Prehistoric animals weren’t all enormous. The horse’s earliest known ancestor, for example, lived around the same time as the giant boa and (at roughly the size of a fox) was much smaller than today’s equine. And though many prehistoric creatures did get very, very large, they didn’t all appear at the same time. The hugest dinosaurs, such as the plant-eating sauropods and the giant predatory theropods, lived during the Jurassic and Cretaceous periods, about 65 million to 200 million years ago. Forty-five million years ago, the earth started seeing a wave of giant mammals, including the rhinolike Uintatherium and the massive Andrewsarchus.* Wooly mammoths and elephant-sized ground sloths, in turn, lived during the last ice age, between 12,000 and 5 million years ago.
Excerpted from ‘A Snake the Size of a Plane,’ posted on Slate, February 5, 2009.