The end-Ordovician mass extinction

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Usually reckoned the second most severe of the five by proportion of genera lost, and structurally unlike the others because it is clearly two pulses separated by several hundred thousand years and driven by opposite conditions. A short, intense glaciation on Gondwana — then positioned over the South Pole — locked water into ice, dropped global sea level and drained the shallow epicontinental seas where most marine diversity lived; that is the first pulse. The second came with deglaciation, sea-level rise and the spread of anoxic water back across the shelves. It is the only one of the Big Five in which cooling is the primary driver rather than warming. The graptolites, brachiopods and trilobites were heavily affected, but the event is notable for removing large numbers of species without dismantling the structure of marine ecosystems — the same kinds of organisms in the same kinds of communities resumed afterwards, which is not true of the end-Permian.

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