Dinosaurs appeared in the Triassic but crocodile cousins ruled it
The Triassic opened after a mass extinction that gutted life on Earth and closed with another. In between, over 50.5 million years, dinosaurs made their debut, yet the dominant land reptiles were relatives of today's crocodiles. Only when a second catastrophe wiped most of those out did dinosaurs take over.
Friedrich Alberti named the period in 1834 from the Greek for triad, after three rock layers widespread in southern Germany: a colourful sandstone at the bottom, a shelly limestone in the middle and coloured clay on top. It runs from about 251.9 to 201.4 million years ago, making it the first and shortest period of the Mesozoic Era and the seventh of the Phanerozoic Eon.
Life split into survivors, brief experiments and future rulers. Reptiles, especially archosaurs, were the main land vertebrates, and among archosaurs the pseudosuchians, the crocodilian lineage, held sway. Other archosaurs took to the air as pterosaurs, the first vertebrates to fly. Therapsids, which had dominated the Permian, enjoyed a short burst as dicynodonts and cynodonts spread, then dwindled until most had vanished by the period's close. Dinosaurs arose in the Late Triassic but had to wait until the Jurassic to dominate.
Almost all land was joined in Pangaea, which straddled the equator in an arc from pole to pole, with the Tethys oceans cupped inside and the vast Panthalassa Ocean beyond. Its interior was mostly hot desert. Subduction zones ringed the supercontinent, while old mountain belts wore down and thinned crust marked where the Atlantic, Indian and Southern oceans would later open. The landmass rotated counterclockwise until roughly 230 million years ago before resuming a westward drift.
Volcanism bracketed the period. The Siberian Traps kept erupting into the Early and Middle Triassic, and by the Late Triassic the Central Atlantic Magmatic Province was active, heralding the seafloor spreading that began the Central Atlantic. In the final Rhaetian stage Pangaea started to split into Laurasia and Gondwana and the climate grew more humid. The closing Triassic–Jurassic extinction then removed most pseudosuchians, clearing the stage for the dinosaurs.
Source: Triassic
