Dimetrodon

Dimetrodon (i/daɪˈmɛtrədɒn/; meaning "two measures of teeth") is an extinct genus of synapsid that lived during the Early Permian, around 299–270 million years ago (Ma). It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds in Texas and Oklahoma. More recently, fossils have been found in Germany. Over a dozen species have been named since the genus was first described in 1878.

Dimetrodon is often mistaken as a dinosaur or as a contemporary of dinosaurs in popular culture, but it went extinct around 40 million years before the appearance of the first dinosaur in the Triassic period. Generally reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than it is to any living reptilian group, though it is not a direct ancestor of any mammals. Dimetrodon belongs to a group traditionally called "mammal-like reptiles", more recently termed "non-mammalian synapsids" because many vertebrate paleontologists today group Dimetrodon together with mammals in an evolutionary group or clade called Synapsida while dinosaurs go together with living reptiles and birds in a separate group, Sauropsida. A single large opening on either side of the back of the skull links Dimetrodon with mammals and distinguishes it from most of the earliest sauropsids, which either lack openings or have two openings. Features such as ridges on the inside of the nasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early tetrapods (four-limbed vertebrates) to mammals.

Dimetrodon was probably one of the top predators in Early Permian ecosystems, feeding on fish and tetrapods, including reptiles as well as amphibians. Smaller Dimetrodon species may have had different ecological roles. The sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation. Some recent studies argue that the sail would have been ineffective at removing heat from the body, and was more likely used in sexual display.