on the origin of man and of stone toolmaking. S. Afr. J. Sci. 57, 3–13.

Rohlf, F.J., 2003. Bias and error in estimates of mean shape in geometric morphometrics. J. Hum. Evol. 44, 665–683.

Schlager, S., 2023. Morpho: Calculations and visualizations related to geometric morphometrics. R package version 2.11. http://cran.r-project.org/web/packages/Morpho/index.html.

Schneider, C.A., Rasband, W.S., Eliceiri, K.W., 2012. NIH image to ImageJ: 25 years of image analysis. Nat. Methods 9, 671–675.

Skinner, M.M., Wood, B.A., Boesch, C., Olejniczak, A.J., Rosas, A., Smith, T., Hublin, J.-J., 2008. Dental trait expression at the enamel-dentine junction of lower molars in extant and fossil hominoids. J. Hum. Evol. 54, 173–186.

Skinner, M.M., Gunz, P., Wood, B.A., Boesch, C., Hublin, J.-J., 2009a. Discrimination of extant Pan species and subspecies using the enamel-dentine junction morphology of lower molars. Am. J. Phys. Anthropol. 140, 234–243.

Skinner, M.M., Wood, B.A., Hublin, J.-J., 2009b. Protostylid expression at the enamel-dentine junction and enamel surface of mandibular molars of *Paranthropus robustus* and *Australopithecus africanus*. J. Hum. Evol. 56, 76–85.

Skinner, M.M., de Vries, D., Gunz, P., Kupczik, F., Klassen, R.P., Hublin, J.-J., Roksandic, M., 2016. A dental perspective on the taxonomic affinity of the Balanica mandible (BH-1). J. Hum. Evol. 93, 63–81.

Schwartz, J.H., Tattersall, I., 2003. The Human Fossil Record. Craniodental Morphology of Genus *Homo* (Africa and Asia), vol. 2. Wiley-Liss, Hoboken.

Skinner, M.M., Bailey, S.E., Gunz, P., Kimbel, W.H., Alemseged, Z., Delezene, L.K., Menter, C., Moggi-Cecchi, J., Kupczik, K., 2018. Below the crown: Examining interspecies variation in postcanine enamel thickness, EDJ, and root form in the *Paranthropus* clade. Am. J. Phys. Anthropol. 165 (S66), 254.

Skinner, M.M., Bailey, S.E., Gunz, P., Kimbel, W.H., Alemseged, Z., Delezene, L.K., Menter, C., Moggi-Cecchi, J., Kupczik, K., in press. Below the crown: Examining interspecies variation in postcanine enamel thickness, EDJ, and root form in the *Paranthropus* clade. In: Constantino, P., Reed, K., Wood, B.A. (Eds.), The Forgotten Lineage(s): Paleobiology of *Paranthropus*. Springer, Dordrecht.

Spoor, C.F., Zonneveld, F.W., Macho, G.A., 1993. Linear measurements of cortical bone and dental enamel by computed tomography: Applications and problems. Am. J. Phys. Anthropol. 91, 469–484.

Spoor, F., Gunz, P., Neubauer, S., Stelzer, S., Scott, N., Kwekason, A., Dean, M.C., 2015. Reconstructed *Homo habilis* type OH 7 suggests deep-rooted species diversity in early *Homo*. Nature 519, 83–86.

Stelzer, S., Gunz, P., Neubauer, S., Spoor, F., 2018a. Using the covariation of extant hominoid upper and lower jaws to predict dental arcades of extinct hominins. J. Hum. Evol. 114, 154–175.

Stelzer, S., Neubauer, S., Hublin, J.-J., Spoor, F., Gunz, P., 2018b. Morphological trends in arcade shape and size in Middle Pleistocene *Homo*. Am. J. Phys. Anthropol. 168, 70–91.

Taylor, A.B., 1997. Relative growth, ontogeny, and sexual dimorphism in gorilla (*Gorilla gorilla gorilla* and *G. g. beringei*): Evolutionary and ecological considerations. Am. J. Primatol. 43, 1–31.

Tobias, P.V., 1991. The Skulls, Endocasts and Teeth of *Homo habilis*. Olduvai Gorge, vol. 4. Cambridge University Press, Cambridge.

Tobias, P.V., von Koenigswald, G.H.R., 1964. A comparison between the Olduvai hominines and those of Java and some implications for hominid phylogeny. Nature 204, 515–518.

Tobias, P.V., Copley, K., Brain, C.K., 1977. Entries for South Africa. In: Oakley, K., Campbell, B., Molleson, T. (Eds.), Catalogue of Fossil Hominids. Part I: Africa, second ed. British Museum (Natural History), London, pp. 95–151.

Towle, I., Irish, J.D., De Groote, I., Fernée, C., Loch, C., 2021. Dental caries in South African fossil hominins. S. Afr. J. Sci. 117, 8705.

Uchida, A., 1998. Variation in tooth morphology of *Gorilla gorilla*. J. Hum. Evol. 34, 55–70.

Ungar, P.S., Grine, F.E., Teaford, M.F., El Zaatari, S., 2006. Dental microwear and diets of African early *Homo*. J. Hum. Evol. 50, 78–95.

Wolpoff, M.H., 1970. The evidence for multiple hominid taxa at Swartkrans. Am. Anthropol. 72, 576–607.

Wood, B.A., 1991. Koobi Fora research Project. Hominin Cranial Remains, vol. 4. Clarendon Press, Oxford.

Zanolli, C., Pan, L., Dumoncel, J., Kullmer, O., Kundrát, M., Liu, W., Macchiarelli, R., Mancini, L., Schrenk, F., Tuniz, C., 2018. Inner tooth morphology of *Homo erectus* from Zhoukoudian. New evidence from an old collection housed at Uppsala University, Sweden. J. Hum. Evol. 116, 1–13.

Zanolli, C., Davies, T.W., Joannes-Boyau, R., Beaudet, A., Bruxelles, L., de Beer, F., Hoffman, J., Hublin, J.-J., Jakata, K., Kgasi, L., Kullmer, O., Macchiarelli, R., Pan, L., Schrenk, F., Santos, F., Stratford, D., Tawane, M., Thackeray, F., Xing, S., Zipfel, B., Skinner, M.M., 2022a. Dental data challenge the ubiquitous presence of *Homo* in the Cradle of Humankind. Proc. Natl. Acad. Sci. USA 119, e2111212119.

Zanolli, C., Kaifu, Y., Pan, L., Xing, S., Mijares, A.S., Kullmer, O., Schrenk, F., Corny, J., Dizon, E., Robles, E., Détroit, F., 2022b. Further analyses of the structural organization of *Homo luzonensis* teeth: Evolutionary implications. J. Hum. Evol. 163, 103124.

Zanolli, C., Bouchet, F., Fortuny, J., Bernardini, F., Tuniz, C., Alba, D.M., 2023. A reassessment of the distinctiveness of dryopithecine genera from the Iberian Miocene based on enamel-dentine junction geometric morphometric analyses. J. Hum. Evol. 177, 103326.

Zimmer, V., Oettlé, A., Hoffman, J., Thackeray, J.F., Zipfel, B., Braga, J., 2022. Revisiting mandibular symphyseal shape in juvenile early hominins and modern humans using a deformation-based approach. Research Square preprint. https://doi.org/10.21203/rs.3.rs-1929855/v1.