Relict Distribution Patterns of Aquatic Animals: Another Tool in Evaluating Late Pleistocene Climate Changes in the Sahara and Sahel

Abstract

Although many kinds of adaptive strategies have evolved, it is generally true that higher aquatic animals can only disperse through water. They differ herein from plants and from the numerous terrestrial animals that are linked one way or the other to plants. A study of the relict distribution of selected groups of aquatic animals in a desert like the Sahara can therefore produce valuable new insights into the past climatic fluctuations of this area. Nonmigrant dragonflies of African origin in the Mediterranean Basin are of two types: species with relicts on the central Mediterranean islands but not on the Iberian Peninsula and the coastal plain of NW Morocco, and species with relicts on the Iberian Peninsula, but not on the Mediterranean islands. It is argued that the first category corresponds to a migration wave around 20,000 BP, and the second to a much more important migration wave (embracing also many species of fish, amphibia, and the Nile crocodile), between 12,000–8,000 BP. Relictisation of these species in the Central Sahara occurred as early as 7,000 BP, while in the Southern Sahara and Sahel all relicts were wiped out by a severe drought around 5,000–4,000 BP. The Tibesti, Ennedi, and Adrar of Mauretania were recolonized by African species thanks to transgressions of Lake Chad and the River Senegal, but the Adrar-des-Iforhas and the Aīr still have a very deficient fauna, indicating that no surface water link with the Sudan has ever been re-established.