Palaeogeographical and palaeoecological aspects of the Cambro–Ordovician radiation of echinoderms in Gondwanan Africa and peri-Gondwanan Europe

Ecology and tempo of the Lower Palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in Cambro–Ordovician faunas from Laurentia and the northern Gondwana margin. The Cambrian ‘agronomic revolution’ triggered a global radiation of echinoderms, with the progressive disappearance of biomat-related lifestyles, and the colonisation of new environments. Both in Laurentia and on the northern Gondwana margin, soft-substrate echinoderm assemblages related to cold and/or deep environments were dominated by blastozoans and stylophorans. These assemblages show a pattern of continuous diversification from the Middle Cambrian to the Middle Ordovician. The major radiation of crinoids in the Lower to Middle Ordovician of Laurentia is only a local diversification correlated with the development of hardgrounds in warm, shallow environments. No comparable major Ordovician radiation event occurred in the cooler environments of the northern Gondwana margin.