It was the T-J Extinction event 201 million years ago that helped ring in the age of dinosaurs. As Pangea began to break apart, enormous volcanic eruptions tied to the Central Atlantic Magmatic Province released greenhouse gases and disrupted climates and oceans. Many large Triassic competitors vanished, especially several crocodile-line archosaurs.

Before the T-J Extinction, dinosaurs existed, but they were not yet the undisputed rulers. The Triassic world was crowded with other powerful reptiles and archosaurs. Our mammalian ancestors were also present, but small. The great synapsid empire had already fallen 50 million years earlier during the P-T Extinction. By the T-J boundary, mammals were survivors in the margins, not rulers.

After the T-J Extinction, dinosaurs expanded fast. The Jurassic opened with opportunity, and dinosaurs filled the vacant niches: giant herbivores, swift predators, armored forms, and eventually birds. Then came the K-Pg Extinction 66 million years ago. In an evolutionary blink, the non-avian dinosaurs were gone. Once again, our small mammalian ancestors were still there — and this time, the world opened for us.