"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.