Dimetrodon was a pelycosaur, one of a number of animals known as Mammal-Like Reptiles (more accurately synapsids) that dominated the Late Permian period. It was a large meat-eater that lived primarily in swampy areas and had a very interesting feature - a large sail on its back. Although often mistaken for a dinosaur, it died out long before dinosaurs evolved and is more closely related to modern mammals. Unlike other pelycosaurs that did not have fins, Dimetrodon was able to warm up in the morning and cool off more efficiently during the heat of the day due to its sail. This method of controlling its body temperature, along with its large and powerful jaws, gave this predator a distinct advantage over the other large carnivores, making it the dominant land predator. In fact, its ability to warm up faster would have enabled it to catch and eat the slower, non-finned Mammal-Like Reptiles. Dimetrodon is easily distinguished from dinosaurs as its legs were positioned on the side of its body while dinosaurs had their legs under their bodies. Even with this body structure, it was probably a very fast runner. It had a large head with very strong jaws and two types of teeth -sharp fang-like teeth for ripping and tearing in the front and smaller cheek teeth in the back that were designed for shredding the meat. [1] This information stood on the collector card from the Kenner toyline: 'Scientists once speculated that Dimetrodon's unique dorsal structure was used as a sail to help the animal swim. Now, however, the more widely accepted explanation is that the spines were involved in regulating body temperature, as blood flowing through the sail could be warmed or cooled more quickly than in the rest of the body.'