Dimetrodons, artwork. Dimetrodon (meaning 'two-measure tooth') lived in the early Permian period, around 270 million years ago. It was not a dinosaur, but a pelycosaur (class Synapsida), a primitive ancestor of the mammals. It was a predator that grew to a length of 3.5 metres and weighed about 250 kilograms. It had a large dorsal 'sail' that was believed to be used to regulate its body temperature. Although cold-blooded, Dimetrodon showed early mammalian traits, such as enlarged openings at the back of the skull (fenestra), sharp canine teeth and shearing teeth. It is thought that Dimetrodons and other pelycosaurs gave rise to the therapsids, which were the direct ancestors of the mammals.