The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


The endocast of Sahelanthropus tchadensis, the earliest known hominid (7 Ma, Chad)

THIBAUT BIENVENU1,2,3, DEAN FALK4,5, KATERINA SEMENDEFERI3, FRANCK GUY2, CHRISTOPH ZOLLIKOFER6, MARCIA PONCE DE LEÓN6, PAUL TAFFOREAU7, HASSANE TAISSO MACKAYE8, ANDOSSA LIKIUS8, PATRICK VIGNAUD2 and MICHEL BRUNET1,2.

1Chaire de Paléontologie Humaine, Collège de France, 2Institut International de Paléoprimatologie, Paléontologie Humaine: Evolution et Paléoenvironnements, Université de Poitiers, 3Department of Anthropology, University of California San Diego, 4Department of Anthropology, Florida State University, 5School for Advanced Research, Santa Fe, 6Anthropologisches Institut, Universität Zürich-Irchel, 7European Synchrotron Radiation Facility, Grenoble, 8Département de Paléontologie, Université de N’Djamena

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Fossil remains assigned to Sahelanthropus tchadensis were found in the Toros-Menalla fossiliferous area (Djurab Desert, Northern Chad), in continental sediments dated to 7 Ma by both radiochronologic and biochronologic methods. These fossils, including the famous male holotype cranium TM 266-01-060-1 nicknamed Toumaï, display a number of derived cranial and dental characters, making Sahelanthropus tchadensis the earliest known hominid (= sister group of Panids) species.

Fossil hominid endocasts are the only direct evidence of human brain evolution. The endocast of TM 266-01-060-1 opens a unique window on the first steps of this evolution just after the split between humans and chimpanzees. As the cranium is filled with highly mineralized matrix, and the inner table of cranial bone is fragmented and distorted, classical methods of endocast extraction could not be applied. Thus, a special protocol was developed for this cranium, involving 3D X-ray synchrotron microtomography, propagation phase contrast, semi-automated segmentation of the inner table, and correction of postmortem distortion based on both rigid and thin-plate spline transformations. The reconstructed endocast of TM 266-01-060-1 is compared with older fossil anthropoid endocasts, modern endocasts (great apes and humans) and australopithecine endocasts, taking volumetric, linear and angular measurements and performing a Generalized Procrustes Analysis.

The endocranial volume of TM 266-01-060-1 confirms the data (360-370 cc) already published for the 3D cranium reconstruction. It falls within the variability range of chimpanzees. The studies are still in progress, but numerous features of the brainstem, occipital lobes, and prefrontal cortex, already confirm the hominid status of Sahelanthropus tchadensis.

This study was funded by the French ANR-09-BLAN-0238 Program, the Swiss National Science Foundation Grant 31003A_135470/1, ESRF (beamline ID17) proposal ec12, and a Grant from the Fyssen Foundation.

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