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Pronounced (die-MET-roh-don) Dimetrodon was a prehistoric animal with a large sail; it was not a dinosaur but a pelycosaur, an early synapsid.

Dimetrodon (meaning two-measure teeth) was a sail-backed, meat-eating animal that lived during the Permian Period, roughly 280 MYA years ago, before the dinosaurs evolved. It was a so-called mammal-like reptile, a pelycosaur; pelycosaurs were the ancestors of the mammals. Dimetrodon was a dominant carnivore during the Permian period, living mainly in swampy areas. Unlike their fellow non-finned pelycosaurs, they warmed up early after sunrise, and cooled off more efficiently during the heat of the day. This efficient thermoregulation along with their large and powerful jaws gave them the advantage over their cohorts, making them dominant. Dimetrodon was about 11.5 feet (3.5 m long and weighed roughly 550 pounds (250 kg. It had a large sail-like flap of skin along its back, dense with blood vessels. The sail was supported by long, bony spines, each of which grew out of a separate spinal vertebra (a bone in the back. The sail may have been a thermoregulatory structure, used to absorb and release heat. The sail may have also been used for mating and dominance rituals, and/or for making Dimetrodon look much larger than it was to predators. This direct cast replica was found in Texas.