Sahelanthropus tchadensis,
Toumaï Shows the Teeth III
Beauvilain and Le Guellec reply
Brunet et al. (1) criticize three main pointsin our article (2) : a) the tectonic origin of the Toros-Menalla scarp, b) the determination of side of an isolated molar that was stuck onto a right mandible attributed to Sahelanthropus tchadensis from siteTM266, and c) the inventory of fossils. We responsif to each of these criticisms in turn and conclude that our original hypothesis and interpretation are likely to be correct.
Geological issues in the Djurab
Brunet et al. (1) report that georadar did not yield any evidence of faulting in the Toros-Menalla region, and they ascribe the formation of the scarp to ‘overdigging’ by wind. Whilst wind deflation does indeed cause depressions in deserts, such basins are seldom linear, usually being undulating in outline or spoon-shaped.
The fact that the scarp in question is rectilinear over a distance of more than 40 km suggests an underlying tectonic origin. Elsewhere in the Chad Basin (3) there is evidence of neotectonic activity including the western shore of Lake Chad, which runs straight NNW-SSE for nearly 250 km, a trend that continues along the Dilia Valley (Niger) over an even greater distance. The Bahr el Ghazal Valley is almost straight NNE-SSW for more than 450 km, with a cliff on the left bank in its lower reaches. This neotectonic activity frequently reactivates ancient fractures (4).
Georadar was indeed used in January 1999 but only at sites east of the Bahr el Ghazal (the Toros-Menalla region is on the west side). On fossil and recent dunes it revealed reflectors only 1 to 2 metres thick (5), and in sandstone it ‘did not yield an image at depths greater than 1 metre.’(5)
This georadar equipment is not designed to locate evidence of faulting, and it was not used to search for faults in the Djourab. Thus failure of the equipment to reveal faults in the region does not provide a sound argument against our view that the linear stuctures that occur there are likely to be of tectonic origin.
We thus maintain our original view that the Toros-Menalla scarp, which yielded all the fossils of Sahelanthropus tchadensis, owes its origins to tectonic activity. It has subsequently been deflated by the wind, but this has only modified its form rather than being the original cause of it.
Restoration and interpretation of Sahelanthropus fossils
The fossils from the Djurab are well mineralized but are often abraded by wind-blown sand. There are strong daily changes in temperature, which can range from 50°C air temperature at ground level in the day to light frost at night. At midday, pebbles and fossils lying on the surface of the ground can be too hot to touch. These changes lead to fissuring of fossils, which usually have planar fracture surfaces rather than strongly curved ones. Crusts covering the fossils are either in the form of a very adhesive, hard grey, chemically- resistant matrix, or as an iron-rich concretion, coloured by manganese, often separated from the fossil itself by a small space occupied by sand (see Fig. 1).
The skull of Toumaï, which was found upside down on the sand, was protected from erosion by the second type of crust, whereas the right hemi-mandible TM 266-02-154-1 had traces of grey matrix (Fig. 2), which also covered the base of the isolated m/3. For this reason we consider that the cranium on the one hand, and the mandible and the m/3 on the other were not fossilized in the same deposits and are thus probably not contemporary.
The simplest reconstruction of the history of the m/3 and the right hemi-mandible is that they were broken before burial and fossilization some 6 to 7 million years ago, as shown by the matrix covering the broken surface of the distal root of the m/3 (Fig. 2). Later on, they became encrusted under near-surface conditions. Recently exhumed by the wind, abrasion has removed much of their cover of matrix and has polished their surface, while thermal variations have resulted in damage to some of the teeth. The m/3 was presumably the last specimen to erode out of the sediments as its surface is well preserved. Conversely, the other right mandible (TM 266-02-203), discovered in March 2002 without teeth a dozen metres from the mandible and m/3, was the first to erode out.
Figs 1-3. Photographs of specimens attributed to Sahelanthropus tchadensis (taken of the cranium at 08:00 on 19 July 2001):
1) the M3/ in the cranium ;
2) oblique lingual view of the roots of the m/3 in the mandible (note in particular the relatively planar fracture surface of the distal root, which curves distally and buccally at a constant level (black arrow), and compare it with the antero-posteriorly curved surface marked by an arrow in the image provided by Brunet et al.1) ;
3) root of the right C1/ in the cranium of Toumaï. (Scale bars: 10 mm.)
Biological considerations of the m/3
One of the arguments put forward by Brunet et al. (1) concerning the determination of side of the isolated m/3 is the wear pattern. However, examination of the last upper molar in the skull of Toumaï (Fig. 1) indicates that we need to be cautious about the interpretation of wear on cusps in general. Curiously, in this particular instance wear on the buccal cusps is closely similar to that on the lingual ones.
Because of this, from the beginning of our study (2) we attached more importance to the grooves, which were less affected by wear during life (and more recently by wind abrasion), and are thus better preserved than the tops of the cusps. Firstly, the similarity between the fossil m/3 and a ‘modern’ human molar is striking. The alignment on a regular curve of the three buccal cusps (protoconid, hypoconid, hypoconulid) and their decrease in size mesio-distally constitute a determining criterion. Secondly, the intercuspid column that we named the metaconulid in our article evokes more a morphological variant of the lingual surface of the tooth than one of the buccal surface. Thirdly, the orientation of the main disto-buccal groove accords precisely in its occlusal articular dynamic relationship with the oblique crest (crista obliqua) [‘pont d’émail’] of the non-working maxillary molarwhenthe opposite side of the jaw is engaged in chewing as in other Anthropomorphes (6).
A second line of argument employed by Brunet et al. (1) to demonstrate that the m/3 fits onto the right mandible was a series of scans in which the broken surface of the base of the tooth and the roots in the mandible were shown to be compatible with each other, being separated only by a thin but continuous layer of glue. It is necessary to recognize the detailed work which allowed the fitting of the m/3 crown so precisely onto the roots of the hemimandible. The hard sandy matrix which covered the base of the isolated molar when it was found first had to be removed and then the space now occupied by glue had to be prepared millimetre by millimetre. A similar operation was required for the parts of the root in the mandible, which were similarly covered in matrix (Fig. 2). In general, a section immediately beneath the cervix of left and right mandibular molars reveals radicular surfaces that may be superposed to within about one millimetre. So, it is not surprising in this particular case that a left tooth seemed to correspond to the roots in a right mandible.
The right upper canine of Toumaï.
We apologize for the possibility that our text regarding Toumaï’s right C1/ could be misinterpreted, the canine root being in situ in the skull as shown in Fig. 3. In contrast we could only write that the canine crown found in November 2001 fits onto the root, we being in the Djourab, and the skull at the University of Poitiers.
Curatorial issues
Writing catalogue numbers on fossil specimens is a daily activity in the field. At such an important site as TM 266, all the fossils were collected. That is why, from July to December 2001, 52 postcranial bones, whose zoological group could not be determined in the field, were catalogued, in the expectation that some of them might belong to Sahelanthropus. Among these fragments, 36 are long bones (tibia, femur, humerus and ulna) including intact specimens and broken diaphyses. Considering the excellent preservation of the Toumaï cranium, a careful examination of these bones should yield interesting information, as we consider it likely that postcrandial fossils of a large primate may be present at the site, although nothing has been reported until now. Note that in the catalogue returned to N’Djamena by the University of Poitiers in December 2001, a single fossil had been added to the field catalogue. It consists of specimen TM 266-01-447 (a right M3 according to Brunet et al. (7), whereas the catalogue entry states that it is ‘Classification primate; Description fragments morceaux racines m/1/m/3 ; Dépôt Poitiers (reliquat tamis)’. These specimens were returned to the CNAR, N’Djamena, on 30 January 2002.
Conclusions
We see no compelling reason to modify radically our hypothesis about the geomorphology of the Toros-Menalla region, nor our interpretation of the isolated left m/3 and damaged right mandible from locality TM 266 attributed to Sahelanthropus tchadensis. Finally, given the excellent preservation of fossils at the site, we consider it likely that the collection of postcranial elements collected there may well contain some specimens that belong to Sahelanthropus.
1. Brunet M. et al. (2004). Sahelanthropus tchadensis: the facts. S. Afr. J. Sci. 100, 443-445.
2. Beauvilain A. and Le Guellec Y. (2004). Further details concerning fossils attributed to Sahelanthropus tchadensis (Toumaï). S. Afr. J. Sci. 100, 142-144.
3. Morin S. (2000). Géomorphologie. In Atlas de la province Extrême-Nord Cameroun, pp. 717. IRDLCA/France MINREST-INC/Cameroun. (see in particular 'Le canevas tectonique' et la figure 7 'Le cadre structural du lac Tchad' - 'The tectonic framework' and figure 7 : 'The structural framework of Lake Chad').
4. Neev D., Hall J.K. and Saul J.M. (1982). The Pelusium Megashear System across Africa and associated lineament swarms. J. Geophys. Res. 87, B2, 1015-1030.
5. Schuster M. (2002). Sédimentologie et paléoécologie des séries à vertébrés du paléolac Tchad depuis le Miocène supérieur. Thesis, University of Strasbourg.
6. Lautrou A., University of Paris V René Descartes, pers. comm. (author of Anatomie Dentaire, Masson, Paris).
7. Brunet M. et al. (2002). A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418, 145-151.