The first mass extinction
85 per cent of species vanish in two quick blows
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The Ordovician–Silurian extinction is the second largest of the Big Five by species lost. The mechanism is well understood: a short, severe glaciation on Gondwana lowered sea level by an estimated 100 metres and removed the shallow shelf seas where nearly all marine life lived. The rapid warming that followed brought anoxia. The coincidence with land plants beginning to spread is striking: weathering driven by plant roots draws CO₂ from the atmosphere, and several researchers see a direct causal chain from the first mosses to the ice.
For children
Gondwana, the great southern continent, drifted over the South Pole and was covered in ice. Sea level fell, the shallow seas dried out, and the life in them died. Then the ice melted just as abruptly, and those that had adapted to the cold died too. Two blows in two million years killed eight species in ten.
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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org