Dimetrodon

Dimetrodon ( or ; ) is an extinct genus of sphenacodontid synapsid that lived during the Cisuralian (Early Permian) epoch of the Permian period, around 295–272 million years ago. With most species measuring long and weighing , the most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It was an obligate quadruped (it could walk only 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 of these coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, its fossils have also been found in Germany and over a dozen species have been named since the genus was first erected in 1878.

Dimetrodon is often mistaken for a dinosaur or portrayed as a contemporary of dinosaurs in popular culture, but it became extinct by the middle Permian, some 40 million years before the appearance of dinosaurs. Although reptile-like in appearance and physiology, Dimetrodon is much more closely related to mammals, as it belongs to the closest sister family to therapsids, the latter of which contains the direct ancestor of mammals. Dimetrodon is traditionally assigned to the paraphyletic group "pelycosaurs", a term now considered obsolete and replaced by terms such as "primitive synapsids" or "basal synapsids"; the name "mammal-like reptiles" is also traditionally used for non-mammalian synapsids due to some of the features shared with modern mammals such as tooth specialization and endothermy, but that term is now also defunct. The Dimetrodon skull has a single opening (temporal fenestra) behind each eye, a feature shared among all synapsids, unlike the skulls of reptiles and birds, both of which belong to the clade Sauropsida, which had diverged from the synapsids by the Late Carboniferous.