When Did the First Mass Extinction Happen?

Around 443 million years ago, at the end of the Ordovician Period, Earth experienced its first of the "Big Five" mass extinctions. A severe ice age caused global sea levels to drop dramatically, destroying shallow marine habitats where most life existed. An estimated 85% of marine species went extinct, including many trilobites, brachiopods, and graptolites. The extinction occurred in two pulses separated by about a million years, corresponding to the advance and retreat of massive glaciers centred on what is now the Sahara region of Africa (which was then at the South Pole).