Abstract: The Cambrian (541–485 million years ago or ‘Ma’) and the Ordovician (485–458 Ma) periods represent a crucial phase in the history of the Earth: they bring the sudden appearance of most major animal groups, together with unprecedented anatomies and functionalities. The initial phase of diversification of metazoans, the ‘Cambrian Explosion’, brought the appearance of very high-level taxonomic groups –Phyla and Classes–, was followed by a second burst of marine biodiversity: the ‘Great Ordovician Biodiversification Event’, or GOBE, which produced an dramatic increase at the Order and Family and Genus levels and a significant rise in the complexity of the ecosystems. This Sprigg Lecture will look at the information provided by Cambrian fossils from such localities as the Burgess Shale in Canada, Chengjiang in China and Emu Bay Shale in Australia, and compare with the new Ordovician soft-bodied faunas discovered in Morocco: the Fezouata and Tafilat biotas.