The oldest major event they identified is the Ordovician-Silurian extinction, which occurred somewhere between 450 and 400 million years ago. Essentially all life was in the ocean back then so most of the species lost were marine-based. This mass extinction—the second most deadly, in which about 85 percent of all species went extinct—occurred in two stages over a period of about 3.5 million years. The cause seems to have initially been lower temperatures and massive glaciations that caused dramatic drops in sea level.
