the giant dinosaurs with the same lungs—take in oxygen while breathing in and out. This means dinosaurs got more oxygen with each breath than a similarly sized mammal. And there’s more: the air sacs extend through the body and even into the bones, acting as an air-conditioning system, and lightening the skeleton.

they even burrowed and nested together in social groups,

Every so often, without warning, the lake would gurgle and a cloud of invisible gas would ascend from its anoxic abyss. Sometimes the gases were volcanic, other times a by-product of bacteria or algae. Regardless, the fumes were toxic and quickly asphyxiated anything in their path: animals swimming near the lake surface, loitering by the shore,

Early on, long before they found success in the Cretaceous, the tribosphenic therians splintered into two

This new molar was a marvelous evolutionary invention, because as its name implies, it serves two functions at once. It can grind and shear.

If the hothouse

Evolving alongside the angiosperms, in a coevolutionary waltz, were their pollinators:

The first group, with their two skull openings, were the diapsids. They would eventually evolve into lizards, snakes, crocodiles, dinosaurs, birds, and turtles (which closed up their holes). The second group, with their single skull opening, were the synapsids. They would diversify into a dazzling array of species, including—more than a hundred

Whatever they were, they weren’t unique to us. Later anatomists identified this same three-bone set in other mammals—but only mammals.