Franck Guy

A new hominid from the Upper Miocene of Chad, Central Africa

The search for the earliest fossil evidence of the human lineage has been concentrated in East Africa. Here we report the discovery of six hominid specimens from Chad, central Africa, 2,500 km from the East African Rift Valley. The fossils include a nearly complete cranium and fragmentary lower jaws. The associated fauna suggest the fossils are between 6 and 7 million years old. The fossils display a unique mosaic of primitive and derived characters, and constitute a new genus and species of hominid. The distance from the Rift Valley, and the great antiquity of the fossils, suggest that the earliest members of the hominid clade were more widely distributed than has been thought, and that the divergence between the human and chimpanzee lineages was earlier than indicated by most molecular studies.

Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad

All six known specimens of the early hominid Sahelanthropus tchadensis come from Toros-Menalla site 266 (TM 266), a single locality in the Djurab Desert, northern Chad, central Africa. Here we present a preliminary analysis of the palaeontological and palaeoecological context of these finds. The rich fauna from TM 266 includes a significant aquatic component such as fish, crocodiles and amphibious mammals, alongside animals associated with gallery forest and savannah, such as primates, rodents, elephants, equids and bovids. The fauna suggests a biochronological age between 6 and 7 million years. Taken together with the sedimentological evidence, the fauna suggests that S. tchadensis lived close to a lake, but not far from a sandy desert, perhaps the oldest record of desert conditions in the Neogene of northern central Africa.

Virtual cranial reconstruction of Sahelanthropus tchadensis

Previous research in Chad at the Toros-Menalla 266 fossiliferous locality (about 7 million years old) uncovered a nearly complete cranium (TM 266-01-60-1), three mandibular fragments and several isolated teeth attributed to Sahelanthropus tchadensis . Of this material, the cranium is especially important for testing hypotheses about the systematics and behavioural characteristics of this species, but is partly distorted from fracturing, displacement and plastic deformation. Here we present a detailed virtual reconstruction of the TM 266 cranium that corrects these distortions. The reconstruction confirms that S. tchadensis is a hominid and is not more closely related to the African great apes. Analysis of the basicranium further indicates that S. tchadensis might have been an upright biped, suggesting that bipedalism was present in the earliest known hominids, and probably arose soon after the divergence of the chimpanzee and human lineages.

New material of the earliest hominid from the Upper Miocene of Chad

Discoveries in Chad by the Mission Paléoanthropologique Franco-Tchadienne have substantially changed our understanding of early human evolution in Africa. In particular, the TM 266 locality in the Toros-Menalla fossiliferous area yielded a nearly complete cranium (TM 266-01-60-1), a mandible, and several isolated teeth assigned to Sahelanthropus tchadensis and biochronologically dated to the late Miocene epoch (about 7 million years ago). Despite the relative completeness of the TM 266 cranium, there has been some controversy about its morphology and its status in the hominid clade. Here we describe new dental and mandibular specimens from three Toros-Menalla (Chad) fossiliferous localities (TM 247, TM 266 and TM 292) of the same age. This new material, including a lower canine consistent with a non-honing C/P3 complex, post-canine teeth with primitive root morphology and intermediate radial enamel thickness, is attributed to S. tchadensis. It expands the hypodigm of the species and provides additional anatomical characters that confirm the morphological differences between S. tchadensis and African apes. S. tchadensis presents several key derived features consistent with its position in the hominid clade close to the last common ancestor of chimpanzees and humans.

Tchad: Découverte d'une faune de mammifères du pliocène inférieur

Abstract In Northern Chad, the site of Kolle in the Djourab erg has yielded a vertebrate fauna including 21 species, 14 of which are Mammals. This fauna provides evidence of a mosaic of environments: fresh-water, woodlands, grasslands. In spite of possible taphonomic or collecting bias, and of some endemicity, this fauna allows us to propose an age in the range 5-4 My.

Symphyseal shape variation in extant and fossil hominoids, and the symphysis of Australopithecus bahrelghazali

The holotype of the species Australopithecus bahrelghazali is a mandibular fragment preserving left C-P(4) and right I(2)-P(4). One of the key features of the A. bahrelghazali mandible is its sagittally and transversally flat anterior region associated with a vertical, bulbous symphysis that is assumed to differ morphologically from the classic, more apelike eastern australopith morphology with its sloping symphysis, developed transverse tori, and distinct genioglossal fossa. Yet, close similarity has been suggested between A. bahrelghazali and A. afarensis. To date, no metrical comparison of the symphyseal morphology of east and west African australopiths has been performed. For the selected characters, this study attempts to test the following null hypothesis: A. bahrelghazali does not present significant differences from A. afarensis (i.e., A. bahrelghazali vs. A. afarensis variation does not depart from expected intraspecific variation for hominoid species). Analysis of the mandibular symphysis is difficult to undertake using conventional linear variables since few landmarks are available to make a precise quantitative assessment of its morphology. In addition, while a few studies have used outline-based techniques of analysis to address symphyseal shape variation, none has integrated data on early hominins. We present here a detailed comparative study of variation in the symphyseal outline of A. bahrelghazali and A. afarensis based on a quantitative characterization by elliptic Fourier coefficients. Original data on symphyseal variation in Pliocene hominins are provided and discussed within a comparative framework of extant and fossil hominoid representatives. We evaluate the relevance of our quantitative data describing the shape of the symphysis for discriminating hominoid taxa, and test for differences in symphyseal shape between A. bahrelghazali and A. afarensis. Elliptic Fourier coefficients appear to be well-suited descriptors for depicting symphyseal variation within hominoids. Our results confirm that symphyseal shape is a good indicator of taxonomic affinity within hominoids, enabling a clear distinction between great apes, early hominins, and modern humans. Furthermore, our results substantiate the unusual pattern of the A. bahrelghazali symphyses compared to A. afarensis and support the specific status of the west African australopith.

Assessing endocranial variations in great apes and humans using 3D data from virtual endocasts

Modern humans are characterized by their large, complex, and specialized brain. Human brain evolution can be addressed through direct evidence provided by fossil hominid endocasts (i.e. paleoneurology), or through indirect evidence of extant species comparative neurology. Here we use the second approach, providing an extant comparative framework for hominid paleoneurological studies. We explore endocranial size and shape differences among great apes and humans, as well as between sexes. We virtually extracted 72 endocasts, sampling all extant great ape species and modern humans, and digitized 37 landmarks on each for 3D generalized Procrustes analysis. All species can be differentiated by their endocranial shape. Among great apes, endocranial shapes vary from short (orangutans) to long (gorillas), perhaps in relation to different facial orientations. Endocranial shape differences among African apes are partly allometric. Major endocranial traits distinguishing humans from great apes are endocranial globularity, reflecting neurological reorganization, and features linked to structural responses to posture and bipedal locomotion. Human endocasts are also characterized by posterior location of foramina rotunda relative to optic canals, which could be correlated to lesser subnasal prognathism compared to living great apes. Species with larger brains (gorillas and humans) display greater sexual dimorphism in endocranial size, while sexual dimorphism in endocranial shape is restricted to gorillas, differences between males and females being at least partly due to allometry. Our study of endocranial variations in extant great apes and humans provides a new comparative dataset for studies of fossil hominid endocasts.

Talar Morphology, Phylogenetic Affinities, and Locomotor Adaptation of a Large-Bodied Amphipithecid Primate From the Late Middle Eocene of Myanmar

A well-preserved fossil talus [National Museum of Myanmar Primates (NMMP) 82] of a large-bodied primate is described from the late middle Eocene Pondaung Formation of central Myanmar. The specimen was collected at Thandaung Kyitchaung, a well-known amphipithecid primate-bearing locality near the village of Mogaung. NMMP 82 adds to a meager but growing sample of postcranial remains documenting the large-bodied primates of the Pondaung Formation. This new talus exhibits a suite of features that resemble conditions found in living and fossil haplorhine primates, notably anthropoids. As such, the phylogenetic signal deriving from the morphology of NMMP 82 conflicts with that provided by NMMP 20, a partial skeleton (including a fragmentary calcaneus) of a second large-bodied Pondaung primate showing undoubted adapiform affinities. Analysis subtalar joint compatibility in a hypothetical NMMP 82/NMMP 20 combination (talus/calcaneus) reveals a substantial degree of functional mismatch between these two tarsal bones. The functional incongruence in subtalar joint morphology between NMMP 20 and NMMP 82 is consistent with the seemingly divergent phylogenetic affinities of these specimens, indicating that two higher level taxa of relatively large-bodied primates are documented in the Pondaung Formation. On the basis of its size and morphology, we refer the NMMP 82 talus to the large-bodied amphipithecid Pondaungia. The occurrence of anthropoid-like tali in the Pondaung Formation obviates the need to invoke homoplasy to explain the shared, derived dental characters that are common to amphipithecids and undoubted anthropoids. Functionally, the NMMP 82 talus appears to have pertained to a primate that is engaged in active quadrupedalism in an arboreal environment along broad and subhorizontal branches. The primate taxon represented by NMMP 82 was capable of climbing and leaping, although it was not particularly specialized for either of these activities.

Three-dimensional analysis of mandibular dental root morphology in hominoids

Although often preserved in the fossil record, mandibular dental roots are rarely used for evolutionary studies. This study qualitatively and quantitatively characterizes the three-dimensional morphology of hominoid dental roots. The sample comprises extant apes as well as two fossil species, Khoratpithecus piriyai and Ouranopithecus macedoniensis. The morphological differences between extant genera are observed, quantified and tested for their potential in systematics. Dental roots are imaged using X-ray computerized tomography, conventional microtomography and synchrotron microtomography. Resulting data attest to the high association between taxonomy and tooth root morphology, both qualitatively and quantitatively. A cladistic analysis based on the dental root characters resulted in a tree topology congruent with the consensus phylogeny of hominoids, suggesting that tooth roots might provide useful information in reconstructing hominoid phylogeny. Finally, the evolution of the dental root morphology in apes is discussed.

Prospective in (Primate) Dental Analysis through Tooth 3D Topographical Quantification

The occlusal morphology of the teeth is mostly determined by the enamel-dentine junction morphology; the enamel-dentine junction plays the role of a primer and conditions the formation of the occlusal enamel reliefs. However, the accretion of the enamel cap yields thickness variations that alter the morphology and the topography of the enamel–dentine junction (i.e., the differential deposition of enamel by the ameloblasts create an external surface that does not necessarily perfectly parallel the enamel–dentine junction). This self-reliant influence of the enamel on tooth morphology is poorly understood and still under-investigated. Studies considering the relationship between enamel and dentine morphologies are rare, and none of them tackled this relationship in a quantitative way. Major limitations arose from: (1) the difficulties to characterize the tooth morphology in its comprehensive tridimensional aspect and (2) practical issues in relating enamel and enamel–dentine junction quantitative traits. We present new aspects of form representation based exclusively on 3D analytical tools and procedures. Our method is applied to a set of 21 unworn upper second molars belonging to eight extant anthropoid genera. Using geometrical analysis of polygonal meshes representatives of the tooth form, we propose a 3D dataset that constitutes a detailed characterization of the enamel and of the enamel–dentine junction morphologies. Also, for the first time, to our knowledge, we intend to establish a quantitative method for comparing enamel and enamel–dentine junction surfaces descriptors (elevation, inclination, orientation, etc.). New indices that allow characterizing the occlusal morphology are proposed and discussed. In this note, we present technical aspects of our method with the example of anthropoid molars. First results show notable individual variations and taxonomic heterogeneities for the selected topographic parameters and for the pattern and strength of association between enamel–dentine junction and enamel, the enamel cap altering in different ways the “transcription” of the enamel–dentine junction morphology.