This paper is based on the concept that the drainage system of c. 7.5-4.6 Ma is still reflected in the geomorphology of northeastern Chad and adjacent areas of Libya. During the Messinian and early Pliocene a large lake was present in the Chad Basin that is termed Neogene Lake Chad. It fluctuated in size in response to the precessional cycle and at times overflowed to the east, NE or north, giving rise to the Sahabi rivers. The Eosahabi flowed during the drawdown of the Mediterranean (late Messinian) and eroded the Erdi and part of the East Tibesti Valley. The post-rift Miocene deposits of the Chad Basin, some several hundred metres in thickness, record a transgression over an irregular and faulted terrain with the deposition of coarse and fine elastic material. Fluviatile and lacustrine environments are represented. At least part of the Miocene succession belongs to a phase of late Miocene lake development. Fluctuating climate during the Messinian and early Pliocene led to repeated changes in the environment of the Chad and Eosahabi Basins with over 100 climatic cycles developed. This would favour the development of animal species with high adaptability, for example to littoral, riparian, woodland and savanna habitats.
