 Dimetrodon is an early member of a group called synapsids , which include mammals and many of their extinct relatives , though it is not an ancestor any mammal ( which appeared millions of year later ) . It is often mistaken for a dinosaur in popular culture , despite having become extinct some 40 million years ( Ma ) before the first appearance of dinosaurs in the Triassic period . As a synapsid , Dimetrodon is more closely related to mammals than to dinosaurs or any living reptile . By the early 1900s most paleontologists called Dimetrodon a reptile in accordance with Linnean taxonomy , which ranked Reptilia as a class and Dimetrodon as a genus within that class . Mammals were assigned to a separate class , and Dimetrodon was described as a " mammal @-@ like reptile " . Paleontologists theorized that mammals evolved from this group in ( what they called ) a reptile @-@ to @-@ mammal transition . 
