The Triassic was a battleground, in which dinosaurs and mammals warred to see who would inherit the niches left empty after the Permian extinction. At first, both sides seemed equally matched, with dinosaurs like Eoraptor (1) competing more less directly with mammals like Cynognathus (2). The two groups rapidly diverged, however.
Mammals grew every larger and more diverse over the course of the Mesozoic, evolving into towering tree-browsers (10), low-grazing giants armed with plates (11), spikes (12), and tusks (13) as well as predators large and ferocious enough to conquer these defenses (6).
Small, nimble dinosaurs were better suited for life in the shadows. They dug under the ground (3) and climbed trees (4) to escape from mammals, only venturing onto the ground at night, aided by dark-adapted vision, smell, and hearing keener than any mammal’s (5, 6). Some even became modest hunters themselves (7).
Where dinosaurs excelled was reproduction. Mammals took care of their young after hatching, but one group of dinosaurs carried their eggs and chicks with them in a mobile nest or pouch evolved from the forelimbs (8,14). It was this lineage that would go on to rule the Earth after the next extinction.