You can’t understand dinosaurs without a sense of time. We need to know when a dinosaur lived to comprehend how it fits into what paleontologist William Diller Matthew called “life’s splendid drama.” But we throw around Deep Time estimates, framed in millions of years, so often that it’s easy to become inured to the wider context of life’s history.

The Mesozoic Era, which lasted from about 250 million to 66 million years ago, is often called the Age of Dinosaurs. As a kid, this brought to mind one endless summer when dinosaurs flourished. And many of the books I read picked one environment from three different periods within the era to represent dinosaur life. Little Coelophysis was the canonical Triassic dinosaur; the huge sauropods and theropods of the Morrison Formation represented the Jurassic, and a Cretaceous Tyrannosaurus versus Triceratops face-off ultimately capped off the succession. With the periods juxtaposed this way, millions of years didn’t seem so very long.

But let’s unpack some of that scenery. Diplodocus, Apatosaurus, Allosaurus, Stegosaurus and their neighbors roamed western North America about 150 million years ago. This slice of time falls in the latter portion of the Jurassic. The traditional representatives of the latest Cretaceous scene—Tyrannosaurus and Triceratops—did not evolve until about 67 million years ago. By themselves, these dates are just labels, but think of them falling along evolution’s timeline. About 83 million years separated Apatosaurus from Tyrannosaurus and Allosaurus from Triceratops. The so-called Age of Mammals—which began when the non-avian dinosaurs were wiped out—has been going on for about 66 million years. Less time separates us from Tyrannosaurus rex than separated T. rex from Stegosaurus.

Consider how much life has changed in the past 66 million years. Archaic mammals flourished and ultimately went extinct long before anything like the world’s modern fauna appeared. Saber-fanged, knobbly-headed herbivores such as Uintatherium, lemur-like primates called adapiforms, razor-jawed carnivores known as creodonts and many other strange forms proliferated and disappeared. Even lineages familiar to us today, such as horses, rhinos and elephants, evolved and diversified and are now represented by just remnants of what once existed.

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