The Neogene history of the Namib Desert is reasonably well understood, thanks to the presence of abundant sediments and fossils in Namaqualand, the Sperrgebiet and the Namib-Naukluft Park. Hyper-arid desert has existed along the west coast of Southern Africa since the Early Miocene. It has been demonstrated on the basis of stable isotope studies of struthious eggshells and mammalian dental enamel, that throughout the Neogene winter rainfall regime occurred in the south and summer rainfall regime in the north, although the boundary zone between the two regimes fluctuated latitudinally by several hundred kilometres through the Miocene. The pre-Miocene geological history of the region is much less well understood. The aim of this contribution is to discuss the palaeoclimate and palaeoenvironment of the richly fossiliferous pre-Miocene deposits related to the recently recognised Lutetian-Bartonian Ystervark Carbonatite Formation in the Sperrgebiet, with a view to throwing light on the development of arid climatic conditions in the subcontinent during the Palaeogene, and the consequences that aridification had on the Southern African biota. In pre-Bartonian time the climate in the Sperrgebiet was relatively humid and tropical with summer rainfall, but by Bartonian time there were signs of increasing aridity and a switch to predominantly winter rainfall was achieved by the Aquitanian. Fluctuations in groundwater levels were driven by the rise and fall of sea levels, resulting in alternating cycles of erosion and deposition in the zone along the coast. Keywords: Namib, Palaeogene, desertification, hypsodonty, grass, evolution, groundwater level.