The
Ordovician–Silurian extinction events, also known as the
Ordovician extinction, were, combined, the second-largest of the five major
extinction events in Earth's history in terms of percentage of
genera that went extinct and second largest overall in the overall loss of life. Between about 447
Ma to 443 Ma (million years ago), two pulses of extinction, separated by four million years, appear to have happened. They were the second biggest extinction of marine life, ranking below only the
Permian–Triassic extinction event, and as they occurred, all known life was confined to the seas and oceans.