When paleontologists refer to the "Big Five", they are referring to the big five mass extinction events:
-The Ordovician-Silurian extinction events, which happened about 450 million years ago, killing almost 85% of the existing species.
-The Late Devonian extinction, which happened 370 million years ago, finishing more than 80% of the existing species.
-The Permian-Triassic extinction event, which happened 250 million years ago, killing more than 95% of the species of that time.
-The Triassic-Jurassic extinction event, that happened 210 million years ago, ending with almost 76% of the existing species.
-The Cretaceous-Paleogene extinction event, which happened 65 million years ago, ending with more than 75% of the existing species.