THE FEZOUATA BIOTA: AN ORDOVICIAN BURGESS SHALE

The Great Ordovician Biodiversification Event was a pivotal episode in the history of life, replacing the Cambrian Evolutionary Fauna by the Paleozoic Evolutionary Fauna which dominated the marine realm until the end-Permian mass extinction. During this Ordovician radiation, most phyla diversified more rapidly than at any other time in the Phanerozoic: diversity increased twofold at the ordinal level, three times at the family level, and nearly four times at the genus level.

Our knowledge of Ordovician communities is based almost entirely on the mineralized fossil record; although exceptionally preserved biotas have an important role to play in unravelling the evolution of Ordovician organisms and ecosystems, their contribution so far has been limited because of their scarcity and the fact that the few known Middle and Late Ordovician assemblages represent restricted marine environments.

The discovery of the Fezouata Biota in south-eastern Morocco reveals the first Ordovician exceptionally preserved biotic complex from a normal, open marine setting. The assemblages range in age from latest Tremadocian to late Floian and represent the only exceptionally preserved fauna documenting the prelude to and early stages of the Ordovician radiation. The Fezouata biota shows considerable diversity and contains a high number of taxa typical of Cambrian Burgess Shale-type faunas in association with more modern forms. The preservation of the fossils, entombment within and below event beds, followed by early diagenetic pyritization, is similar to that of the Early Cambrian Chengjiang fauna of China.

This discovery marks the first occurrence of Burgess Shale-type faunas after the Middle Cambrian, and shows that their perceived absence from younger deposits is a taphonomic artifact rather than the result of extinction and replacement of these biotas. Burgess Shale-type taxa continued to impact the diversity and ecological structure of deeper marine communities well after the Middle Cambrian. This questions the concept of a sudden dramatic turnover between the Cambrian and Paleozoic Evolutionary Faunas; at the same time, the presence of a number of advanced, typically post-Cambrian elements among the nonmineralized taxa indicates that significant diversification had already occurred prior to the Ordovician.