Evidence for the relatedness between odd-toed ungulates, including horses, rhinos, and tapirs, is the structure of their lower limbs, especially the large middle toe on each hoof. They also have distinctive teeth and are “hind-gut fermenters” with a digestive system that can process large amounts of low-quality vegetation. While they are not diverse today, these animals were abundant and diverse for millions of years. They first appear in the fossil record 58 million years ago (late Paleocene). During the 15 million years that followed, they diversified into many species, including ancestral horses that were the size of dogs, tapirs similar to those living today, and a variety of rhinos. An extinct rhino (Paraceratherium sp.) was the largest land animal that has ever lived. It weighed as much as 20 tons (eight times as much as an elephant). Odd-toed ungulates declined though the Oligocene and Miocene (about 34-5.5 million years ago), while even-toed ungulates rose to dominance, for reasons that are debated among paleontologists and paleoecologists.