Transcript: Species have been developing and becoming extinct throughout the history of life on Earth. Extinction is not unusual. However, there have been certain periods of time when the extinction rate increases dramatically. Essentially large fractions of the number of plant and animal species on the entire planet become extinct within a time span that is geologically short. There have been five major mass extinctions in the history of the last half billion years. Before this time there simply wasn't a good enough fossil record to know what happened in terms of the extinction rate. At the late Ordovician period, at the beginning of the Silurian, four hundred and twenty million years ago, about fifty percent of all species became extinct. Then again, about three hundred and eighty million years ago in the late Devonian period, about twenty-five percent of all species became extinct. Large extinction of half the species occurred at the end of the Permian era about two hundred and forty million years ago and then again relatively shortly afterwards about two hundred and twenty million years ago at the end of the Triassic and the beginning of the Jurassic. Perhaps the most famous mass extinction occurred sixty-five million years ago at the boundary between the Cretaceous and the Tertiary period when the dinosaurs and many other species of plants and animals became extinct in a relatively short period of time.