Mass extinction

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A mass extinction is conventionally defined as a loss of a large fraction of species in a geologically short interval, across many unrelated groups and across a wide geographic range. Each clause is doing work. Extinction is continuous — the background rate never stops — so what distinguishes a mass extinction is rate and breadth rather than the fact of loss. "Many unrelated groups" excludes the collapse of a single clade, and "wide geographic range" excludes a regional catastrophe. The measurement is harder than the definition. Percentages quoted for species loss are estimates reverse-engineered from counts of genera or families in a fossil record that is spatially uneven, taxonomically inconsistent and biased toward shelly marine animals — which is why the figure for the end-Permian has been revised from around 96 per cent of marine species on the classic 1979 analysis to around 81 per cent on a 2016 reanalysis without any new fossils being required. The "Big Five" grouping itself comes from a statistical treatment of the marine record and is a useful convention rather than a natural category; some workers argue the Late Devonian belongs in a different class, being a failure of speciation more than a spike of extinction.

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