Dimetrodon belongs to a group called the synapsids, which includes all living mammals and a menagerie of extinct animals extending back to about 305 million years ago. Being an ancient animal that looks different from modern-day mammals, non-mammalian synapsids are often mistaken for reptiles and dinosaurs. Although synapsids and reptiles descend from a common ancestor, neither group is directly ancestral to each other. Plus, Dimetrodon became extinct about 60 million years before the first dinosaurs even evolved. That’s almost the same amount of time that separates humans from Tyrannosaurus.