Today’s whales are the biggest mammals that have ever lived. On land, however, mammals hit their top size between 37 and 23 million years ago in the form of Paraceratherium. This huge, hoofed mammal belonged to a group of hornless rhinoceros called hyracodonts, and Paraceratherium itself looked something like a rhino impersonating a giraffe. The herbivore stood about 18 feet high at the shoulder and could have reached its head 25 feet off the ground. Not even the most massive of the mammoths grew quite so large.