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The Triassic period is the first period of the Mesozoic Era, which lasted for about 180 million years. Mesozoic means "middle life", the time between the ancient Paleozoic and the modern Cenozoic. The Triassic itself was from about 251 to 199 million years ago.
The Triassic period marked an unusual time — life was recolonizing the Earth after the greatest mass extinction ever, the Permian-Triassic extinction, during which 96% of marine genera, 70% of terrestrial genera and 99.5% of individual organisms were annihilated. The precise cause of the Permian-Triassic extinction is not known, but it was so devastating that biologists informally call it "the Great Dying."
Throughout the Triassic period, the world was defined by one super-continent, Pangaea, and one super-sea, Panthalassa. In the center of Pangaea, there was a gigantic desert, the largest the Earth has ever seen. The general climate everywhere was very hot and dry, presenting a challenge for colonization.
In the oceans, modern corals first appeared, resuming the vast reef-building activities which had ceased during the Silurian almost 150 million years prior. Ammonites flourished, diversifying from a single line that had survived the Great Dying. A number of fish and reptile-like fishes flourished in the sea, including ichthyosaurs, plesiosaurs, and many others. By the end of the Triassic period, some of these, particularly ichthyosaurs, developed to dinosaur-sized proportions. Echinoderms evolved in the seas.
On land, the real winners in the Triassic period, as well as throughout the rest of the Mesozoic, were the reptiles. This is why the Mezoic is often called the "Age of Reptiles." Truly modern insects, such as dragonflies, first evolved in the Triassic. Among dinosaurs, the archosauromorphs (cold-blooded) reptiles flourished the most, over their endothermic (warm-blooded, mammal-like) relatives, probably because the cold-blooded reptiles adapted better to the arid environments. The great continent Pangaea was mostly a desert, spotted with oases and a thin ring of coastal life.
The swamp-like trees and ferns of the Paleozoic, which gave rise to the greatest coal beds on Earth, required moisture to thrive and therefore didn't do so well during the dry Triassic. The evergreens, such as conifers and other gymnosperms, dominated the forests of the Triassic period.