Dimetrodon
© Jerry Lofaro/FineArtAmerica Dimetrodon is an extinct synapsid, mammal-like reptile. It could reach 4.6 metres in length and could have weighted up to 250 kilograms. It was probably one of the apex predators, feeding on fish and tetrapods. The sail of Dimetrodon may have been used to stabilize its spine and to heat and cool its body as a form of the thermoregulation. Although it is often thought of as a dinosaur, probably because of its large size and carnivorous habits, it lived at least 40 million years before the first dinosaurs appeared on Earth. In fact, it is much more closely related to the ancestors of mammals, although there is little mammal-like about its structure or habits. Dimetrodon is a large quadrupedal animal with a sprawling posture like a modern crocodile, but unlike a modern crocodile, had a tall, narrow skull with huge canine teeth and a single opening on each side behind the eyes that marked it as a mammal-like reptile. Its most conspicuous feature is...