A REINTERPRETATION OF THE ORDOVICIAN RADIATION

It has long been recognized that the Laurentian upper Cambrian features three closely spaced (ca 3-5 my) and significant mass extinctions, terminating the Marjuman, Steptoean, and Sunwaptan stages. A wealth of new field data demonstrates that these phenomena continued into the Ordovician, with mass extinctions of similar intensity terminating the Skullrockian and Stairsian stages. A comprehensive global database of all nearly 23,000 trilobite species indicates, as far as data exist, that these events were global in scope and effect. This sequence of five mass extinctions, each ranking among the most proportionally severe of the Phanerozoic, coincides almost exactly with a long interval of extreme volatility in the carbon isotopic record. Trilobites diversified rapidly between extinction events, but global species richness was progressively and severely reduced even as higher (family group level) taxonomic diversity was largely maintained. Following the fifth and final extinction event near the end of the Tremadocian (Laurentian end-Stairsian), radiation proceeded unchecked. Diversification following this event involves the previously discovered Whiterock Fauna, that component of Trilobita which participated in the Ordovician Radiation. Many different potential agents have been investigated as possible causes of or contributors to the Ordovician Radiation. The most important factor governing the timing and nature of the diversification may simply be the cessation of cyclical extinctions. Marine life diversified rapidly through much of the Cambrian, stagnated or slightly declined at higher taxonomic levels during a period featuring five closely spaced Late Cambrian and Early Ordovician extinction events, then resumed rapid diversification once the extinctions ended.