Period· The columns of time
Triassic
252 Ma → 201 Ma
The Triassic, from 252 to 201 million years ago, was the recovery after the end-Permian extinction, in which dinosaurs and mammals appeared, and it closed with another volcanic province.
Period of recovery after the end-Permian extinction, in which dinosaurs and mammals appear, and closed by a further volcanic province.
What the evidence shows
It begins after the Great Dying, and the reconstitution of ecosystems takes several million years. Reptiles return to the sea and establish themselves there: within a few million years their limbs become paddles and the interior of their bones becomes spongy, the signature of an animal that no longer returns to land. Their appearance follows the extinction so closely that they appear to have passed through it rather than to have occupied the space it left. Midway, between 234 and 232 million years, the Carnian pluvial episode replaces an arid regime with a humid one for about two million years, in step with a volcanism of its own; a third of marine genera disappear, and the conifers, the crocodile lineage and the dinosaurs emerge from that interval. The oldest animals universally accepted as dinosaurs lived around 233 to 230 million years in what is now Argentina and Brazil, small, bipedal and the size of a large dog. Mammaliaforms follow, the size of a shrew and probably nocturnal. Pangaea finally begins to fracture, the rift filling with basalt in four pulses over some 600,000 years.
Bounds and conventions
A golden spike opens it; its top is dated by uranium-lead on the basalts of the Central Atlantic Magmatic Province, giving a precision better than a hundred thousand years. The first appearance of dinosaurs is by contrast a moving target, and not only towards greater age: African finds published in 2022 are of comparable age to those of South America, while dating of the beds carrying their immediate precursors has made them five to ten million years younger, which compresses the origin of the group into less than five million years.