Triassic

The Triassic is the first period of the Mesozoic era of geologic time. It extends from 251 to 200 million years ago. The Triassic period is preceded by the Permian and followed by the Jurassic.

The Triassic period began with the third and most devastating extinction event on Earth, when nearly ninety five percent of all species on land and in the seas were wiped out. The cause of this event is not known.Space.com: The Five Worst Extinctions in Earth's History (September 7, 2000) Surviving species diversified to fill ecological niches left open by the disaster. Lizards, crocodiles, turtles, frogs, and the ancestor of all mammals first appeared in the Triassic. Early proto-dinosaurs also emerged in the Triassic and gradually competed with mammal-like reptiles and early mammals for supremacy.Science Daily: Rise Of Dinosaurs in Late Triassic More Gradual Than Once Thought (July 20, 2007)

When the Earth's fourth great extinction occurred at the end of the Triassic 200 million years ago, the dinosaurs emerged as the dominant land life form. They remained so for 140 million years.BBC: Rise and Fall of the Dinosaurs