# A new fossil buffalo from the Shungura Formation (Ethiopia) reveals the role of heterochrony in the evolution of Syncerus

Faysal Bibi, Jean-Renaud Boisserie

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Faysal Bibi, Jean-Renaud Boisserie. A new fossil buffalo from the Shungura Formation (Ethiopia) reveals the role of heterochrony in the evolution of Syncerus. Peer Community In Paleontology, 2025, 10.31233/osf.io/eh9s8_v3. hal-05461149

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RESEARCH ARTICLE

A new fossil buffalo from the Shungura
Formation (Ethiopia) reveals the role of
heterochrony in the evolution of *Syncerus*

Faysal Bibi<fnref n="1" /> ID & Jean-Renaud Boisserie<fnref n="2" />,<fnref n="3" /> ID

**Cite as:** Bibi F and Boisserie JR (2025). A new fossil buffalo from the Shungura Formation (Ethiopia) reveals the role of heterochrony in the evolution of *Syncerus*. *Paleorxiv* eh9s8, ver. 5, peer-reviewed by PCI Paleo. DOI: 10.31233/osf.io/eh9s8_v5

Published: 22nd October 2025

Recommender:

Vera Weisbecker

Reviewers:

Rex Mitchell and one anonymous reviewer

Correspondence:

faysal.bibi@mfn.berlin

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THIS ARTICLE WAS PEER-REVIEWED BY PCI PALEO

Read the Editor's recommendation and Referees' reports at DOI:10.24072/pci.paleo.100399

Abstract

Variation in the timing and rate of development (heterochrony) is a key driver of morphological diversification. The African buffalo (*Syncerus caffer*) is the sole surviving species of a once diverse Pleistocene lineage and shows striking eco-morphological variability that may be the result of heterochronic shifts. Heterochrony has also been suspected to account for the past diversity of *Syncerus*, but the early evolution of this clade has remained obscure due to limited fossil evidence. Here we describe new material from the Shungura Formation (southern Ethiopia), including a nearly complete cranium dated to ~2.6 Ma, which we designate as the type of a new species of *Syncerus*. This species is smallbodied with short, divergent horns, resembling immature and small-bodied extant *Sy. caffer* (forest or 'nanus' ecomorphs). Three-dimensional geometric morphometrics reveal that fossils of *Ugandax* and *Syncerus* (including the new cranium) broadly follow the ontogenetic size-shape allometry of *Sy. caffer*, suggesting cranial diversification along a conserved allometric trajectory, and that the new species may represent an ancestral form from which later *Syncerus* evolved. In contrast, the extreme horn length of the extinct giant buffalo *Sy. antiquus* departs from this pattern, indicating lineage-specific innovation. These findings provide the earliest well-preserved evidence of the African buffalo lineage and highlight the role of conserved allometry and heterochrony in the evolutionary history of *Syncerus*. They also underscore the importance of new fossil discoveries and comparative analyses for understanding the origins of morphological diversity.

Keywords: *Syncerus*, Bovidae, Pliocene, Pleistocene, Africa, Geometric morphometrics, Heterochrony

<footnote n="1">¹ Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science - Berlin, Germany</footnote>

<footnote n="2">² Centre Français des Études Éthiopiennes, CNRS & Ministère de l'Europe et des affaires étrangères - Addis Ababa, Ethiopia</footnote>

<footnote n="3">³ Laboratoire Paléontologie Évolution Paléoécosystèmes Paléoprimatologie, UMR 7262 PALEVOPRIM, CNRS & Université de Poitiers - Poitiers, France</footnote>
## RÉSUMÉ

Les variations du rythme et de la durée du développement (hétérochronies) constituent un moteur essentiel de la diversification morphologique. Le buffle d'Afrique (*Syncerus caffer*), seul survivant d'une lignée pléistocène autrefois diversifiée, présente une remarquable variabilité écomorphologique qui pourrait résulter de décalages hétérochroniques. Les hétérochronies ont également été proposées pour expliquer la diversité passée de *Syncerus*, mais l'évolution initiale de ce clade est restée méconnue en raison de la rareté des fossiles. Nous décrivons ici du matériel inédit provenant de la Formation de Shungura (sud-ouest de l'Éthiopie), dont un crâne sub-complet daté d'environ 2,6 Ma, que nous désignons comme type d'une nouvelle espèce de *Syncerus*. Cette espèce est de petite taille et porte des cornes courtes et divergentes, rappelant les individus juvéniles et de petite taille de l'actuel *Sy. caffer* (écotype forestier ou « nanus »). Les analyses morphométriques géométriques tridimensionnelles révèlent que les fossiles d' *Ugandax* et de *Syncerus* (y compris le nouveau crâne) suivent de manière générale l'allométrie ontogénétique liant la taille et la forme chez *Sy. caffer*, ce qui suggère une diversification crânienne suivant une trajectoire allométrique identique, et que la nouvelle espèce pourrait représenter une forme ancestrale dont les représentants ultérieurs de *Syncerus* auraient dérivé. En revanche, la longueur extrême des cornes du buffle géant éteint *Sy. antiquus* s'écarte de ce schéma, indiquant une innovation spécifique à cette lignée. Ces résultats apportent le plus ancien témoignage bien préservé de la lignée du buffle d'Afrique et mettent en évidence le rôle de l'allométrie et des hétérochronies dans l'histoire évolutionnaire de *Syncerus*. Ils soulignent également l'importance de nouvelles découvertes fossiles et d'analyses comparatives pour comprendre les origines de la diversité morphologique.

Mots clefs : *Syncerus*, Bovidae, Pliocène, Pléistocène, Afrique, Morphométrie géométrique, Hétérochronie
# INTRODUCTION

Changes in organismal form are often tightly linked to changes in size, with allometric relationships explaining much of the morphological variation observed within and among species (Huxley 1932; Gould 1977; Klingenberg 2016; Cardini 2019). Such patterns frequently arise through heterochrony—shifts in the rate or timing of development—that move individuals or lineages along pre-existing size-shape trajectories and generate novel phenotypes (Simons & Frost 2021; Tejero-Cicuéndez et al. 2023). Although heterochrony is an important mechanism for morphological diversification, its role in shaping evolutionary lineages is often difficult to evaluate in the fossil record, especially where well-preserved specimens suitable for morphometric analysis are rare (Bhullar et al. 2012; Gomes Rodrigues et al. 2018; Krone et al. 2019; Kammerer et al. 2020).

The African buffalo, *Syncerus caffer*, provides a compelling case study. As the only living representative of an otherwise diverse Plio-Pleistocene radiation of African bovins, it exhibits striking phenotypic variability across its geographic range, from large, dark savanna buffalo with massive horns to small, reddish forest forms with short horns (Grubb 1972; Prins & Sinclair 2013). This remarkable variation has raised questions about the developmental and evolutionary processes underlying the lineage. Some evidence suggests that both extinct and extant *Syncerus* species may have diversified along conserved allometric trajectories, but the early history of the clade remains poorly documented due to the scarcity of well-preserved specimens.

Here we report a nearly complete cranium from the Shungura Formation (~2.6 Ma), which, together with associated horn cores, allows us to describe the earliest recognized species of *Syncerus*. This discovery not only clarifies the origins of the lineage but also provides a rare opportunity to evaluate the role of heterochrony and allometric scaling in the diversification of African buffalo. Using 3D geometric morphometrics, we assess cranial shape and size relationships across extant and extinct members of *Syncerus* and its relatives, testing whether their morphological disparity can be explained by shared ontogenetic and evolutionary allometries. Specifically, we tested the hypothesis that species of *Syncerus* are 'scaled variants' of each other (Simons & Frost 2021). This predicts that fossil and recent *Syncerus* individuals share a common ontogenetic and evolutionary allometry and that the new fossil cranium aligns with the same size-shape relationship defined by extant *Sy. caffer*.
## BACKGROUND

Bovidae (Cetartiodactyla, Ruminantia) is the most diverse family of terrestrial large mammals and makes up the majority of large herbivore species in tropical and subtropical ecosystems across Eurasia and Africa. A recent study of 96 bovid species (Bibi & Tyler 2022) showed that ~10% of this family's cranial shape diversity (excluding horns) could be explained by a common size-shape allometry. Size was positively correlated with facial elongation, a pattern documented across a wide range of placental mammals (Cardini 2019). In herbivores, selection for both increased size and facial elongation could have promoted repeated diversification into grazing niches, particularly since larger size and longer jaws provide metabolic and mechanical advantages when it comes to the ingestion of grasses, which are often less protein-rich and more abundant in abrasives such as phytoliths and dirt, than herbaceous shrubs or tree leaves (Damuth & Janis 2011; Codron *et al.* 2019).

The largest bovids, past and present, are bovins (tribe Bovini). These include all extant buffalo and oxen, including domestic and wild forms of the genera *Bos* (including *Bison*), *Bubalus* (including *Anoa*), and *Syncerus*. Extant bovins are largely Asian in distribution, and the African buffalo (*Syncerus caffer*) is the only living endemic African bovin. Although commonly called the Cape buffalo, its range extends across much of sub-Saharan Africa, occupying habitats from semi-arid savannas to moist tropical forests, wherever permanent water and grass are available (Prins & Sinclair 2013).

In the Pliocene and Pleistocene, however, African bovins were more diverse. Two or more species often coexisted, distributed among genera such as *Simatherium*, *Pelorovis*, and *Ugandax*. In the late Quaternary, *Sy. caffer* is often found sympatrically with the extinct giant buffalo *Sy. antiquus*, which likely survived into the middle Holocene (Gautier & Muzzolini 1991; Gentry 2010). However, the earlier evolution of the clade is not well understood. The oldest fossils attributable to *Syncerus* are known from between 3 Ma and 2.5 Ma, but these are fragmentary and of uncertain species identity (Gentry 1985, 2010; Bibi *et al.* 2017). Their attribution to *Syncerus* is based on horns that are more strongly divergent, inclined, mediolaterally compressed, with a more anteriorly-located posterolateral keel, than in its supposed ancestor *Ugandax* (Gentry 2006). A better understanding of the evolutionary origins and development of the *Syncerus* clade has been hindered by a lack of well-preserved material, especially from the 3–2 Ma time interval.
Extant *Sy. caffer* is also notable for its exceptional phenotypic variation across its geographic range. Savanna populations include massive individuals (~500-800 kg) with dark pelage and long horns that sweep downward from thick bases and often form a fused boss. In contrast, forest-dwelling individuals are much smaller (~250-320 kg), reddish in color, and bear shorter, simpler horns (Grubb 1972). The stark phenotypic differences among extant African buffalo populations have been attributed to environmental differences, leading to the idea that the various morphs represent ecophenotypes. This ecophenotypic plasticity, expressed across habitats from savanna to forest, provides a modern analog for understanding the developmental flexibility that may also have contributed to morphological diversity in the fossil record. Furthermore, these striking differences exceed the level of ecophenotypic variation typical of other large mammals and have led to a complicated taxonomic history (reviewed below). Heterochrony has been invoked to explain this variation; several authors (Geist 1971; Estes et al. 1974; Vrba 1996) emphasized the role of slowed growth (neoteny) in bovid development, while Vrba et al. (1994) documented evidence for pedomorphosis along evolutionary allometries in reduncins. Such findings suggest that heterochronic processes may also underlie the disparity observed within *Syncerus*.

Over 50 species names were proposed historically to accommodate this variation (Grubb 2005), but since Lydekker (1913) consensus has generally recognized all African buffalo as a single, highly variable species (reviewed in Grubb 1972). This variability was long suspected to reflect environmental differences, with subspecies interpreted as ecophenotypes. Vrba (1996, 2004) proposed that size differences follow climatic gradients, invoking Bergmann's and Allen's rules: the large-bodied, long-horned savanna morph (*Sy. caffer caffer*) would result from prolonged growth in cooler environments at higher latitudes and altitudes (see also Sinclair 1977).

The pronounced variation among living populations has also led to competing subspecies schemes. Four morphotypes have often been recognized: *Sy. c. caffer* (large, long-horned savanna form of eastern and southern Africa), *Sy. c. nanus* (small, short-horned forest form of central and western Africa), *Sy. c. brachyceros* (intermediate-sized, short-horned savanna form of West Africa), and *Sy. c. aequinoctialis* (large, long-horned savanna form of central Africa)(Grubb 1972; Prins & Sinclair 2013). However, molecular work showed that *nanus*, *aequinoctialis*, and *brachyceros* are not monophyletic, and that the distinctive small 'nanus' forest morphotype evolved repeatedly from larger ancestors (Smitz *et al.* 2013). These results supported dividing *Sy. caffer* into two main subclades, *Sy. c. caffer* and *Sy. c.*
*nanus* (the latter including both ‘aequinoctialis’ and ‘brachyceros’), with all extant buffalo populations descending from a most recent common ancestor dating to ~149–449 ka.

The fossil record contributes little to this recent history. Definite occurrences of *Sy. caffer* before the Late Pleistocene are rare, with only scattered, poorly preserved crania from the Middle Pleistocene (Gentry 2010; occurrences tabulated by Faith *et al.* 2019). By contrast, *Sy. antiquus* is abundant and well documented from the Middle and Late Pleistocene, and even into the Holocene, from southern to northern Africa (Gautier & Muzzolini 1991; Klein 1994). *Syncerus acoelotus* was described from the Early Pleistocene of Olduvai Gorge, and remains confidently recorded only from that site, and from a handful of specimens (Gentry & Gentry 1978). Aside from *Sy. antiquus* and *Sy. acoelotus*, no other extinct species of *Syncerus* have been formally described. Older fossils assigned to the genus remain uncertain: Gentry (1985) attributed material from the Shungura Formation (~3–2.5 Ma) to *Sy. ?acoelotus*, while Bibi *et al.* (2017) referred two ~2.6 Ma specimens from Ledi-Geraru to cf. *Syncerus*. Gentry (2010) later assigned the Shungura material to *Simatherium shungurense*, described by Geraads (1995) based on a cranium with long, slender, and straight horns. However, we favor Gentry’s initial identification of the early Shungura material to *Syncerus*, as explained previously (Bibi *et al.* 2017), and below. Until now, however, the Shungura material remained taxonomically indeterminate, pending discovery of more complete specimens.

The Shungura Formation is a sequence of fossiliferous Plio-Pleistocene deposits, divided into 12 Members (Basal, then A to L, omitting I), dated between ~3.75 Ma and 1.09 Ma. It was first investigated in 1933 by Arambourg (1947), then in 1967-1976 by the International Omo Research Expedition (Arambourg *et al.* 1967; Howell 1968), and since 2006 by the Omo Group Research Expedition (OGRE, Boisserie *et al.* 2008). Stratigraphy and geochronology were described in detail by de Heinzelin (1983) and revised by McDougall and Brown (2006, 2008), McDougall *et al.* (2012), and Kidane *et al.* (2014). Most recently Gardin *et al.* (2024, in their supplementary data) integrated all available chronological data to establish an updated age model for the formation.

## MATERIALS AND METHODS
Specimens were studied at the Ethiopian Heritage Authority (EHA, formerly Authority for Research and Conservation of Cultural Heritage) at the National Museum of Ethiopia in Addis Ababa, Ethiopia; the National Museums of Kenya in Nairobi, Kenya; and the Museum für Naturkunde in Berlin, Germany. Measurements were taken with standard digital calipers, measuring tape, and angle measure. Horn core lengths were measured using measuring tape along the external (convex) curvature surface. Horn lengths for some extant *Sy. caffer* specimens were measurable only on the external horn sheath, and horn core length was estimated as 0.64 x sheath length, based on two specimens at the Museum für Naturkunde, Berlin, in which both core and sheath lengths were measurable (and which gave ratios of 0.64 and 0.63). Specimens were classified as adult based on permanent cheek teeth (P2-M3) erupted and in wear, or as immature based on presence of DP-DP4, or if P2-P4 and M3 were still in the process of erupting. All individuals included in this study had M1-M2 already erupted (i.e. no very young individuals were sampled). Individuals of *Sy. caffer* were classified as either ‘caffer’ or ‘nanus’ morphotype based on large size and long horns, or small size and short horns, respectively. Full specimen details are given in a supplementary file (TaxonClassifiers.csv)(Bibi & Boisserie 2025).

Shungura Formation specimen numbers generally take the form of an ‘OMO’ or ‘L’ prefix, followed by locality and specimen numbers. We followed Gentry (1985) in giving the stratigraphic member of Shungura Formation after the specimen number in the text, though this does not form part of the actual specimen number. For example, in ‘L 444-10005 C’, L 444 is the locality, 10005 is the specimen number for that locality (IORE numbering starts at 1, OGRE numbering starts at 10001), and ‘C’ refers to Member C of the Shungura Formation.

Fossil specimens not described below but included in the cranial analyses are KNM-ER 524, GAW-VP-3/210, Atbara 20-88, and NHM PV M 34563. KNM-ER 524 is the holotype of *Pelorovis turkanensis* and was described by Harris (1991). Specimens GAW-VP-3/210 and Atbara 20-88 are undescribed but have been studied by one of us (F.B.). They are included in this study to provide further taxonomic and chronological context and therefore only a brief description of their provenance is given here. GAW-VP-3/210 derives from the Haradaso Member (~4.8 Ma, Renne et al. 1999) of the Middle Awash study area in Ethiopia (T. White, pers. comm.). It is similar to material of *Ugandax demissum*, initially described as *Simatherium demissum* by Gentry (1980) from Langebaanweg, South Africa, and is attributable to the same species. Atbara 20-88 was collected from Pleistocene deposits at Wadi Turk along the middle Atbara River in eastern Sudan (Abbate et al. 2010; Bibi et al. in
prep.). Luminescence dating at the nearby site of Al Sharafa established a range of ~224 ka–17 ka for the Pleistocene sequences in the area (Tsukamoto et al. 2022). Analyses to further constrain the age of the Wadi Turk site are underway (Tsukamoto, Bibi et al. in prep.), but for now we assume an age within this timeframe. Atbara 20-88 is much larger than *Sy. caffer*, with horn core bases directed anterolaterally, and is attributable to the extinct giant buffalo, *Sy. antiquus*. NHM PV M 34563 is a cast of the holotype of *Homoioceras singae* (a synonym of *Sy. antiquus*, (Gentry 2010)) described from Singa, Sudan, by Bate (1951).

These fossil specimens all exhibit differing degrees of diagenetic deformation, including dorsoventral and/or mediolateral compression. However, all retain a high degree of bilateral symmetry. Sensitivity analyses were conducted to exclude any significant influence of bilateral fossil deformation or missing coordinate estimation on our results. These included a set of analyses with all fossil specimens removed (specimens of *Bos primigenius* were not considered as fossil for this analysis); one using only the symmetric shape component of the landmark coordinates, which forces bilateral coordinates on the left and right sides of the cranium to be mirrored, artificially removing any bilateral asymmetry; one with landmark coordinates that were missing in more than five specimens deleted; and one with all landmarks which were missing in Omo 53-10001 deleted. Other fossils included in the analysis included six crania of *Bos primigenius* and one of *Bison latifrons*—however, these are much younger and better preserved than the African fossil specimens. Given its relatively recent extinction, and its survival in the form of domesticated cattle, *Bos primigenius* was not considered a fossil taxon in the sensitivity analyses.

For the 3D geometric morphometric analysis, 43 3D surface models were generated using the Artec Spider and Eva surface scanners, the CREALITY Raptor scanner, from CT scans, or obtained from published or otherwise available datasets (Table 1). Nine models derive from Farke's (2010) dataset (scan data kindly provided by A. Farke), and seven models were obtained from sketchfab.com. We used the same landmark scheme as Bibi & Tyler (2021) and Baird et al. (2023), with 53 landmarks chosen to capture cranial shape excluding horns (Fig. 1). These were digitized on each skull using iDAV Landmark Editor v.3.645, Stratovan Checkpoint v.r23.05.30, and Slicer v.5.8.1. All geometric morphometric analyses were conducted in R v.4.5.0 and R Studio v. 2025.05.0+496 (Posit team 2025; R Core Team 2025), mainly using the *geomorph* v.4.0.1 and *RRPP* v.1.0 packages (Collyer & Adams 2018, 2021; Baken et al. 2021; Adams et al. 2023). Two main sets of analyses were conducted, one including all Bovini, and one including just specimens of *Ugandax* and *Syncerus* species. For
each analysis, specimens were Procrustes-transformed to account for rotation, translation, and size differences, and missing landmark coordinates were estimated using the *estimate.missing* function in *geomorph*, with the thin-plate spline option. UPGMA cluster analyses and principal components analyses were conducted on the Procrustes-transformed coordinates. The regression score of shape on size was calculated by projecting the Procrustes-transformed coordinates onto the axis of the shape variables that had the maximum covariation with log centroid size (Drake & Klingenberg 2008; Klingenberg 2016). The regression score was then plotted against centroid size to visually assess the relationship between size, shape, and ontogenetic age (i.e. the allometric trajectories). Tests relied mainly on ANOVA comparisons of the Procrustes-transformed coordinates using the *procD.lm* function in the *geomorph* package. Reference meshes showing the extremes of the first three principal components were made by warping a specimen of *Syncerus caffer* that lay close to the overall mean shape (ZMB Mam 19143) to the maximum and minimum values of each component.

All raw data and R scripts to replicate all figures and analyses are provided as supplementary files (Bibi & Boisserie 2025).

Dental size comparisons and plots used measurement data from the Mammal Dental Metrics Database (Bibi 2023), namely deriving from: (Lönnberg 1933; Cooke & Coryndon 1970; Churcher 1972; Vrba 1976; Gentry 1980, 1985, 1987; Harris *et al.* 1988; Gentry 1990; Janis 1990; Harris 1991; Harris *et al.* 2003; Harris 2003; Gilbert 2008; Haile-Selassie *et al.* 2009; Everett 2010; Gentry 2011; Rowan *et al.* 2015; Suwa *et al.* 2015; Faith *et al.* 2015; Bibi *et al.* 2017; Faith *et al.* 2020; Rowan *et al.* 2022). Horn core size comparisons and plots used data from: (Pomel 1893; Lönnberg 1933; Bate 1951; Gentry 1967, 2006; Gentry & Gentry 1978; Harris 1991; Gilbert 2008; Geraads *et al.* 2012; Bibi *et al.* 2017).

ABBREVIATIONS

<table><tr><td>AMNH</td><td>American Museum of Natural History, New York</td></tr><tr><td>IMNH</td><td>Idaho Museum of Natural History</td></tr><tr><td>MfN</td><td>Museum für Naturkunde Berlin (previously ‘Zoological Museum of Berlin’), mammalian collection</td></tr><tr><td>YPM</td><td>Yale Peabody Museum, New Haven</td></tr><tr><td>NHMUK</td><td>Natural History Museum United Kingdom (London)</td></tr><tr><td>NME</td><td>Ethiopian Heritage Authority/National Museum of Ethiopia, Addis Ababa</td></tr><tr><td>KNM</td><td>National Museums of Kenya, Nairobi.</td></tr></table>
<table><caption>TABLE 1. – Specimens used in the 3D geometric morphometric analysis.</caption><thead><tr><th>Museum</th><th>Specimen</th><th>Fossil/ Extant</th><th>Abbreviation</th><th>Genus</th><th>Subgenus</th><th>Species</th></tr></thead><tbody><tr><td>YPM</td><td>YPM MAM 9022</td><td>Extant</td><td>Bo bis</td><td>Bos</td><td>Bison</td><td>bison</td></tr><tr><td>YPM</td><td>YPM MAM 9023</td><td>Extant</td><td>Bo bis</td><td>Bos</td><td>Bison</td><td>bison</td></tr><tr><td>IMNH</td><td>IMNH R-163</td><td>Extant</td><td>Bo bis</td><td>Bos</td><td>Bison</td><td>bison</td></tr><tr><td>MfN</td><td>ZMB Mam 85047</td><td>Extant</td><td>Bo gau</td><td>Bos</td><td>Bos</td><td>gaurus</td></tr><tr><td>MfN</td><td>ZMB Mam 14802</td><td>Extant</td><td>Bo gau</td><td>Bos</td><td>Bos</td><td>gaurus</td></tr><tr><td>AMNH</td><td>M-113755</td><td>Extant</td><td>Bo jav</td><td>Bos</td><td>Bos</td><td>javanicus</td></tr><tr><td>IMNH</td><td>IMNH-48001 1770</td><td>Fossil</td><td>Bo lati</td><td>Bos</td><td>Bison</td><td>latifrons</td></tr><tr><td>MfN</td><td>MB Ma 671</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>MfN</td><td>MB Ma 226</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>NHMUK</td><td>NHMUK ARC 1972 5062</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>NHMUK</td><td>NHMUK ARC 1977 5012</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>NHMUK</td><td>NHMUK PV OR 50086</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>NHMUK</td><td>NHMUK ARC 1972 5068</td><td>Fossil</td><td>Bo pri</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>NHMUK</td><td>NHMUK 1981_984</td><td>Extant</td><td>Bo tau</td><td>Bos</td><td>Bos</td><td>primigenius</td></tr><tr><td>MfN</td><td>ZMB Mam 48424</td><td>Extant</td><td>Bu arn</td><td>Bubalus</td><td>Bubalus</td><td>arnee</td></tr><tr><td>MfN</td><td>ZMB Mam 14801</td><td>Extant</td><td>Bu arn</td><td>Bubalus</td><td>Bubalus</td><td>arnee</td></tr><tr><td>AMNH</td><td>M-152856</td><td>Extant</td><td>Bu dep</td><td>Bubalus</td><td>Anoa</td><td>depressicornis</td></tr><tr><td>AMNH</td><td>M-61146</td><td>Extant</td><td>Bu dep</td><td>Bubalus</td><td>Anoa</td><td>depressicornis</td></tr><tr><td>MfN</td><td>ZMB Mam 73488</td><td>Extant</td><td>Bu dep</td><td>Bubalus</td><td>Anoa</td><td>depressicornis</td></tr><tr><td>AMNH</td><td>M-40046</td><td>Extant</td><td>Bu min</td><td>Bubalus</td><td>Bubalus</td><td>mindorensis</td></tr><tr><td>AMNH</td><td>M-99339</td><td>Extant</td><td>Bu min</td><td>Bubalus</td><td>Bubalus</td><td>mindorensis</td></tr><tr><td>MfN</td><td>ZMB Mam 73510</td><td>Extant</td><td>Bu min</td><td>Bubalus</td><td>Bubalus</td><td>mindorensis</td></tr><tr><td>MfN</td><td>ZMB Mam 73496</td><td>Extant</td><td>Bu qua</td><td>Bubalus</td><td>Anoa</td><td>quarlesi</td></tr><tr><td>MfN</td><td>ZMB Mam 73484</td><td>Extant</td><td>Bu qua</td><td>Bubalus</td><td>Anoa</td><td>quarlesi</td></tr><tr><td>KNM</td><td>KNM-ER 524</td><td>Fossil</td><td>Pe tur</td><td>Pelorovis</td><td></td><td>turkanensis</td></tr><tr><td>NME</td><td>OMO 75/S-1969-2733</td><td>Fossil</td><td>Si shu</td><td>Simatherium</td><td></td><td>shungurense</td></tr><tr><td>MfN</td><td>Atbara 20-88</td><td>Fossil</td><td>Sy ant</td><td>Syncerus</td><td></td><td>antiquus</td></tr><tr><td>NHMUK</td><td>NHM PV M 34562</td><td>Fossil</td><td>Sy ant</td><td>Syncerus</td><td></td><td>antiquus</td></tr><tr><td>MfN</td><td>ZMB Mam 35750</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr><tr><td>YPM</td><td>YPM MAM 11649</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr><tr><td>YPM</td><td>YPM MAM 9227</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 19143</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 29536</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 35239</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td></td><td>caffer</td></tr></tbody></table>
<table><tr><td>MfN</td><td>ZMB MAM 41820</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 108867</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 108931</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 108991</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 109009</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 41819</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>MfN</td><td>ZMB Mam 41818</td><td>Extant</td><td>Sy caf</td><td>Syncerus</td><td>caffer</td></tr><tr><td>NME</td><td>OMO 53-10001</td><td>Fossil</td><td>Sy sp nov</td><td>Syncerus</td><td>sp. nov.</td></tr><tr><td>NME</td><td>GAW-VP-3/210</td><td>Fossil</td><td>Ug dem</td><td>Ugandax</td><td>demissum</td></tr></table>
<figure><img src="image_10.png" /><figcaption>FIG. 1. — Landmark scheme used for the cranial geometric morphometric analysis (following Bibi & Tyler 2021). Specimen shown is *Syncerus caffer* ZMB Mam 35239.</figcaption></figure>

SYSTEMATIC PALEONTOLOGY

CETARTIODACTYLA Montgelard, Catzeflis & Douzery, 1997

BOVIDAE Gray, 1821

BOVINI Gray, 1821

*Syncerus* Hodgson, 1847

TYPE SPECIES. — *Syncerus caffer* (Sparrman, 1779).

GENERIC DIAGNOSIS (modified from Gentry & Gentry 1978). — Moderate to large sized bovins with wide skulls and short faces; horn cores emerging just behind the orbits, strongly divergent, short to very long, weakly to greatly compressed dorsoventrally (or mediolaterally), with triangular or subtriangular cross-section, often with keels and weak heteronymous torsion. Characters shared with other bovins: females with horns, basioccipital process short and narrower anteriorly than posteriorly, with large anterior tuberosities,
occipital broad and low, becoming broader in later forms by way of increased robusticity of the mastoid region, supraorbital pits quite close together, molars with large basal pillars; lower fourth premolar with paraconid and metaconid growing towards one another or fused. *Syncerus* and *Bubalus* are distinguished from *Bos* (including *Bison*) by horns that emerge just behind the orbits, with keels, flat surfaces, a triangular cross-section, weak curvature, and heteronymous torsion, less hypsodont teeth, and molars with less complicated central cavities and weaker ribs between the styles, and less protruding orbits; but these are probably all plesiomorphic traits in Bovini. Compared to *Ugandax* (from which it likely evolved), *Syncerus* has horns with larger bases, greater divergence, greater dorsoventral (mediolateral) compression, broader and more flattened surfaces (especially the medial or dorsal surface), often with a more triangular basal cross-section, weaker medial/dorsal curvature, and a narrower bulla. Also, in early *Syncerus*, the anterior horn core edge is weakly curved backwards, and the posterior edge is straight, while in *Ugandax* both anterior and posterior edges are either weakly curved backwards (as in the holotypes of *U. coryndonae* and *U. gauthieri*) or relatively straight. Horns of larger species and individuals of *Syncerus* also reach much greater lengths than those of *Ugandax*, which generally do not surpass 400 mm in length. Horns of *Syncerus* are differentiated from those of *Pelorovis* (*P. turkanensis*, *P. oldowayensis*), which bear no keels, have rounded basal cross-sections, emerge from the back of the cranium and curve strongly anteriorly, and show varying degrees of homonymous torsion. Horns of *Syncerus* are also differentiated from the type and only specimen of *'Pelorovis' kaisensis*, a horn core which is much larger, straighter, without keels, and with a rounded cross-section (Geraads & Thomas 1994).

*Syncerus ekosowan* sp. nov.

*Syncerus ?acoelotus* Gentry & Gentry, 1978: 313; pl. 2. – Gentry 1985: 139; pl. IV; figs 5-8.

*Simatherium shungurense* Geraads, 1995: 89; pl. I [partim]. – Gentry 2010: 753.

Likely also: cf. *Syncerus* Hogdson, 1847. – Bibi *et al.* 2017: 3; fig. 2B-C.

REFERRED SPECIMENS. — **Holotype:** OMO 53-10001, an almost complete cranium with both horn cores and right and left M¹-M³ (Fig. 2, Table 2). Discovered by Kampiro Kayrento during OGRE field season 2015. Excavated from Unit C-8 of Member C of the Shungura Formation, with an age of around 2.568 Ma (age model of Gardin *et al.* 2024). **Other specimens:** New specimens: L 193-10027 C-7, right and left horn cores (Fig. 3); L 444-
10005 C-9, right and left horn cores, both almost complete (Fig. 3). Specimens previously described by Gentry (1985) and re-examined for this study: OMO 18-1967-114 C, left horn core; OMO 18-1967-914 C, right horn core; OMO 18-1969-2725 & 2726 C, frontlet with right horn core base (2725) and basal left horn core fragment (2726), same individual; OMO 18/sup-1967-153, 911, and 912 C, left (911-912) and distal right (153) horn cores, attributed to the same individual. Specimens previously described by Gentry (1985) and presumably still at the Natural History Museum in London: L 1-238 B-9/C, juvenile partial cranium with both horn cores, frontals, left orbit, and left upper dP²-M¹; L 2-26 B-10, partial cranium; L 16-10 G-4, horn core fragment, side indeterminate (Gentry 1985 pl. III fig. 1); L 53-3 C, right horn core and partial left horn core; L 147-48a E, fragmentary horn core; L 607-1 G-5, partial cranium; L 744-1 C, right and left horn cores and ‘fragmentary skull part’; L 837-2 C, right horn core; OMO 23-1967-722 C, right horn core fragment; OMO 58-1968-2316 E/F, left horn core base.
<figure><img src="image_11.png" /><figcaption>FIG. 2. – Holotype cranium of *Syncerus* sp. nov., OMO 53-10001. Clockwise from top left: dorsal, ventral, anterior, posterior, left lateral, and occlusal views. Scale bar for crania is 10 cm total, that for teeth is 5 cm.</figcaption></figure>
<table><caption>TABLE 2. – Measurements on OMO 53-10001, holotype cranium of *Syncerus ekosowan* sp. nov.</caption><thead><tr><th>Measurements</th><th>OMO 53-10001</th></tr></thead><tbody><tr><td>Horn core DAP x DT</td><td>R: 83.2 x 63.4<br>L: 79.6 x 61.4</td></tr><tr><td>Horn core total curved length (along anterior surface)</td><td>R: 285<br>L: 275</td></tr><tr><td>Inclination angle of horn core against parietals</td><td>45°</td></tr><tr><td>Divergence angle of horns</td><td>150° in anterior view<br>120° in dorsal view</td></tr><tr><td>Width across dorsal orbits</td><td>175</td></tr><tr><td>Width across external pedicels</td><td>165</td></tr><tr><td>Internal distance between horn cores</td><td>125</td></tr><tr><td>Width between supraorbital foramina</td><td>69e</td></tr><tr><td>Cranio-facial angle</td><td>125°</td></tr><tr><td>Parietal-Occipital angle</td><td>90° against lateral occipital face<br>130° against the median nuchal crest</td></tr><tr><td>Minimum distance between temporal lines</td><td>61.4</td></tr><tr><td>Width across mastoids</td><td>173 (195 with temporal tubercles)</td></tr><tr><td>Occipital height from dorsal foramen magnum</td><td>61.1</td></tr><tr><td>Occipital height from ventral foramen magnum (basion)</td><td>87.6</td></tr><tr><td>Width across posterior tuberosities</td><td>55.2</td></tr><tr><td>Width across anterior tuberosities</td><td>27.1</td></tr><tr><td>Distance from posterior to anterior tuberosities</td><td>53.7</td></tr><tr><td>Bulla length</td><td>ca. 34</td></tr><tr><td>Upper molar row length</td><td>R: 73.7 (70.6 without the metastyle)<br>L: 72.6 (69.1 without the metastyle)</td></tr></tbody></table>
<figure><img src="image_12.png" /><figcaption>FIG. 3. — Horn cores of *Syncerus ekosowan* sp. nov. L 193-10027 and L 444-10005. Both are right (R) and left (L) horn cores shown in dorsal (top) and anterior (bottom) views. Basal cross-sections shown as if looking into the horn core from the base. Scale bar equals 10 cm total.</figcaption></figure>

**TYPE LOCALITY, STRATA, AND AGE.** — Shungura Formation, members B to G, specifically units B-9 or B-10 to G-5 (3.03 Ma or 2.98 Ma to 2.13 Ma; De Heinzelin 1983; Gardin *et al.* 2024).

**ETYMOLOGY.** — From the Nyangatom word for buffalo, in honor of the Nyangatom people who live in the region and have contributed to scientific expeditions to the Shungura Formation over many decades.

**DIAGNOSIS.** — A small *Syncerus* about the size of forest populations of extant *Sy. caffer*. Horn cores are short and strongly divergent, emerging from the skull almost horizontally, dorsoventrally compressed, and curving backwards weakly to moderately. The horn core basal cross-section is triangular with two to three keels, and a flattened anterior surface (Fig. 4). The posterior keel is usually the best defined followed by the anterior keel, while the ventral keel (homologous with posterolateral keel in bovids with less divergent horns) is much more rounded, and is located almost as far anteriorly as the anterior keel. Pedicels are not swollen, and the anterior keel does not continue anteriorly along the pedicel as in *Ugandax*. Rugosities continue a short way onto the dorsal surface of the frontal medial to the horn core, indicative of some medial extension of the horn sheath (perhaps incipient horn bossing). Frontal sinuses are expanded and hollow out the pedicel but do not extend into the horn core beyond the very base (like *Ugandax* and unlike *Sy. caffer*). Occipital is broad and
low, with a rounded dorsal outline, as in *Ugandax*, with less robust and laterally protruding mastoid regions than in *Sy. caffer*. Temporal lines are moderately spaced apart. Basioccipital is long (longer than in *Sy. caffer*), with prominent anterior tuberosities that are much closer together than the posterior ones. Bulla is medium sized and strongly compressed mediolaterally (as in *Sy. caffer*, and unlike the enlarged bullae in *Ugandax*). A large, laterally projecting tubercle is present above the acoustic meatus. Tooth row located more anteriorly than in *Sy. caffer*, but like *Ugandax* (with posterior border of the M³ located beneath the anterior orbital orbit). The lateral palatal indentations reach the level of the posterior M³ and are slightly further anterior than the median one. In many ways this is a species slightly more derived than *U. coryndonae*, but more primitive than later *Syncerus* like *Sy. caffer* and *Sy. antiquus* and perhaps also *Sy. acoelotus*. Further differences from these species are given below.

<figure><img src="image_13.png" /><figcaption>FIG. 4. – Basal cross-sections of horn cores of *Syncerus ekosowan* sp. nov. All shown in distal view (as if looking into the cranium). Dorsal is to the top, ventral to the bottom, anterior to the sides, and posterior to the midline of the figure. L, left; R, right. Scale bar equals 10 cm total.</figcaption></figure>
DESCRIPTION

The holotype cranium OMO 53-10001 (Fig. 2) is the most complete specimen known of an early *Syncerus*. Horn cores arise just behind the orbit, are strongly inclined (45° against the parietals) and strongly divergent (150° in anterior view, and 120° in dorsal view), arising almost straight sideways and curving backwards (homologous to the medial curvature in more upright horns such as those of *Ugandax*). They are of moderate length, about two-thirds the total length of the cranium. Cross-section is triangular with three keels, posterior keel is quite sharp, and the anterior (anterodorsal) keel is rounded but well defined. The posterolateral (ventral) keel is the least well defined and quite rounded. It is located almost as far anteriorly as the anterior keel, which results in a flat anterior surface to the horn core. The medial (dorsal) and posterolateral (ventral) surfaces are also quite flat, but less so than the anterior surface. Horns are mediolaterally (dorsoventrally) compressed. Wide longitudinal grooves cover the horn core and are most regularly expressed on the dorsal and anterior surfaces. The anterior keel continues ventrally as a rugosity onto the frontal, and there are weak rugosities on the pedicel and frontal medial and anterior to the horn core, indicative of the extension of the horn sheath onto the frontal bones as in extant *Syncerus*. Torsion is basically absent, though there is a hint (perhaps 10° from base to tip) of heteronymous torsion on the right horn core especially.

Features of the cranium are common to most bovins, especially of the buffalo group (Bubalina) and extant *Syncerus*: dorsal braincase at the parietals is flat; temporal fossae are well developed below the horn cores and to either side of the braincase; temporal lines remain wide apart, at their closest still being wider than the foramen magnum. Also common to extant *Sy. caffer* and probably also Pleistocene African bovins, there is a very large tubercle projecting laterally just dorsal to the auditory meatus. The occipital surface is low and wide in outline, quite flat, with extremely narrow and sharp nuchal ridge (dorsal border). The basioccipital is long; posterior tuberosities are very prominent, anterior tuberosities less prominent and much closer together than posterior ones. The bulla is strongly compressed mediolaterally, with a wide space between it and the posterior tuberosities of the basioccipital (similar to many bovins, but not to *Si. shungurense* or *Ug. coryndonae*, which have wide bullae). The zygomatic has an extremely prominent, ventrally projecting masseteric ridge and deep masseteric fossa on its ventral surface, extending onto the posterior maxilla. The zygomatic bar is very narrow at its anterior end below the anterior orbit. The anterior edge of the orbit is located above the posterior edge of the M³. The lateral palatal indentations reach
the level of the posterior edge of M³, while the median indentation is located slightly more posteriorly. The cranium preserves left and right first to third molars only, and these are in quite advanced wear (late middle to early late wear). As expected for Bovini, these have rounded lingual cusps with large basal pillars, labial cusps with prominent styles and ribs, complexly shaped internal enamel cavities, small oval central enamel islands. The M³ has a tall and prominent metastyle. Lengths of the right and left upper molar rows are 74.3 mm and 72.8 mm respectively.

The remaining assigned horn cores match those of the cranium in their basal size and length, weak posterior curvature, triangular cross-section, mediolateral (dorsoventral) compression (though this is weaker in most specimens than in the type), flattened surfaces and three more or less-defined keels. As previously described (Gentry & Gentry 1978; Gentry 1985) for both the Shungura *Syncerus* and for Olduvai *Sy. acoelotus*, the frontal sinuses only reach into the very base of the horn core and no further. The pedicel is hollowed into a single open chamber without bony struts (similar to the alcelaphin condition and unlike extant bovins).

Two specimens show significant variation. The OMO 18-1969-2725 & 2726 horn cores (same individual) differ in having an ovate to diamond cross-section, whereby even the dorsal surface is rounded (Fig. 4). Furthermore, there is no indication of a posterolateral keel, and their dorsoventral compression is much greater. However, the cross-section is still wider anteriorly than posteriorly, and the divergence, surface texture, and basal rugosities of the horn cores, and the overall size, frontal morphology, and limited extent of the frontal sinuses all match the other specimens of *Syncerus ekosowan*. The basal horn core anteroposterior diameter (and probably also length) is of similar size, frontals are thick, and the interfrontal and frontoparietal sutures all indicate an adult individual. L 193-10027 is a pair of short horn cores that differ from all other specimens referred here in having pronounced torsion and strong dorsally directed curvature. What is interesting is that the torsion is homonymous, entirely opposite to the condition in buffaloes (Bubalina), or even the Bovinae in general (Bovini, Tragelaphini, Boselaphini). Homonymous horn torsion is normally characteristic of bison and oxen (Bovina, *Bos* including *Bison*). L 193-10027 also lacks keels, and the horn core cross-section is ovate, with a narrower posterior end and wider anterior end. L 193-10027 resembles the short-horned crania attributed to *Syncerus* sp. and *Sy. acoelotus* from Daka, Middle Awash (Gilbert 2008) and Olduvai (Gentry & Gentry 1978 pl. 4 fig. 2). These
are further discussed below. It is possible that OMO 18-1969-2725 & 2726 and L 193-10027, with their weaker keels and more ovate cross-sections, represent female individuals.

Gentry (1985) gave the length of three horn cores, L 837-2, L 1-238 B, and L 744-1, as ~200 mm, ~210 mm, and 240 mm, respectively. This is shorter than the 275 mm-320 mm range measured on the OGRE and IORE specimens described here. L 837-2 and L 1-238 B are on the smaller end of referred specimens in terms of basal dimensions. The partial cranium L1-238 B appears to be slightly larger in overall size than OMO 53-10001, but with smaller and shorter horns. Occipital regions in partial crania L 2-26 B and L 607-1 G are also of similar to slightly larger size than in OMO 53-10001.

While there is abundant isolated dental material attributable to Bovini from the Shungura Formation, it is difficult to distinguish among different, similarly sized species, such as *Sy. ekosowan* and *Ug. coryndonae*, or possibly even smaller individuals of *Si. shungurense*. Therefore, the only dental material attributed with confidence to *Sy. ekosowan* are for the moment just the teeth attached to the holotype cranium. Similarly, postcranial remains – even when these are collected – are very difficult to assign to species with confidence, and for the time being are not considered here. Further work may refine the situation.

## COMPARISONS

*Syncerus ekosowan* differs from *Ugandax coryndonae*, best known from Late Pliocene deposits at Hadar (Gentry 2006; Geraads et al. 2012), by its smaller size, horn cores with greater divergence, greater mediolateral (dorsoventral) compression, stronger curvature, and more forward location of the posterolateral keel resulting in a flattened anterior surface. These horn features also generally characterize the genus *Syncerus*, being more or less common to all later species, i.e., *Sy. acoelotus*, *Sy. antiquus*, and *Sy. caffer*. However, horns of *Sy. ekosowan* and *U. coryndonae* both show a large range of variation, and isolated horn cores might in some cases be difficult to distinguish. In *Ugandax*, the anterior pedicels are swollen, but not in *Syncerus*. In *Ugandax* the anterior keel continues down the pedicel as a discrete linear rugosity, while in *Syncerus* any pedicel or frontal rugosities are less well defined and, while best developed anteriorly, are present medial to the horn core (possibly reflecting incipient horn sheath bossing). Supraorbital foramina in *Ugandax* are associated with shallow round or elongated depressions (pits or grooves), while these depressions appear absent in OMO 53-10001, though the frontal surface is not well preserved. The basioccipital is very similar in proportions, but OMO 53-10001 has larger anterior tuberosities than
generally observed in specimens of *U. coryndonae*. The Hadar *Ugandax* has very large bullae (Gentry 2006 referred to these as 'moderately large'), while in OMO 53-1000 these are smaller and more mediolaterally-compressed, and more similar to the condition in extant Bovini.

*Simatherium shungurense* was described based on a cranium from Member G of the Shungura Formation (holotype: Omo 75/s-1969-2924 G; Geraads 1995) and other crania almost certainly attributable to the same species are known from the Middle Awash and Koobi Fora (discussed by Bibi 2009). *Syncerus ekosowan* differs from *Si. shungurense* in much shorter horns (Fig. 6) that are much more divergent. In *Si. shungurense*, the horn cores are much longer, more slender, quite straight, arising above or just behind the orbits, strongly inclined posteriorly, and only weakly divergent. Cranial metrics of *Si. shungurense* are mostly larger than in OMO 53-10001 (e.g. upper molar length of 88 mm compared to ~73 mm, respectively). In *Si. shungurense* the basioccipital tuberosities are not very prominent, and project less than in the smaller OMO 53-10001 cranium. *Simatherium shungurense* also has very wide auditory bullae that almost contact the posterior tuberosities, in contrast to the Shungura (and extant) *Syncerus*, in which the bullae are compressed mediolaterally, with a large space between them and the posterior tuberosities. In *Si. shungurense* the lateral palatal indentations are placed far posterior to the M³, and level with the median indentation, while in OMO 53-10001 they are slightly anterior to the median indentation. The slenderness of the horns in *Si. shungurense* led Gentry (2010) to suspect that it might have been a female, and he further proposed that the Shungura *Syncerus* might be attributable to *Si. shungurense* instead (and specifically that the horn core L 16-10 G could belong to a male *Si. shungurense*). However, fundamental differences in size, as well as in the shape, length, and orientation of the horns between *Sy. ekosowan* (possibly including L 16-10 G, though we were not able to examine this specimen) and *Si. shungurense* cannot be explained by sexual dimorphism. Similarly, the differences between *Si. shungurense* and *Pelorovis* spp., and *Sy. acoelotus* are too great to permit explanation by sexual dimorphism.
<figure><img src="image_14.png" /><figcaption>FIG. 5. – Basal horn core anteroposterior (DAP) and transverse (DT, dorso-ventral in *Syncerus*) diameters in *A*, African fossil and extant bovins, and *B*, *Syncerus* spp. The dashed line is the 1:1 line (i.e. no horn core compression). *Ugandax* shows the least transverse compression. *Pelorovis* and *Syncerus* have different size-shape allometries: in *Pelorovis*, small specimens exhibit greater transverse compression than larger specimens, while the opposite is true in *Syncerus*. Within *Syncerus*, *Sy. caffer* exhibits the greatest degree of transverse compression. Specimens of *Syncerus ekosowan* sp. nov. (diamonds) overlap with both *Ugandax* and *Sy. caffer* in compression. Units are in mm. Axes are log-scaled.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>FIG. 6. – Horn core length plotted against basal anteroposterior diameter (DAP) in *A*, Bovini, and *B*, *Syncerus* spp. *Syncerus caffer* horns are relatively short for their basal size. Relative horn length in *Sy. ekosowan* sp. nov. (diamonds) is intermediate between *Ugandax* and *Sy. caffer*. Horn length in *Sy. antiquus* is much greater than predicted by the length to basal size relationship in other species of *Syncerus*. Units are in mm. Axes are log-scaled.</figcaption></figure>

*Syncerus ekosowan* differs from the holotype of *Sy. acoelotus* (a calvarium with horns from Upper Bed II in Olduvai Gorge, Gentry & Gentry 1978) in smaller size, horn cores that are shorter, less divergent, and with a more triangular cross-section (Fig. 6). The lack of penetration of the horn core by the frontal sinuses is common to both. Gentry & Gentry (1978 pl. 4 fig. 2) also attributed a shorter-horned specimen to *Sy. acoelotus*, and another specimen was later described by Gilbert (2008 fig. 3.15) from the Daka Member in the Middle Awash (~1 Ma). The Daka specimen, BOU-VP-1/36, is large and heavy-set, with horns that are short, strongly compressed mediolaterally (dorsoventrally), and oval in cross-section, with
strong medial curvature. Some specimens of *Sy. ekosowan* (e.g., L 139-10027, OMO 18-1969-2725) resemble these shorter-horned forms in their more ovate cross-sections, but differ in being longer, in having much larger bases with more triangular cross-sections, and stronger posterior curvature. Furthermore, the Daka cranium, despite having smaller horns, is much larger in size than *Sy. ekosowan*, with broader frontals and parietals that are flattened and almost in the same plane, an occipital surface that is much wider, with a long and horizontal dorsal border, and an extremely enlarged mastoid region. These features suggest that the Daka short-horned *Syncerus*, and likely the Olduvai specimen too, might be subadult individuals of large species such as *Sy. acoelotus* or *Sy. antiquus*. The consistency of the morphology across the assemblage of *Syncerus ekosowan*, which spans members C to G of the Shungura Formation, and the lack of a greater variation of lengths, shapes, and sizes in the fossil record argues against the possibility of *Sy. ekosowan* being an eco-phenotypic variant of another highly variable species. It differs significantly from all its contemporaries or near-contemporaries (*Si. shungurense*, *U. coryndonae*, *Sy. acoelotus*), and the fossil record includes no intermediate forms that could bridge the morphological differences between *Sy. ekosowan* and any of the other species.

*Syncerus ekosowan* differs from all varieties of extant *Sy. caffer* in that the frontals between the horn cores are not raised into a broad ridge (intercornual torus, even present in diminutive forest buffalo individuals), in having a taller (less wide) occipital surface, and a longer basioccipital; in a palate that is located slightly more anteriorly (with the posterior edge of the M³ located below the anterior orbit, while it is below the central orbit in *Sy. caffer*); in frontal sinuses that are less extensive only reaching the basal horn core, while these extend deep into the horn core, the posterior frontals, parietals, and possibly even the occipitals in some individuals of *Sy. caffer* (Farke 2010). Most or all these features in OMO 53-10001 are also shared with *Ug. coryndonae*, suggesting they are all plesiomorphic for the lineage. OMO 53-10001 further differs from the larger (i.e., savanna) individuals of *Sy. caffer* in much smaller size, smaller, shorter, and less divergent horn cores that are less dorsoventrally compressed above the base and lack expanded bases and bossing (Figs 5; 6). It is of similar to slightly smaller size as small, forest-dwelling individuals (*Sy. caffer 'nanus'*), but has horns that are relatively much larger, located less far back on the cranium, more divergent and less posteriorly directed, less inclined and remaining above the level of the dorsal cranium (dropping below in *Sy. c. nanus*), with a much better developed triangular
cross-section and weaker mediolateral (dorsoventral) compression, and less evidence of a frontal boss.

<figure><img src="image_16.png" /><figcaption>FIG. 7. – Molar length and width in African bovins (fossil and extant). The only teeth attributable with confidence to *Syncerus ekosowan* sp. nov. are those on the holotype cranium OMO 53-10001 (i.e., uppers) and these are similar in size to small specimens of *Syncerus*. Teeth of *Syncerus* are similar in size to those of *Ugandax*, but smaller than those of *Pelorovis*, though overlap is high. Teeth of OMO 53-10001 (plotted with diamond outlines) are wide in relation to their length, but it is not certain that this is common to all *Sy. ekosowan*. Note x and y axes are log-scaled.</figcaption></figure>

Dental differences are difficult to identify since only the holotype preserves teeth that can be confidently attributed to the new species. Compared to other *Syncerus* spp., the upper molars in OMO 53-10001 are small and wide relative to their length (Fig. 7). They differ from those of *Sy. caffer* in smaller size and possibly in having less prominent labial ribs and basal pillars and lower-crowned teeth. Lengths of the right and left upper molar rows are comparable to the smallest individuals of extant *S. caffer nanus*, and similar in size to a maxilla of *Syncerus acoelotus* from Olduvai (Gentry & Gentry 1978). An estimate of premolar length on the missing upper premolars of OMO 53-10001 is ~45–55 mm, giving a
premolar to molar length ratio of ~65-70%, which is similar to that in extant individuals of *Sy. caffer* (59-70%, n=17, supplementary data). Although no lower dentitions can be attributed confidently to *Sy. ekosowan*, a large number are known from the Shungura Formation, and these differ from extant *Sy. caffer* in longer premolar rows; Gentry (1985) reported a lower premolar to molar length ratio of 66% for a Shungura mandible and a range of 55% - 62% for six mandibles of *Sy. caffer*. Furthermore, lower fourth premolars retain the plesiomorphic condition of posteriorly slanted metaconids and open anterior lingual valleys, while in *S. caffer*, the metaconid is anteriorly expanded and the anterior lingual valley is closed (Gentry 1985; Bibi pers. obs.).

GEOMETRIC MORPHOMETRICS

ANALYSIS OF BOVINI

In the UPGMA cluster dendrogram (Fig. S1A), *P. turkanensis* was strongly differentiated from all other bovins, Bubalina and Bovina were distinguished from each other (although not consistently), and all fossil specimens besides *Pelorovis* grouped together in a position closer to Bubalina than to Bovina. The cluster results only provide general confirmation of the resemblance of the cranium of *Sy. ekosowan* OMO 53-10001 to both Asian and African buffaloes. Analyses of the Procrustes-transformed shape coordinates indicate that genera differ significantly in shape (Procrustes ANOVA, *p* = 0.001, *Z* = 7.1). Cranial shape differences among *Bos*, *Syncerus*, and *Bubalus* are all significant (Pairwise Procrustes ANOVA, *p* < 0.05, *Z* > 2.1). Pairwise comparisons among genera were all significant, except for the comparison of *Pelorovis* and *Simatherium*, and all comparisons against *Ugandax*. However, all three of these fossil genera are represented here by a single specimen each, so these results might reflect low statistical power, rather than an actual lack of shape differences.

Among Bovini, the first four principal components of cranial shape explained 32%, 19%, 8%, and 6% of the variation, respectively (Figs 8; S2). Cranial shape changes associated with the first three components are shown in Fig. 9A. Bubalina and Bovina separate along PC1, with dorsoventrally deeper crania, shorter faces, less protruding orbits, and more anteriorly located horns in the Bubalina (Figs 8; 9). PC1 is strongly correlated negatively with log centroid size (Spearman's rank correlation test, *p* < 0.001, *rho* = -0.76, R² = 0.58),
with larger individuals exhibiting more elongated faces (Fig. 10A). This relationship is
however not significant within any of the three extant genera (Bos, *Syncerus*, *Bubalus*, p >
0.05). *Bos* displays much higher disparity than *Syncerus* and *Bubalus*, exemplified by the
large differences between *Bos primigenius* and bison species along PC2, with deeper and
broader crania, shorter faces, and more protruding orbits in bison (Fig. S2). The holotype of
*Sy. ekosowan*, OMO 53-10001, consistently groups close to other specimens of *Syncerus*.
*Pelorovis turkanensis*, *Si. shungurense*, and *Sy. antiquus* are located at the positive ends of
both PC2 and PC3, reflecting crania that are narrow in both dorsoventral and medio-lateral
dimensions. Sensitivity analyses indicate that the primary shape components identified in the
PCA are not strongly affected by deformation or the estimation of missing landmark
coordinates (Figs. S3–S7, S9, S11).

Among all Bovini, shape was highly correlated with log centroid size (Procrustes ANOVA, p = 0.001, Z = 4.5, R² = 0.23), as well as in genera *Bubalus* (p = 0.004, Z = 2.38, R²= 0.23) and *Syncerus* (p = 0.015, Z = 2.16, R² = 0.15), but not in *Bos* (p = 0.075, Z = 2.10, R² = 0.14). The relationships of shape and size among *Bos*, *Bubalus*, and *Syncerus* were significantly different in intercept, reflecting baseline shape differences (p = 0.001, Z = 4.2, R²= 0.18), but not in slope (interaction of size and genus, p = 0.11, Z = 1.3, R² = 0.04). Visualizing the regression score of shape on size against log centroid size (Fig. 11A) confirms similar slopes but large differences in intercept, particularly between *Syncerus* and *Bubalus* on one hand, and *Bos* (including *Bison*) on the other.
<figure><img src="image_17.png" /><figcaption>FIG. 8. – The first four principal components of cranial shape in Bovini, with 95% ellipses. Percentages indicate the amount of variation explained by each component. *Bubalus*, *Syncerus*+*Ugandax*, and *Bos* (including *Bison*) are best differentiated along PC1. OMO 53-10001 (shown with diamonds) plots with other *Syncerus* specimens. See supplementary Fig. S2 for version with taxon labels.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>FIG. 9. – Shape changes associated with the first three principal components. **A**, all Bovini; **B**, *Ugandax* and *Syncerus* spp. Crania made by warping a mesh of *Syncerus caffer* located closest to the total sample mean shape. See Fig. S7 for version with fossils removed prior to the analysis.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 10. PC1 plotted against centroid size. A, all specimens (Bovini). B, *Syncerus* and *Ugandax* only. In A, bovins show increasing facial elongation and gracilization (more positive PC1 values) with increasing size. The relationship is not significant within any of the three extant genera. In B, *Syncerus* shows the opposite relationship, through the amount of change is much smaller. Note that one specimen of *Sy. antiquus* is an extreme outlier (bottom right). Dashed line is the fit for all data points. Solid line in B is the fit for *Sy. caffer* only. X-axes are log-scaled. See Fig. S8 for version with taxon and specimen labels and Fig. S9 for version with fossils removed prior to the analysis.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. 11. The regression score of cranial shape on log centroid size plotted against centroid size. A, all specimens (Bovini). B, *Syncerus* and *Ugandax* only. In A, the buffaloes (*Bubalus*+*Syncerus*) differ in intercept from oxen (*Bos* including *Bison*). *Syncerus ekosowan* sp. nov. lies closest to the size-shape relationship of subadults and small, short-horned ('nanus') adults of *Syncerus caffer*. X-axes are log-scaled. See Fig. S10 for version with taxon and specimen labels and Fig. S11 for version with fossils removed prior to the analysis.</figcaption></figure>

## ANALYSIS OF UGANDAX AND SYNCERUS

In the analysis including just *Syncerus* spp. and *Ugandax demissum*, the first four principal components explained 31%, 15%, 12% and 9% of the variation, respectively (Fig. 12). The cranial shape changes associated with the extremes of first three principal components are shown in Fig. 9B. Both the estimation of missing landmark coordinates and the removal of fossils had little effect on the principal components space (Figs. S7, S9, S11, S13).

PC1 and PC3 reflect facial elongation, cranial depth, and the flatness of the dorsal cranial surface. (PC1 negative end and PC3 positive end, Fig. 9B). *Ugandax demissum*, *Sy. ekosowan*, and one specimen of *Sy. antiquus* (NHM PV M 34562) are close to or within the
95 % range of *Sy. caffer* along the first four principal components. The other specimen of *Sy. antiquus* (Atbara 20-88) is strongly separated from *Sy. caffer* along both PC1 and PC3.

In contrast to the relationship in Bovini, PC1 is correlated positively with log centroid size (Spearman's rank correlation test, p < 0.05, rho = 0.50, R² = 0.25), indicating that facial elongation decreases with increasing size (Fig. 10B). This relationship is even stronger when only individuals of *Sy. caffer* were considered (p < 0.01, rho = 0.78, R² = 0.61). Atbara 20-88, the largest cranium and that with the most negative PC1 score, is a prominent outlier to this relationship, possibly reflecting an artifact of distortion (dorsoventral compression) during fossilization. Analyses excluding fossil specimens confirm this relationship in *Sy. caffer* (Fig. S7, S9).

A plot of the regression score of shape on size shows that *Syncerus ekosowan*, *Sy. antiquus*, and *U. demissum* fall close to the size-shape relationship of *Sy. caffer* (Fig. 11; Fig. S8). Even though it is an adult, OMO 53-10001 is in the size range of immature specimens of *Sy. caffer*. The fossil crania of *Sy. antiquus* are as large or larger than the largest *Sy. caffer* in our sample. At least one of these (NHM PV M 34563) is located within the confidence interval of the allometric regression for *Sy. caffer* (Fig. 11B). Cranial shape and log centroid size were correlated among all *Syncerus* and *Ugandax* specimens (Procrustes ANOVA, p = 0.04, Z = 1.8, R² = 0.11) and in *Sy. caffer* alone (p = 0.001, Z = 2.9, R² = 0.23).
<figure><img src="image_21.png" /><figcaption>FIG. 12. – The first four principal components of cranial shape in *Syncerus* and *Ugandax*. The new cranium is similar to subadult and short-horned ('nanus') adult individuals of *Sy. caffer*. The position of one specimen of *Sy. antiquus* at the extremes of PC1 and PC3 likely reflects deformation (dorsoventral compression) during fossilization. See Fig. S12 for version with specimen labels and Fig. S13 for analysis without fossil specimens.</figcaption></figure>
# DISCUSSION

## THE FIRST SYNCERUS

The discovery of OMO 53-10001 C provides a basis for the naming of the oldest and most primitive species of *Syncerus*. The main diagnostic features allying the new species with *Syncerus* and distinguishing it from the more primitive *Ugandax* are the more inclined, divergent, and compressed horns with a more anteriorly located posterolateral keel, and perhaps also a reduced auditory bulla. *Syncerus ekosowan* also displays relatively stouter horns than in *Ugandax* species (basal dimensions are similar, though cranial size is smaller). Morphological variation is high, however, and isolated horn cores or cranial fragments can be difficult to distinguish. Our findings support Gentry's (1985) initial assignment of the Shungura Formation material to *Syncerus*, and the establishment of a new species.

## CONSERVED CRANIAL ALLOMETRY IN SYNCERUS

Our results suggest that cranial diversification within *Syncerus* occurred largely along a conserved allometric trajectory. Both fossil and extant specimens—including the new Shungura cranium—lie on or close to the size-shape relationship defined by *living Sy. caffer*. (Fig. 11). The new cranium falls outside the 95% confidence intervals but so do several specimens of *Sy. caffer* and one of two specimens of *Sy. antiquus*. Larger samples are required to know whether these deviations represent significant biological differences, a wide range of intraspecific variation, or the effects of fossil deformation (e.g. dorsoventral compression). Nonetheless, the high similarity of all fossil specimens of *Ugandax* and *Syncerus*, including the new Shungura cranium, to the size and shape of *Sy. caffer* provides support for the hypothesis that cranial morphological disparity in this clade reflects scaling of a single underlying developmental program, rather than repeated independent innovations. *Syncerus ekosowan* may therefore represent an ancestral ‘template’ for the origin of later *Syncerus* forms.

The consistency of the Shungura remains suggest that its small body size and short, divergent horns are unlikely to reflect miniaturization, but rather an early stage from which larger-bodied, long-horned species later evolved. While delayed maturation of body and horn growth relative to reproductive maturity is a classic example of neoteny (reviewed in Gould 1977), the evolutionary trajectory from a small, short-horned ancestor such as *Sy. ekosowan* to large, long-horned taxa like *Sy. acoelotus*, *Sy. antiquus*, and *Sy. caffer* is better
interpreted as peramorphosis. In this case, the evidence suggests hypermorphosis—extension of the growth period—rather than acceleration of developmental rates, consistent with observations in other bovid species (Estes *et al.* 1974; Vrba 1996).

## ECOMORPHS AND SHUNGURA HABITATS

In analogy to extant *Sy. caffer 'nanus'*, Gentry (1985) suggested that the small Shungura *Syncerus* indicated the presence of woodland or forest habitats: "It looks as if the *Syncerus* lineage could long have shown a degree of morphological plasticity as an adaptive response to different environments, and one might conclude that forest or woodland environments were more widespread in the Omo area during member C time than before or after that time" (Gentry 1985 p.142). However, as noted above, we do not see a large range of morphological variation as might be expected if *Sy. ekosowan* was a highly plastic species. Unlike in *Sy. caffer*, the habitat preferences of *Sy. ekosowan* cannot be assumed from its size and cranial form but require paleoecological analyses. Stable carbon isotope data indicate that Shungura bovins between Members C and G had C₄-dominated—i.e. most likely grass-dominated—diets (Negash *et al.* 2020). While isolated dental specimens cannot be assigned with confidence to species, *Sy. ekosowan* is by far the most abundant bovin in the Shungura Formation based on horn core specimens and so presumably some or most of the bovin teeth analyzed by Negash *et al.* (2020) belonged to this species. Grass-eating does not, however, preclude forest or wooded habitats, as even forest buffalo today prefer grazing in clearings or along roads and rivers (Bekhuis *et al.* 2008; Prins & Sinclair 2013), though individuals from the Ituri Forest were found to have pure C₃ diets (Cerling *et al.* 2004). It has been suggested that the ancestral niche for *Sy. caffer* was in open marshy areas namely at the interface between savanna and rainforest (Bekhuis *et al.* 2008; Prins & Sinclair 2013), a plausible hypothesis also for *Syncerus ekosowan*.

## PHENOTYPIC DIVERSIFICATION IN BOVINI

Our geometric morphometric analyses also provide insights into phenotypic diversification in the Bovini clade. Similar cranial shape and size-shape allometries in *Syncerus* and *Bubalus* confirm the greater similarity of African and Asian buffaloes (i.e. Bubalina) to each other than to *Bos* (Bovina). As in *Syncerus*, size-shape allometry appears to have been equally important in the phenotypic diversification of *Bubalus*, explaining much (23%) of the cranial shape variation between mainland water buffalo (*Bu. arnee*) and island dwarfs such as the
tamaraw and anoas (Bu. mindorensis, Bu. depressicornis, Bu. quarlesi). Further studies could investigate whether the proposed ancestors of Bubalus, the fossil forms Hemibos and Proamphibos, lie along the same size-shape regression. Our analysis also revealed a much greater degree of cranial disparity in Bos than in Syncerus and Bubalus. This is interesting considering that Bos diversified more recently than the buffaloes (Hassanin et al. 2012, 2013; Bibi 2013), implying faster rates of phenotypic evolution. The extinct Pelorovis turkanensis shows several aspects of cranial shape in common with species of Bos. (Figs 8; 10), and the proposal that it should be reassigned to this genus is worth further consideration (Martínez-Navarro et al. 2007). However, the claim by these same authors that it is directly ancestral to Bos primigenius (including Bos taurus) is contradicted by autapomorphic features in the skull of Pelorovis, the existence both past and present of diverse Bos lineages in Eurasia, and molecular phylogenies, which indicate a divergence of Bos primigenius and Bos javanicus during the Early or Middle Pleistocene, with a probable Asian origin (Hassanin et al. 2013).

## CRANIOFACIAL ALLOMETRY IN BOVINI

Species of Bovini follow the rule of cranial evolutionary allometry (CREA)(Cardini 2019), whereby larger species have relatively longer and more gracile faces (as represented by PC1, Figs. 9A, 10A). Because facial gracility results in a weaker bite force relative to size, the trend to longer faces with increasing size in Bovini likely reflects consistent bite force demands (functional homogeneity) across the allometric gradient (Zelditch & Swiderski 2023; Mitchell et al. 2024). This might be expected in grazers, given that grasses are more homogeneously nutritious and overall less fibrous and lignified than browse (Jarman 1974). The correlation of size and facial elongation is, however, not significant within genera (Bos, Bubalus, Syncerus, Fig. 10A). Interestingly, when Syncerus and Ugandax were analyzed separately, the relationship was inverted, with facial shortening accompanying increasing size (Fig. 10B). The presence of a fossil specimen as an extreme outlier along PC1 (Sy. antiquus Atbara 20-88) made interpretation of the relationship across Syncerus, however, unclear (Fig. 10B). Examining the ontogenetic allometry in Sy. caffer confirmed the correlation of larger size with shorter faces in this species (Figs. 10B, S9B). Inverted growth and evolutionary allometries were also noted in several rodent lineages, and likely reflect growth constraints, namely the fact that infants are born without fully formed teeth and jaw muscles, and increase in facial robusticity with age (Zelditch & Swiderski 2023).
While inverse craniofacial allometry might represent selection for greater bite force requirements with increasing size (Mitchell *et al.* 2024), there seems to be no evidence for dietary differences between young and adults of *Sy. caffer* (Prins & Sinclair 2013). Whether an inverse CREA rule characterizes the evolutionary allometry of *Syncerus* as whole should be established with further work using larger fossil sample sizes.

## DEPARTURES FROM SHARED ALLOMETRY

A large proportion of cranial shape variation is, however, not determined by size, leaving room for species-specific innovations, particularly in horn morphology. While our examination of horn core length relative to basal size in bovins showed that the new Shungura species shares the relatively shorter horns of other *Syncerus* species, they revealed that another species departs significantly from the length-size allometry of *Syncerus*. Horn cores of the giant buffalo *Sy. antiquus* can reach over 1.5 m in length, far greater than predicted by allometry in the *Syncerus* clade (Fig. 6B). This contrasts with the large antlers of the giant elk, which, as Gould (1974) famously demonstrated, can be predicted by the general allometric relationship of body size and antler length in deer. Strong selective pressures might have been behind the evolution of the great horn span of the giant buffalo, resulting in a significant departure away from the conserved length to size relationship in the *Syncerus* clade. This supports the proposal that secondary sexual characters such as horns and other weapons or ornaments can evolve rapidly, especially following population isolation (West-Eberhard 1983; Vrba 1995; Emlen 2008).

The extreme horn length of *Sy. antiquus* likely reflects prolonged post-maturational growth of horn cores, analogous to patterns seen in extant buffalo, sheep (*Ovis*), and in other continuously growing structures such as elephant tusks (*Loxodonta*, *Elephas*) and giraffe ossicones (*Giraffa*), with such extension of growth duration in secondary sexual characters typically associated with territoriality and living in large groups (Estes *et al.* 1974). However, while living bovins are gregarious, they are all non-territorial, and it is reasonable to infer that the giant buffalo shared this social system. The selective context for such extreme weaponry therefore remains uncertain, but any interpretation would also have to explain other extreme examples such as the long-horned bison (*Bison latifrons*), where escalation in size and weaponry also likely represents lineage-specific departures from otherwise conserved allometry.

## CONCLUSIONS
The new *Syncerus* material from the Shungura Formation provides the earliest well-preserved evidence of the African buffalo genus. These fossils, including a nearly complete cranium, document a small-bodied, short-horned species, *Sy. ekosowan*, that predates the better-known Pleistocene representatives of the genus. Geometric morphometric analyses suggest that *Sy. ekosowan* and other early *Syncerus* specimens follow the same cranial allometric trajectory as extant *Sy. caffer*, tentatively supporting the view that much of the morphological diversification within the clade arose through conserved growth patterns. This conserved trajectory helps to explain the striking eco-morphological variation among modern African buffalo, in which forest and savanna morphotypes can be understood as heterochronic outcomes along a shared developmental pathway. At the same time, the extreme horn length of *Sy. antiquus* represents a clear departure from this pattern, highlighting the potential for lineage-specific innovations in secondary sexual traits.

More broadly, these findings underscore both the value of new fossil discoveries and the need for larger comparative samples. Expanding the fossil record and integrating it with developmental analyses will be essential for describing the full range of cranial and horn diversity in *Syncerus*, and for uncovering the developmental mechanisms by which evolutionary constraint and innovation combined to shape the clade. Equally important is further investigation of ecophenotypic plasticity in extant *Sy. caffer*, and the search for traces of such plasticity in the fossil record. This will require not only new and larger fossil samples, but also a critical revision of existing museum collections.

## ACKNOWLEDGEMENTS

This study was supported by German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) project number 282995372 to F.B. and conducted within the framework of the Omo Group Research Expedition (OGRE) field and lab work. The OGRE is a joint program of PALEVOPRIM, the CFEE and the EHA principally funded by the Ministry of Europe and Foreign Affairs, the National Research Agency, the Région Nouvelle-Aquitaine, CNRS INEE, PALEVOPRIM, and the Fyssen Foundation. The OGRE is extremely grateful to the Ethiopian Heritage Authority, the SNNPR, the South Omo Zone, the Nyangatom and Dassanetch Weredas and their people for their help and reception. The specimen from the Middle Atbara, Sudan, was collected as part of fieldwork funded by the German Research Foundation (DFG project 387794796 to F.B.), and that from Singa studied
with the support of the European Research Council (ERC Consolidator Grant, PALEONILE, project 101045217). We are indebted to the hundreds of people who participated in the fieldwork missions of the IORE, OGRE, and other research projects, who managed collections and databases, prepared specimens, contributed to their study, provided financial support, helped with administrative processes, and provided advice and moral support. We thank the National Museum of Ethiopia/Ethiopian Heritage Authority (formerly ARCCH, Ministry of Tourism) for allowing access to the collections and research, NME/EHA staff for support with collection study, Tim White and the Middle Awash Project for access to specimens, and A. Gigliotti, L. Janzen, K. Mahlow, P. Mahlow, L. Rose, K. Salih, and J. Tyler for assistance with 3D scanning. We thank V. Weisbecker (editor), R. Mitchell, and a second anonymous reviewer for comments that significantly improved this manuscript.

## DATA, SCRIPT, AND CODE AVAILABILITY

Raw data files and R scripts for reproducing all analyses are available at:
DOI 10.17605/OSF.IO/WCAVJ

## AUTHOR CONTRIBUTIONS

J.-R.B led field research and new specimen acquisition; F.B. designed the study; J.-R.B. curated specimen data; F.B. performed the analyses and interpreted the results; F.B. and J.-R.B. acquired funding; F.B. wrote the manuscript with comments and revisions from J.-R.B.

## CONFLICT OF INTEREST DISCLOSURE

Both authors are recommenders at PCI Paleo. The authors otherwise declare that they have no financial conflict of interest with the content of this article.

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WEST-EBERHARD M.J. 1983. — Sexual Selection, Social Competition, and Speciation. *The Quarterly Review of Biology* 58 (2): 155–183. https://doi.org/10.1086/413215

ZELDITCH M.L. & SWIDERSKI D.L. 2023. — The Predictable Complexity of Evolutionary Allometry. *Evolutionary Biology* 50 (1): 56–77. https://doi.org/10.1007/s11692-022-09581-1
SUPPLEMENTARY MATERIALS

In addition to raw data and R scripts (see Data, script, and code availability section above), this manuscript also contains the following supplementary materials: Supplementary Figures S1–S13.
<figure><img src="image_22.png" /><figcaption>FIG. S1. – UPGMA cluster dendrograms of the cranial landmark coordinates for (A) all specimens (Bovini), and (B) *Ugandax* and *Syncerus*. African fossil taxa highlighted in bold. OMO 53-10001 plots close to individuals of *Sy. caffer* in A. The high similarity of *Pelorovis*, *Simatherium*, and one specimen of *Sy. antiquus* to each other in A suggests artifacts of cranial deformation might be affecting the cranial shape of these specimens.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>FIG. S2. – The first four principal components of cranial shape in Bovini. Same as Fig. 8, but with taxon labels. See Table 1 for explanation of abbreviations.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>FIG. S3. – The first four principal components of cranial shape in Bovini. Same as Fig. 8 but using the bilaterally symmetric shape component. Similar results indicate that the inclusion of fossil specimens (which might have some degree of bilaterally asymmetric deformation) is not significantly affecting the results.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>FIG. S4. – The first four principal components of cranial shape in Bovini. Same as Fig. 8 but with fossil specimens removed prior to the analysis. Similar results indicate that the inclusion of fossil specimens (some with deformation) is not significantly affecting the results.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>FIG. S5. – The first four principal components of cranial shape in Bovini. Same as Fig. 8 but with landmarks for which data was missing for >5 specimens removed prior to the analysis (a reduction from 53 to 43 landmarks). Similar results indicate that the estimation of missing data is not significantly affecting the results.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>FIG. S6. – The first four principal components of cranial shape in Bovini. Same as Fig. 8 but with all landmarks which were missing in Omo 53-10001 removed prior to the analysis (largely from the anterior snout region, a reduction from 53 to 43 landmarks). Similar results indicate that the estimation of missing landmarks is not significantly affecting the results.</figcaption></figure>
<figure><img src="image_28.png" /></figure>

FIG. S7. – Shape changes associated with the first three principal components. **A**, all Bovini; **B**, *Syncerus* spp. Same as Fig. 9 but with extant species and specimens only (fossil specimens removed prior to the analysis). Note the polarity of PC1 in the analysis with *Syncerus* only has inverted, but otherwise shape change in both this and the analysis of all Bovini are largely similar.
<figure><img src="image_29.png" /><figcaption>Fig. S8. PC1 plotted against centroid size in A, Bovini, B, *Syncerus* and *Ugandax*. Same as Fig. 10, but with taxon and specimen labels.</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>Fig. S9. PC1 plotted against centroid size in A, Bovini, B, *Syncerus* and *Ugandax*. Same as Fig. 10, but with extant species and specimens only (fossil specimens removed prior to the analysis). Note the polarity of PC1 has inverted (also in Fig. S7), but the relationships among specimens remain largely the same.</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>Fig. S10. The regression score of cranial shape on log centroid size plotted against centroid size in **A**, Bovini, **B**, *Ugandax* and *Syncerus*. Same as Fig. 11, but with taxon and specimen labels.</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>Fig. S11. The regression score of cranial shape on log centroid size plotted against centroid size in *A*, Bovini, *B*, *Syncerus*. Same as Fig. 11, but extant species only (fossil specimens removed prior to the analysis).</figcaption></figure>
<figure><img src="image_33.png" /><figcaption>FIG. S12. – The first four principal components of cranial shape in *Ugandax* and *Syncerus* spp. Same as Fig. 12, with data points labelled by specimen number.</figcaption></figure>
<figure><img src="image_34.png" /><figcaption>FIG. S13. – The first four principal components of cranial shape in *Syncerus*. Same as Fig. 12, but with *Syncerus caffer* only (fossil specimens removed prior to the analysis).</figcaption></figure>
<figure><img src="image_35.png" /></figure>

# On the horns of allometry: buffalo fossils provide new insights into bovine cranial scaling and morphological divergence

**Vera Weisbecker** based on peer reviews by **Rex Mitchell** and 1 anonymous reviewer

Faysal Bibi, Jean-Renaud Boisserie (2025) A new fossil buffalo from the Shungura Formation (Ethiopia) reveals the role of heterochrony in the evolution of *Syncerus*. PaleorXiv, ver. 5, peer-reviewed and recommended by Peer Community in Paleontology. https://doi.org/10.31233/osf.io/eh9s8_v5

Submitted: 10 February 2025, Recommended: 27 October 2025

**Cite this recommendation as:**

Weisbecker, V. (2025) On the horns of allometry: buffalo fossils provide new insights into bovine cranial scaling and morphological divergence. *Peer Community in Paleontology*, 100399. https://doi.org/10.24072/pci.paleo.100399

Published: 27 October 2025

Copyright: This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/

The evolution of the mammalian herbivore skull has long been a treasure trove in search for the processes behind the diversity of mammalian cranial adaptation. Artiodactyl (even-toed ungulates) have a particularly prominent position in this area, having produced several diverse radiations with often extensive fossil records linking their ecology and evolution (Janis, 2007). They have particularly been the subject of studies trying to discern how development and adaptation can work together to produce their cranial diversity, with the added bonus of their highly diverse horns and antlers that have been a staple for studies of heterochrony-driven evolution (McNamara, 2012).

The paper by Bibi and Boisserie (2025) adds a deep-time and extended developmental perspective on previous finds including the lead author (Bibi and Tyler, 2022) that bovid artiodactyls (antelopes, cattle, sheeps/goats, and relatives) display strong cranio-facial allometry (changes of shape with size), where larger species have longer faces than smaller ones. The present study describes a new fossil of the morphologically diverse buffalo genus *Syncerus*, represented today only by the morphologically highly diverse African buffalo species *Syncerus caffer*. Adding this unusually small and ancient species to superb existing extant and fossil data, using geometric morphometrics of the cranium and linear morphometrics of the horns, allows the authors to ask how buffalo cranial diversification could have been driven by allometry and heterochrony (differences in rate and timing).

The results overall confirm a common pattern of craniofacial allometry in the data including buffaloes and other bovines, where larger species have longer faces. This is consistent with the concept that biomechanical trade-offs allow larger species more gracile skulls when the mechanical properties of food remain unchanged
(Mitchell et al., 2024). However, there is an interesting twist that allometric trajectories within the *Syncerus* genus (as well as other bovine genera) is potentially the opposite (with larger body masses coinciding with shorter faces). The study also makes the exciting find that horn shape and proportions are far less allometric than cranial shape and a major contributor to cranial diversification away from a common allometric trajectory among bovids.

I find the paper exciting because it took the rare opportunity to integrate excellent fossil data in an ideal artiodactyl system, allowing co-analysis of deep-time evolutionary patterns and integration of Evo-Devo. Aside from the important insights on the origin and diversification of the *Syncerus* genus, what really stands out to me is the cranium versus horn dichotomy. To me, this is a great example of how the cranium is not a homogenous entity and shape variation patterns will vary depending on the kind of selection acting on them (Marcy et al., 2024). It is also fascinating how larger-scale patterns appear to not apply at shorter time scales, showing how important datasets like these with good coverage of evolutionary divergences are. I hope that more fossils will be found that can provide more insights into this fascinating genus, but this thought-provoking and meticulously analysed study is a fantastic base with implications far beyond the taxonomic system it investigates.

**References:**

Bibi, F., and Boisserie, J.-R. (2025). A new fossil buffalo from the Shungura Formation (Ethiopia) reveals the role of heterochrony in the evolution of *Syncerus*. PaleorXiv, eh9s8, ver. 5, peer-reviewed by PCI Paleo. https://doi.org/10.31233/osf.io/eh9s8_v5

Bibi, F., and Tyler, J. (2022). Evolution of the bovid cranium: Morphological diversification under allometric constraint. Communications Biology, 5(1), 69. https://doi.org/10.1038/s42003-021-02877-6

Janis, C. M. (2007). Artiodactyl paleoecology and evolutionary trends. In D. R. Prothero and S. E. Foss (Eds.), The Evolution of Artiodactyls (pp. 292–302). Johns Hopkins University Press, Baltimore. https://doi.org/10.56021/9780801887352

Marcy, A. E., Mitchell, D. R., Guillerme, T., Phillips, M. J., and Weisbecker, V. (2024). Beyond CREA: Evolutionary patterns of non-allometric shape variation and divergence in a highly allometric clade of murine rodents. Ecology and Evolution, 14(7), e11588. https://doi.org/10.1002/ece3.11588

McNamara, K. J. (2012). Heterochrony: The evolution of development. Evolution: Education and Outreach, 5(2), 203–218. https://doi.org/10.1007/s12052-012-0420-3

Mitchell, D. R., Sherratt, E., and Weisbecker, V. (2024). Facing the facts: Adaptive trade-offs along body size ranges determine mammalian craniofacial scaling. Biological Reviews, 99(2), 496–524. https://doi.org/10.1111/brv.13032

## Reviews

### Evaluation round #2

DOI or URL of the preprint: https://doi.org/10.31233/osf.io/eh9s8_v3

Version of the preprint: 3

### Authors’ reply, 10 October 2025

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**Decision by Vera Weisbecker ID, posted 08 September 2025, validated 08 September 2025**

Thank you for responding to the reviewer comments so comprehensively and also adding the additional analyses. The introduction and discussion had a great re-write that has added depth and more information. Just make sure the yellow-highlight link will be turned into a link to a code repo.

My last, easily fixed, quibble is the lack of review of modern publications (such as suggested by R1) and broader treatment of allometry. The focus on conserved allometric scaling now receives more emphasis in the background and discussion, but it is nearly exclusively focused on heterochrony and development. But the papers currently cited are slightly older references (Cardini, Damuth & Janis, Codron), much older, development-heavy classics such as Huxley, Klingenberg, and Gould, or focus entirely on *Syncercus*. The developmental focus is completely fine, but its contextualisation omits a lot of recent literature (none of R1s suggested publications were added). In addition, although ecomorphological correlates of allometry are mentioned, the paper does not touch upon the pool of more recent allometry-related publications (including my own, but they don't have to be the ones cited!) that could have been used, with view to how longer-faced forms at larger sizes might carry an ecological selective advantage. This makes the paper seem to solely focus on a developmental allometric program or constraint, when in fact there is every chance that the heterochrony described here has a clear functional advantage as well. Download recommender's annotations

**Evaluation round #1**

DOI or URL of the preprint: https://doi.org/10.31233/osf.io/eh9s8_v3

Version of the preprint: 1

**Authors' reply, 19 August 2025**

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**Decision by Vera Weisbecker ID, posted 03 March 2025, validated 03 March 2025**

Both reviewers are overall positive about the manuscript and the contribution of the new species of *Synceros*, as well as the rigorous way that the sensitivity of the analyses is described. It is also excellent to see the transparency of R scripts and data (which hopefully will be published as well?).

However, both reviewers raise concerns about the emphasis on the allometric patterns, which I agree with. R1 suggests that the allometric pattern and its implications for developmental shifts is not interpreted appropriately. R2 notes that the landmarks that tend to be most affected by allometry in bovids (the premaxillary region) are missing and might have been misestimated (with large amounts of missing landmarks for *Ugandax* as well). These concerns are related because the specimen might have adult proportions at small sizes because of the estimated landmarks, not because it really had them. This will need to be addressed, and it might be necessary to add more nuance to the discussion and/or reduce the focus on allometry. R2 also raises some doubts about the horn core proportions are interpreted that warrant consideration.

**Reviewed by Rex Mitchell, 03 March 2025**

**Title and abstract**

Does the title clearly reflect the content of the article? Yes

Does the abstract present the main findings of the study? Yes
**Introduction**

Are the research questions/hypotheses/predictions clearly presented? Yes

Does the introduction build on relevant research in the field? Yes

**Materials and methods**

Are the methods and analyses sufficiently detailed to allow replication by other researchers? Yes

Are the methods and statistical analyses appropriate and well described? No, because I think there might be some issues with the landmarking of the sample and what this means for the major discussion points drawn from artificial landmark estimates.

**Results**

In the case of negative results, is there a statistical power analysis (or an adequate Bayesian analysis or equivalence testing)? No, statistical power is mentioned but not tested.

Are the results described and interpreted correctly? Yes

**Discussion**

Have the authors appropriately emphasized the strengths and limitations of their study/theory/methods/argument? No, I would appreciate more discussion about whether estimating landmarks might have influenced results.

Are the conclusions adequately supported by the results (without overstating the implications of the findings)? If there are indeed problems resulting from the estimated landmarks of the titular specimen, the stated insights of this specimen with regards to understanding allometry in the genus should probably be reduced.

[See attached review report]

**Download the review**

**Reviewed by anonymous reviewer 1, 19 February 2025**

"A new fossil buffalo from the Shungura Formation (Ethiopia), and the role of size-shape allometry in the diversification of Syncerus" by Bibi & Boisserie introduces and describes a new fossil buffalo and uses 3D geometric morphometric analyses to discuss its role in explaining the diversification of the tribe Bovini. I think that this is a solid study describing a clearly important and interesting fossil, and that this paper certainly is deserving of publication, but there are some inconsistencies in their interpretations of allometry analyses that need to be addressed.

On line 72-73, the authors say that 'the evolution of lineages using fossil specimens remains unexplored' which isn't really true, the authors even cite Gould's 1974 study on the extinct Irish Elk's antler allometry. I think this sentence should be re-worded. Further, based on figures 5 and 10, it doesn't appear clear to me that the new species is following the same allometric trajectories as the established extant species. The anteroposterior and transverse diameters of horn cores of this new species are not highly correlated in the way that Syncerus caffer is. In figure 10, the new Syncerus specimen is clearly farther away from the Synceus/Ugandax allometry than any of the other Syncerus/Ugandax specimens. It's possible that this specimen does follow the allometric trajectory that's conserved in the rest of Bovini, but the authors present these results as if it is a certainty that it does, which I don't think is appropriate. For example, in the last sentence of the caption to figure 10, they say 'the type cranium of Sy. Sp. nov [has] the size-shape relationship of juveniles of the living dwarf forest buffalo, even though it is a fully grown adult', but really the new specimen just shares the size of a juvenile S. caffer specimen, given the shape of the new specimen and the allometry of S. caffer we would expect the new specimen to have a centroid of approximately 1000, putting it well within the adult size range of S. caffer. A smaller species having the same morphology as an adult of another species would be an example of heterochrony (peramorphosis via predisplacement or acceleration), but the authors assert that their interpretation of the results (the new specimen following a conserved allometric trajectory) support Vrba's 1996 hypothesis that heterochrony played an important role in buffalo cranial diversification. Species sliding along an allometric trajectory to evolve certain phenotypes with accompanying sizes is not an example of heterochrony, this is not an allometric shift. So, the authors are asserting that this new Syncerus specimen
follows a common Bovini allometric trajectory even though its shape would suggest an adult size, and that this
conservation of allometry is evidence for a heterochrony (a shift in allometry, lines 719-722) – this is inconsistent
to me.

My other big suggestion is to provide more discussion in the methods section of the role the authors think distortion is affecting their results. Specifically, the ways in which certain fossils are distorted should be discussed before results are presented. The authors assert on line 243 that ‘distortions are not significant’ and therefore no retrodeformation was made, but any distortion will affect the results of a geometric morphometric analysis. On lines 678-683 the authors discuss the ways in which Syncerus antiquus might be deformed and how there needs to be more specimens of this species to adequately interpret the significance of its morphology. This directly conflicts with the previously mentioned sentence on line 243. It’s great that the authors performed sensitivity analyses and got encouraging results from them, but I do think it’s necessary to not downplay the effects of distortion in the methods section if it will be brought up later.

Misc/Line-by-line comments

- In figure 5-8 & 10, the new species of Syncerus is highlighted using crosshairs, but these crosshairs strongly resemble confidence intervals. It would improve clarity to change the shape of the point used to highlight the S. sp. nov. specimens.

- On line 729 the authors use the word plasticity suddenly. Based on the previous sentence it seems like they are conflating ontogenetic allometry and plasticity, unless there is any actual shape plasticity in the buffalo this word should not be used. This sentence also needs a citation.

- Period after 'range' on line 93

- Line 239: also removing positional variation by translating to a common centroid

- Flip the second and third sentences of the paragraph starting on line 251, I think you should explain how the regression was performed before you say you plotted it.

**Title and abstract**

Does the title clearly reflect the content of the article? [X] Yes, [ ] No (please explain), [ ] I don't know

Does the abstract present the main findings of the study? [X] Yes, [ ] No (please explain), [ ] I don't know

**Introduction**

Are the research questions/hypotheses/predictions clearly presented? [X] Yes, [ ] No (please explain), [ ] I don't know

Does the introduction build on relevant research in the field? [X] Yes, [ ] No (please explain), [ ] I don't know

**Materials and methods**

Are the methods and analyses sufficiently detailed to allow replication by other researchers? [X] Yes, [ ] No (please explain), [ ] I don't know

Are the methods and statistical analyses appropriate and well described? [X] Yes, [ ] No (please explain), [ ] I don't know

**Results**

In the case of negative results, is there a statistical power analysis (or an adequate Bayesian analysis or equivalence testing)? [X] Yes, [ ] No (please explain), [ ] I don't know

Are the results described and interpreted correctly? [X] Yes, [X] No (please explain), [ ] I don't know

**Discussion**

Have the authors appropriately emphasized the strengths and limitations of their study/theory/methods/argument? [ ] Yes, [X] No (please explain), [ ] I don't know

Are the conclusions adequately supported by the results (without overstating the implications of the findings)?
[X] Yes, [X] No (please explain), [ ] I don't know