 The geologic history of reefs is interrupted by five major extinction events. During these events, large numbers of the earth's species died out. The best known of these events, at the Cretaceous-Tertiary boundary, was caused by an asteroid or comet impact that eliminated dinosaurs as well as many marine species. Causes of older extinction events are still being studied
Each major extinction event eliminated reefs from the world oceans. Following an extinction event, millions of years passed before reefs re-evolved. The newly evolved reef communities often differed in biotic composition from those that preceded them.
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