The Triassic period began approximately 248 million years ago and ended 206 million years ago; although, some sources date it to be between 251 and 199 million years ago. It was followed by the Jurassic period; it was the first period of the Mesozoic era.
The Triassic followed a massive extinction event at the end of the Permian period. The event killed 90 percent of all life on Earth. Only 5 percent of the ocean life survived, making Triassic oceans rather uniform in regard to their ecology. Among the surviving land animals were two different branches of vertebrates. The therapsids were the branch that later developed into mammals, and they dominated the early Triassic. Later, the archosaurs, which developed into modern reptiles, pterosaurs, dinosaurs and birds, became the dominant land animals. They remained so throughout the rest of the Mesozoic.