<Li> Dimetrodon maximus Romer, 1936 </Li> <Li> Eosyodon hudsoni Olson, 1962 </Li> <P> Dimetrodon (/ daɪˈmiːtrədɒn / (listen) or / daɪˈmɛtrədɒn /, meaning "two measures of teeth") is an extinct genus of synapsids that lived during the Cisuralian (Early Permian), around 295--272 million years ago (Ma). It is a member of the family Sphenacodontidae . The most prominent feature of Dimetrodon is the large neural spine 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 of 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 . </P> <P> Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs . Reptile - like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than to modern reptiles, though it is not a direct ancestor of mammals . Dimetrodon is assigned to the "non-mammalian synapsids", a group traditionally called "mammal - like reptiles". This groups Dimetrodon together with mammals in a clade (evolutionary group) called Synapsida, while placing dinosaurs, reptiles and birds in a separate clade, Sauropsida . Single openings in the skull behind each eye, known as temporal fenestrae, and other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids . </P>

Dinosaur with fin on back walks on 4 legs