Africa

Marine fossils, notably tribolites dating from the Cambrian period (505-570 million years ago; the first period of the Paleozoic era) have been found in southern Morocco and Mauritania. Rocks from the succeeding period, the Ordovician (500-425 million years ago) consist of sandstones with a variety of fossilized marine organisms; these rocks occur throughout northern and western Africa, including the Sahara.

The Ordovician was characterized by the development of brachiopods (shellfish similar to clams), corals, starfish, and some organisms that have no modern counterparts, called sea scorpions, conodonts, and graptolites. At the same time the African crust was extensively deformed. The continental table of the central and western Sahara was lifted up almost a mile (1.6 km). The uplifting alternated with crustal subsidings, forming valleys that were periodically flooded.