Chad, Djourab, Toros-Menalla Fossiliferous Zone

At the outset, palaeontological surveys in the zone of contact between the Djourab Desert and the lowlands (Pays Bas) of the Chad Basin were based largely on the « Hydrogeological Reconnaissance Map at a scale of 1/500,000. Explanatory note of the sheet Pays Bas - Largeau" by Jean-Louis Schneider, published by the BRGM in 1968. The map by Jean-Louis Schneider indicates the presence of fossiliferous sites in the northern fringe of the Djourab Erg where it borders the depression of the Bas Pays (Toros, …). For this reason, Mission number 7, in January 1997, made a brief reconnaissance to this sector. But it was during Mission 11, from 11th to 27th November, 1997, led by Alain Beauvilain, that went due north from Salal bypassing the west side of the Djourab Erg in order to prospect systematically this zone from west to east, using the three topographic maps at a scale of 1/200.000 of the IGN : NE 33 V (Toros), NE 33 VI (without a name), NE 34 I (Koro Toro). Here they are fused with map NE 33 VI.

This mission was a great success, with the collection of fossils from 35 sites. At some sites, fossils that were too large to be collected were left in place. Following a diagonal WNW-ESE trajectory, this expedition covered the ensemble of the zone known today as the Toros-Ménalla. The sites yielded thousands of fossils among which there are are numerous holotypes of new taxa.

The topographic sheets of this sector have no contours and have only one spot height (206 m at the wells of Toro Timi -16° 19’ N, 16° 50’ E). While the dunes, essentially comprised of barchans, seifs and several ghourds, are the main components of the countryside there are two scarps (talus) providing relief. A small talus about 1200 metres long is indicated north of 16° 11’ N, 17° 29’ E but above all there is a talus almost 26 km long, which attracted our attention in a region which is not flat but planar. Often with more than 10 metres of relief, it comprises the main element of a series of scarps which lie parallel to it. Scenes from Google Earth indicate that it continues to the south-east where it is partly obscured by mobile dunes. On the ground, there are numerous small breks-in-slope a few tens of centimetres in height which correspond to alternating hard and soft sedimentary beds.

The altitudes taken from Google Earth indicate that there the land surface decreases in altitude from south to north and from the east-south-east to the west-north-west, from the highest point indicated (261 metres) to the lowest (199 metres). Following these altitudes, the orientation of the main scarp and the sediment beds exposed by the wind, reveals that in this flat region there is a succession of superposed floors between which there is usually a sand cover of variable thickness. TM266, at about 226 metres altitude, is at the footslope of the main scarp which is about a dozen metres tall, and in its turn it overlooks the plain which is some 20 metres lower, which to its north is subjected to strong aeolian deflation. All the hominid fossils found in March 2002 were discovered at the foot of this scarp, or in the case of TM247, on a narrow flat part interrupting the slope of the scarp.

Finally, even if the active dunes move rapidly, it seems that for the past three quarters of a century (comparing maps based on aeriel photographs taken in 1956 and recent data from Google Earth) the barchan zone and the denuded surfaces occur more-or-less in the same places. Between the dunes and the bare ground there are vast zones covered in a variable thickness of sand (from centimetres to decimetres deep). In the maps shown on this page, the numbers correspond to fossiliferous sites, with few exceptions numbered in the order in which they were found. In subsequent expeditions, some sites were revisited without giving a new number to the same point, but could have a new number nearby depending on the movement of barchan dunes which previously hindered proper evaluation of the site relative to the previously known one. The principal employed was that two fossils found either side of a dune were numbered as coming from different sites. The maps show that the ensemble of sites located between January, 1997 and March, 2002 comprise several dozen occurrences close to or parts of neighbouring sites. As a result, on the ground the distance between two sites can be as little as several dozen metres (on the ground a second of latitude, as for longitude, represents a distance of about 30 metres).

For example, during the expedition of July, 2001, there are several sites close to each other because they were found while walking. However, TM247 fully justifies a separate number because it occurs on a flat bench interrupting the scarp slope and overlooking neighbouring sites. For the same reason, the various sites located south and south-east of TM266 were located while walking between dunes.

The accuracy of the positioning of sites may be affected by the presence of distortion when the maps were drawn from the aerial photographs, or to similar distortion in scenes from Google Earth as well as to inaccuracies in GPS readings which at the time of the surveys varied over several dozen metres. These sources of error must be considered in the context of the maps, because the diameter of the points cover six hundred metres on the ground…

The organisation of the expeditions explains in large part the greater or lesser quantity of sites discovered during each survey. In effect, with the exception of Mission 11, from 11th to 27th November, 1997, until Mission 18, in February, 1999, the Toros Ménalla zone was surveyed for brief periods, being more or less reconnaissance runs from base camps at Kollé (KL) or Kossom Bougoudi (KB). It was from Mission 20, from 5th to 22nd February, 2000, that expeditions were focussed purely on the Toros-Ménalla area. The lack of discoveries east of 17° 30’ E after the expedition of 19th November to 6th December, 2000, is not because the potential for finding fossils in this region is thought to be exhausted but simply because access to the Toros-Ménalla region no longer passes via Kouba Olanga but takes a more direct route from Salal, thereby maximising the research time in the central and western parts of the Toros-Ménalla area.

TM95 does not exist.

This brief summary and the accompanying maps show how the systematic surveys of the region were carried out using topographic maps and aeriel photographs taken in the late 1950’s, culminating in the rapid discovery of hominids in the heart to the Djourab Desert. The ensemble of maps as well as satellite images from Google Earth, indicate the enormous scientific potential that this region possesses.