Artwork showing the relative scale of humans and Dimetrodon ('two measures of teeth'). This animal is frequently mistaken for a dinosaur. Yet it lived in North America and other locations around the world some 295-272 million years ago, at least 35 million years before the first appearance of dinosaurs. Dimetrodon was rather a sail-backed synapsid - a very old ancestor of mammals (although it itself was not a mammal). Dimetrodons ranged in size depending on the species, from 0.6 m (2 ft) to 4.6 m (15 ft) long. The specimen shown here is D. angelensis, one of the larger varieties.