Though commonly grouped with dinosaurs in children's books and other media, the Dimetrodon was actually a synapsid, not a dinosaur. Synapsids are a group of animals that includes all mammals, as well as early mammal-like reptiles such as Dimetrodon. The large spiny sail on its back is its most distinctive feature, though its exact function remains unknown. History: Dimetrodon lived during the Early Permian Period, long before the dinosaurs it would come to be so closely associated with. It was more closely related to mammals than to dinosaurs, or to any modern-day reptile. Unlike dinosaurs, it walked with its legs splayed out to the side instead of held straight below the body Scientific Name: DimetrodonÂ (meaning two sizes of teeth) Characteristics: Our professionally painted Dimetrodon toy figure features the splayed stance, rounded skull, and unique teeth that Dimetrodon is known for, not to mention its most obvious identifier: the large spiny sail along its back.