Dimetrodon

Dimetrodon (meaning "two measures of teeth") is an extinct genus of predatory synapsid ('mammal-like reptile') genus that flourished during the Permian Period, living between 280–265 million years ago. It was more closely related to mammals than to true reptiles such as lizards.

Dimetrodon was not a dinosaur, despite being popularly grouped with them. Rather, it is classified as a pelycosaur. Fossils of Dimetrodon have been found in North America and Europe, as well as a significant discovery of Dimetrodon footprints in southern New Mexico by Jerry MacDonald.

Description
Dimetrodon was an apex predator, among the largest of its day. It grew to up to 3 m in length. The name Dimetrodon means 'two-measures of teeth', so named because it had a large skull with two different types of teeth (shearing teeth and sharp canine teeth), unlike reptiles. It walked on four side-sprawling legs and had a large tail. Dimetrodon may have moved in a manner similar to present-day lizards.

Sail
The most distinctive characteristic of Dimetrodon was the spectacular sail on its back (other pelycosaurs such as Edaphosaurus, Ianthasaurus, and Sphenacodon also have this trait). The sail, which was dense with blood vessels, was probably used to regulate body temperature; the surface area would allow it to warm up or cool off more efficiently. This adaptation was important because it would give the animal more time to hunt prey. The sail may also have been used in mating rituals and to warn off other predators. The sail was supported by neural spines, each one sprouting from an individual vertebra. Bramwell and Fellgett (1973) calculate that a 200 kg Dimetrodon would heat up from 26° C to 32° C in 205 minutes without a sail and in only 80 minutes with a sail.

Relationship with modern mammals
As a synapsid, Dimetrodon was distantly related to modern mammals. Synapsids were the first tetrapods to evolve differentiated (or heterodont) teeth. Whereas reptiles hardly chew their food, simply gulping it down, synapsids like Dimetrodon developed teeth to help shear meat into smaller pieces for easier ingestion. These 'two-measure teeth' eventually gave rise to the various kinds of teeth present in modern mammals.