what are two major extinctions?
['Ordovician-silurian Extinction: 440 million years ago.', 'Devonian Extinction: 365 million years ago.', 'Permian-triassic Extinction: 250 million years ago.', 'Triassic-jurassic Extinction: 210 million years ago.', 'Cretaceous-tertiary Extinction: 65 Million Years Ago.']