Dimetrodon grandis skeleton at the
National Museum of Natural History
Dimetrodon (was a predatory synapsid ('mammal-like reptile') genus that flourished during the Permian Period, living between 280 and 265 million years ago. It was more closely related to mammals than to true reptiles (Sauropsida), like dinosaur, lizards and birds.
Dimetrodon was not a dinosaur, despite being popularly grouped with them.  Rather, it is classified as a pelycosaur. It is surprisingly well-known to the general public and makes an appearance in the James Mason film Journey to the Center of the Earth.  Fossils of Dimetrodon have been found in North America and Europe. The climate of Europe and North America in the Early Permian, was probably arid to continental, so Dimetrodon was probably adaptable. 
Dimetrodon was a dominant carnivore, the largest known of its day.  It grew to up to 3 meters (10 feet) 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. Dentition showing this differentiation of teeth is called heterodonty. It walked on four side-sprawling legs and had a large tail. Dimetrodon may have moved in a manner similar to present-day lizards.
The most distinctive characteristic of Dimetrodon was 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. In this way, it could have easily picked off slower prey that was still warming up. 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 humans and all other 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. For more information on human ancestry, see timeline of human evolution.
In popular culture
In the television documentary Walking With Monsters (called Before the Dinosaurs in the U.S.), baby Dimetrodon were shown hatching with sails, fully independent. In fact, no Dimetrodon eggs have yet been found and it's entirely possible that the sail, which would be hard to store in an egg, was either absent or not rigid upon hatching. The program also stated that Dimetrodon consumed 90% of a carcass, while lions today eat 70% and that Dimetrodon would only eat intestines after shaking out the waste inside, since "dung is one thing a Dimetrodon can't stomach." Hatchlings also had to sprint towards trees after hatching in order to escape cannibalistic adults. These behaviours seem to be based on the Komodo Dragon and thus attributing them to Dimetrodon is an almost entirely speculative move on the part of the originators, who have presented the characteristics as 'fact', rather than speculation, in order to heighten the sense of realism.
Dimetrodon is often mistaken for Spinosaurus, due to the fact that they both had 'sails'. Spinosaurus however, was a dinosaur and bipedal, whereas Dimetrodon was not a dinosaur and was quadrupedal.
- Dimetrodon appeared in the film Journey to the Centre of the Earth.
- In Power Rangers: Dino Thunder, one of Black Ranger's zords was a Dimetrodon.
- Dimetrodon is often included in dinosaur toy sets, despite the fact that it was not a dinosaur.
- In Dinosaucers, the character Dimetro is an anthropomorphic Dimetrodon.
- Dimetrodon made an appearance in The Land Before Time (1988).
- Bramwell, C. D. and Fellgett, P. P., 1973, Thermal regulation in sail lizards. Nature, v. 242, p. 203-205.
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