THE SKELETON COAST DESERT, SOUTHWEST AFRICA, FROM CRETACEOUS TO RECENT TIMES

The northward tectonic movement of Africa through the descending limb of the southern Hadley Cell has controlled the position of the Skeleton Coast Desert along the southwest margin of Africa since the middle portion of the Cretaceous Period, causing an apparent southward migration of coastal desert conditions of nearly 10 degrees since the opening of the South Atlantic.  Fossil sand dunes in the Huab Basin, Namibia, underlie the Etendeka Basalts (132±1 Ma), indicating that arid conditions predated the Skeleton Coast.  Eighty million-year-old tree trunks currently submerged below sea level in the Orange Basin, South Africa, indicate a temperate paleoenvironment at 40 degrees South latitude at that time.  Marine vertebrate fossil localities in Angola and the distribution of type II kerogen suggest productive upwelling along the African coast since the Cretaceous.  The present day Skeleton Coast has a unique suite of plants and animals with a characteristic distribution of species within the desert.  The gnetalian genus Welwitschia, whose distribution largely defines the Skeleton Coast Desert, may be a Cretaceous remnant with roots extending back to the initial rifting of the South Atlantic.  Most mammalian genera are concentrated in areas of greater available surface moisture, as were, presumably, dinosaurs inhabiting the Cretaceous Skeleton Coast.  New appearances of these groups in the fossil record mostly reflect dispersal into the desert from the north, although exceptions are found, especially among rodents.  Other groups of extant biota more independent of surface water, especially among insects and plants, display diversity hotspots in the desert indicative of in situ evolution.  Thus, the history of the Skeleton Coast is tied to a broad pattern of tectonic movement that maintained desert conditions and produced a persistent environment in which relics remain, new taxa invaded along stretches of greater moisture availability or during intervals of higher precipitation, and speciation within the desert facilitated by geographic barriers greatly increased biotic diversity.