Early Oligocene. Brule Formation, White River Badlands, South Dakota, Nebraska, and Wyoming, USA. The most common White River vegetarians were the oreodonts. They’re extinct now, but for millions of years, this family of leaf-eaters was more numerous than all others across North America. There’s no mammal family today that compares. The body was a cross between a dog and a small deer. The paws were four-toed and built something like a coyote’s, with sharp claws good for excavating burrows. Petrified evidence proves that oreodonts sought refuge in underground bunkers. Perfect skeletons are often dug from fossil burrows, and sometimes mother oreodonts are buried side by side with their pups.