End-Ordovician Mass Extinction
The first of the "Big Five" mass extinctions wiped out an estimated 85% of marine species.
The End-Ordovician mass extinction, occurring approximately 445 million years ago, was the first of the five great mass extinctions in Earth's history. It is believed to have been caused by a severe ice age that led to dramatic sea level drops, destroying shallow marine habitats. An estimated 85% of marine species were eliminated. The extinction occurred in two pulses, separated by about a million years, and dramatically reshaped the trajectory of life on Earth.