Description: It is estimated that 75% of plant and animal (including dinosaur) species went extinct after the Earth was hit with a massive asteroid 66 million years ago. Evidence includes a thin layer of rock containing iridium (rarely found on Earth but common in asteroids) around the planet. Scientists have also discovered a large impact crater. The cause of the extinction is fairly clear but the details of the effects leave much to be explored.
Web Resource: Cretaceous–Paleogene extinction event - Wikipedia