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Late Ordovician mass extinction

Event
The Late Ordovician mass extinction, sometimes known as the end-Ordovician mass extinction or the Ordovician-Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 million years... Wikipedia
Occurred: 450 million years ago - 440 million years ago
Percent of species extinct: 60% – 70%