LIFE

Dinosaurs Start Small

~233–230 million years ago

Phanerozoic · Mesozoic · Triassic

The oldest animals anyone calls a dinosaur without argument were small, two-legged and fast, and they lived in what is now Argentina and Brazil. Herrerasaurus and Eoraptor stood about as tall as a large dog. Nothing about them announces a group that would hold the land for 160 million years, and in their own valley they were already the main carnivores and the small herbivores.

Why it matters

How they took over is less obvious than it looks. The usual story has them stepping into ground cleared by an extinction, but in the best-dated section the older herbivores disappear one lineage at a time, and dinosaur diversity does not rise as they go. What kept dinosaurs penned in for their first thirty million years was probably climate rather than competition: they are missing from the tropics of Pangaea, and where the latitude suited them, in Africa as in South America, the same fauna turns up.

Dating & uncertainty

Uranium-lead ages on volcanic ash in the Argentine and Brazilian basins place these animals between 233 and 230 million years ago, among the tighter constraints on any origin in this timeline. Less clear is whether that is where dinosaurs began: African rocks of similar age hold them too, and their scarcity outside a narrow band of latitudes may reflect climate rather than a point of origin.

Sources

1. Martínez et al. (2011), Science · A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea
2. Marsicano et al. (2016), PNAS · The precise temporal calibration of dinosaur origins
3. Langer et al. (2018), Gondwana Research · U-Pb age constraints on dinosaur rise from south Brazil
4. Griffin et al. (2022), Nature · Africa's oldest dinosaurs reveal early suppression of dinosaur distribution

Earth history as one day

22:46:431 h 13 min before midnight

if Earth’s 4.54 billion years were compressed into a single day