Around 252 million years ago, the Permian-Triassic extinction was the largest mass death in Earth's history: roughly 96 percent of marine species and perhaps 70 percent of terrestrial vertebrates vanished. For surviving organisms, however, this catastrophe was simultaneously an opportunity — vacated ecological niches were open territory waiting to be filled.

In the Triassic's first half, lizard-like rauisuchians and aetosaurs took greatest early advantage. But in the middle Triassic, around 230 million years ago, a group of small, agile, bipedal or quadrupedal reptiles emerged: dinosaurs. Eoraptor lunensis and Herrerasaurus ischigualastensis, found in South America's Ischigualasto Formation in what is now Argentina, are among the oldest known dinosaurs.

The first dinosaurs were unimpressive: a few kilograms at most, dwarfed by the dominant land animals of their time. What set them apart was not size but posture and physiology. Unlike other reptiles, their limbs were positioned directly beneath the body rather than splaying to the sides — a stance that enabled faster movement and greater energy efficiency.

A second major extinction at the close of the Triassic (c. 201 Ma) eliminated most of their remaining competitors and removed the last obstacle to dinosaur expansion. By the early Jurassic they were unambiguous rulers — and would remain so for 165 million years through extraordinary diversity, from titanic sauropods to flying pterosaurs to clawed theropods, until the end of the Cretaceous. Their descendants — birds — still rule the skies today.