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.

On Earth today, mammals, particularly humans, are a common life form in many different habitats. But mammals were not always common or diverse. The first mammals, which evolved during the middle Triassic (about 240 million years ago), were small compared to many of their contemporaries, including the earliest dinosaurs. No larger than shrews or mice, these early mammals probably lived in burrows or other refuges when dinosaur and other reptilian predators roamed the Earth. Though they remained relatively small, mammals evolved to fulfill many ecological roles during the long reign of the dinosaurs. When dinosaurs (except birds) disappeared at the extinction event 66 million years ago, mammals survived and began to expand into ecological spaces (niches) vacated by the larger dinosaur herbivores and predators. By about 55 million years, ago, all modern groups of mammals had evolved. The Cenozoic (the Era in which we live) is known as the Age of Mammals, thanks to their successful radiation into a huge variety of habitats on Earth.