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It's often mistaken for a dinosaur, but Dimetrodon lived at least 50 million years before the first dinosaurs evolved--280 million years ago, during the Permian period. An example of a pelycosaur--one of the first families of reptiles to live entirely on land--Dimetrodon went extinct about 250 million years ago, along with countless other species, during the Permian-Triassic extinction event. (See 10 Facts About Dimetrodon and a gallery of Dimetrodon pictures.)
Dimetrodon's most spectacular feature was its large sail, which ran along the entire length of its back. Paleontologists speculate that the main purpose of this sail was temperature regulation: the flap of skin soaked up sunshine during the day (much like a solar panel), and also helped dissipate energy at night. (Of course, this theory presumes that Dimetrodon possessed an ectothermic, or cold-blooded, metabolism.) Dimetrodon's sail may also have served as a mating display, and it also helped its owner look a lot bigger in the eyes of any hungry carnivores lurking nearby.
While Dimetrodon was certainly the most notable pelycosaur of the Permian period, it did have some close relatives that merit further study. The similarly sailed Edaphosaurus was smaller than its famous cousin, and it was also a plant-eater, meaning that it may have figured on the lunch menu of the carnivorous Dimetrodon. And one of the strangest pelycosaurs of all time was Casea (Greek for "cheese"), a squat, tubular reptile that looked like Dimetrodon shorn of its sail.