Triassic Period (252 to 201 million years)
Following the great extinction event at the end of the Permian Period, life on Earth gradually reestablished itself both on land and in the oceans through succession. Scleractinians (modern corals) replaced earlier forms as dominant reef-forming organisms. On land, reptilian therapsids (an order related to the distant ancestors of mammals) and archosaurs (ancestors of dinosaurs and modern crocodillians) became the dominant vertebrates. New groups evolved in the middle to late Triassic Period including the first dinosaurs, primitive mammals, and flying vertebrates (pterosaurs) but these families did not flourish until after another global extinction event at the close of Triassic time. Current thought is that ancestral forms of both mammals and dinosaurs first appear in the fossil record in Late Triassic time, about 200 million years ago.
During the middle Triassic, the supercontinent of Pangaea began to rift apart into separate landmasses, Laurasia to the north and Gondwanaland to the south. With the breakup of Pangaea, terrestrial climates gradually changed from being mostly hot and dry to more humid condition. Another mass extinction in the fossil record marks the end of the Triassic Period.