Period· The columns of time

Triassic

252 Ma → 201 Ma

A world rebuilt from almost nothing, in which dinosaurs and mammals both begin, small and unremarkable, before another eruption ends it.

What the evidence shows

It opens in the wreckage of the Great Dying, and the recovery takes millions of years. Reptiles walk back into the sea and stay: within a few million years their legs are paddles and the inside of their bones has gone spongy, the signature of an animal that never comes ashore. They appear so soon after the extinction that they look like survivors of it rather than newcomers behind it. Halfway through, between 234 and 232 million years ago, a dry world turns humid for about two million years, perhaps in step with a volcanic episode of its own, a third of marine genera go, and out the other side come the conifers, the crocodile line and the dinosaurs. The oldest animals anyone calls a dinosaur without argument lived about 233 to 230 million years ago in what is now Argentina and Brazil, small, two-legged and about as tall as a large dog. Mammals follow, shrew-sized and probably nocturnal. Then Pangaea starts to split, the crack fills with basalt in four pulses over some 600,000 years, and the period ends.

Bounds and conventions

A golden spike opens it; its top is dated by uranium and lead in the basalts of the Central Atlantic province, which is why it is known to better than a hundred thousand years. The dinosaurs' first appearance is a moving target, and it does not only move earlier: African finds published in 2022 are of comparable age to the South American ones, while dating the beds that hold the dinosaurs' immediate precursors made them five to ten million years younger than thought, which compresses the origin of the group into less than five million years.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Martínez et al. (2011), Science · A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea
3. Marsicano et al. (2016), PNAS · The precise temporal calibration of dinosaur origins
4. Dal Corso et al. (2020), Science Advances · Extinction and dawn of the modern world in the Carnian (Late Triassic)
5. Blackburn et al. (2013), Science · Zircon U-Pb geochronology links the end-Triassic extinction with the Central Atlantic Magmatic Province
6. Kear et al. (2023), Current Biology · Earliest Triassic ichthyosaur fossils push back oceanic reptile origins
7. Luo et al. (2001), Science · A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics