Dimetrodon was a prehistoric predacious reptile that flourished during the Permian period, living between 280 and 260 million years ago. It is known as the ancestor of both reptiles and mammals, having had early characteristics from both. Dimetrodon was not a dinosaur; rather, it is classified as a pelycosaur.

Dimetrodon was a dominant carnivore, the largest of its day. It grew to up to 2 meters in length. The word dimetrodon means "two-measures teeth"; it is so named because it had a large skull with two different types of teeth (shearing teeth and sharp canine teeth). It walked on four side-sprawling legs and had a large tail.

Dimetrodon's most distinctive characteristic is the spectacular sail on its back. The sail was probably used to regulate body temperature; the surface area would allow it to warm up or cool off more efficiently. It may also have been used in mating rituals and to ward off predators. The sail was supported by bony spines, each one sprouting from an individual spinal vertebra.

Dimetrodon was discovered in Texas by Edward Drinker Cope.

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