By the end of the Permian period 250 million years ago, the supercontinent Pangaea began to break up, eventually forming today's landscape, including the Great Plains bordered by the Rockies.

The end of the Permian also witnessed a mass extinction, where 95% of marine species and 75% of terrestrial vertebrates vanished.

The Permian was followed by the Mesozoic era - the age of reptiles.