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Earliest known Old World monkey skull

Abstract

Similarities of the skull are commonly used to support hypotheses of ancestor–descendant relationships between fossil and living ape genera, especially between the late Miocene apes Sivapithecus and Dryopithecus from Eurasia and the living orang-utan (Pongo) from Borneo and Sumatra1,2,3,4. Yet determining whether craniofacial traits shared by extant and Miocene apes are primitive or derived is severely hampered by the rarity of well-preserved fossil crania, particularly of early members of their closest outgroup, the Old World monkeys (Cercopithecoidea). The discovery of a complete and undistorted skull of Victoriapithecus at middle Miocene deposits from Maboko Island, Kenya, provides evidence of intact cranial-vault and basicranial morphology, brain size and craniofacial hafting for a primate from between 32 and 7 million years ago. Victoriapithecus represents a branch of Old World monkey that is intermediate between extant cercopithecids (Colobinae and Cercopithecinae) and the common ancestor they shared with apes (Hominoidea)5,6,7,8. The skull preserves traits widely thought to be derived for extant and fossil members of a proposed Sivapithecus/Pongo clade, but which now appear to be primitive features of ancestral Old World higher primates in general.

Main

Victoriapithecus cranium KNM-MB 29100 was found in situ during excavation of sediment at Maboko Island that has been radiometrically dated as older than 14.7 Myr (ref. 9), and biostratigraphically dated as younger than 16 Myr (ref. 10). The cranium has preserved left male canine and postcanine teeth on both sides (Fig. 1, Table 1). Anatomy of the teeth and skull show Victoriapithecus to be a catarrhine (Old World higher primate) of modern aspect, and more specifically an Old World monkey (Fig. 2). The absence of at least 30 derived dental features shared by the living Old World monkey subfamilies Colobinae and Cercopithecinae demonstrates that Victoriapithecus belongs to neither extant group, but represents the sister taxon (Victoriapithecidae) of modern Cercopithecidae5,6,7,8.

Figure 1
figure 1

Anterior (a), posterior (b), left lateral (c), superior (d) and inferior (e) views of KNM-MB 2.9100.

Table 1 Table 1 Cranial measurements (mm) and comparative indices (%) of KMN-MB 29100
Figure 2: Victoriapithecus shares a tubular ectotympanic with modern catarrhines17 (node A).
figure 2

Features linking the Victoriapithecus skull with Old World monkeys include continuation of the mesial sulcus onto the C1 root and absence of a maxillary sinus7,15,16,17,22,23 (node B). Teeth of living cercopithecines and colobines share several derived features not found in Victoriapithecus, including: dp3,4 and M1,2,3 possessing a distal transverse loph, lacking a crista obliqua, and being longer than wide; well-developed dp3 hypoconid, entoconid and transverse protolophid; presence of dp4 hypolophid; absence of dp4 and M1–2 hypoconulids; P4 aligned with the molar row; and relatively longer M1 (refs 5, 7, 8) (node C).

The cranial vault of Victoriapithecus differs from those of all extant Old World monkeys in being narrower and lower relative to length (Table 1), a configuration found only in the early Pliocene African colobine Libypithecus11. Victoriapithecus and Libypithecus also share unusually well-developed sagittal and nuchal crests. Relative to a body-weight estimate of 4.5 kg for large male postcrania12, the cranial capacity of KNM-MB 29100 at 54 cm3 falls between regressions for extant anthropoids and strepsirhines, and is significantly higher than that of the Oligocene archaic catarrhine Aegyptopithecus13 (Fig. 3). The relative brain size of Victoriapithecus is more similar to that of the red colobus than to any other living cercopithecoid14.

Figure 3: Bivariate plot of log10-transformed mean brain and body weight data for extant hominoids (H), cercopithecoids (C), platyrrhines.
figure 3

(P), tarsiers (T), lemuroids and lorisoids (L)14, Victoriapithecus and Aegyptopithecus. Superimposed best fit lines for extant anthropoids (higher) and strepsirhines (lower) are based on least-squares linear regression equations.

The craniofacial hafting and upper facial morphology of Victoriapithecus are unique among Old World monkeys, adding to the list of traits which demonstrate that victoriapithecids represent the sister taxon of modern cercopithecids. The face is only slightly ventrally deflected (klinorhynch) relative to the basicranium. Its profile is steep and unusually linear, with the bridge of the nose lying along a straight line connecting prosthion, rhinion and glabella. This linear profile is combined with a slight dorsal orientation of the orbits and zygomatic frontal process, with their inferior borders positioned well anterior to their superior margins. Of the living catarrhines, the upper facial morphology of Victoriapithecus is most similar to that of Pongo (the orang-utan), in having tall and narrow orbits (Table 1), well-developed supraorbital costae, which together with anteriorly convergent temporal lines and absence of a postglabellar depression contribute to the formation of a frontal trigon, and presence of three zygomatic foramina positioned well above the inferior orbital rim on the frontal process of the zygomatic. In addition, the cheek region of the zygomatic is tall relative to facial height, and its root extends only a short distance above the molar alveolae, as in Miocene apes but not modern catarrhines15.

The skull confirms previous assessments based on isolated facial bones6,15 that aspects of the face of Victoriapithecus resemble those of modern cercopithecines in having a narrow interorbital septum, low position of the frontozygomatic suture, narrow nasal bones, a low and narrow nasal aperture, a moderately long and anteriorly tapering snout, and a moderately long premaxilla with upper first incisor (I1) positioned anterior to upper second incisor (I2) (Table 1). Many of these features are adaptations for frugivory16, a diet indicated for Victoriapithecus by dental evidence7, and which is characteristic of cercopithecine monkeys. Because parsimony indicates that morphology shared by victoriapithecids and either or both cercopithecid subfamilies should be primitive for Cercopithecoidea7,12,15,17, a predominantly cercopithecine-like cranium and frugivorous diet characterized the earliest Old World monkeys. This position is confirmed by strong cranial resemblances between Victoriapithecidae and the basal catarrhine outgroup Propliopithecidae18,19. The fossil colobine Libypithecus11 retains much of this primitive morphology, including a moderately long snout, although its teeth are adapted for folivory. Other cercopithecids, and especially modern colobines, are comparatively derived.

Because many primitive cercopithecoid cranial features shared by Victoriapithecus and the propliopithecid Aegyptopithecus are also found among Miocene hominoids (especially Afropithecus20,21), they are probably primitive characteristics for Old World higher primates in general. Such primitive features include low cranial vault height, a well-developed sagittal crest, a frontal trigon created by distinct supraorbital costae and anteriorly convergent temporal lines, slight ventral deflection of the face, a linear facial profile, tall and narrow orbits, a moderately long snout, and a relatively tall cheek region. Three of these primitive catarrhine features (supraorbital costae, a frontal trigon, and tall cheek regions) are present in the facial skeletons of both Sivapithecus1,2 and Dryopithecus3,4, and tall and narrow orbits are characteristic of the former.

The new evidence refutes the idea that ancestral Old World anthropoids had high and rounded braincases and short faces, as reconstructed from presumed conservative resemblances between colobines and gibbons16,17,22,23. The skull morphology of Victoriapithecus demonstrates that many aspects of the craniofacial anatomy of late Miocene large-bodied hominoids are primitive catarrhine features, rather than derived indicators of affinity with the great ape and human clade. In this way, the Victoriapithecus skull shows that the anatomy of fossil cercopithecoids is as important as that of hominoids for deciphering the evolutionary history of Old World higher primates.

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Acknowledgements

Excavations at Maboko were conducted with permission of the Office of the President, Republic of Kenya and in collaboration with the National Museums of Kenya. We thank the field crew (especially B. Onyango, V. Oluoch and S. Gitau) and M. G. Leakey for assistance, and E. Delson, M. Kohler, S. Moya-Sola and D. Pilbeam for comments and advice. This work was supported by NSF, L.S.B. Leakey Foundation, National Geographic Society, Wenner Gren Foundation for Anthropological Research, Fulbright, and Boise Fund.

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Correspondence to Brenda R. Benefit.

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Benefit, B., McCrossin, M. Earliest known Old World monkey skull. Nature 388, 368–371 (1997). https://doi.org/10.1038/41078

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