Dimetrodon is mistaken for a dinosaur more often than any other prehistoric reptile--but the fact is that this creature lived tens of millions of years before the first dinosaurs had even evolved. Here are 10 facts you may or may not have known about this sail-backed pelycosaur. (See also a gallery of Dimetrodon pictures.)
1. Dimetrodon wasn't actually a dinosaur...
Although it looked superficially like a dinosaur, Dimetrodon was actually a type of prehistoric reptile known as a pelycosaur. The pelycosaurs were themselves more closely related to the therapsids, or "mammal-like reptiles," than to the archosaurs from which dinosaurs evolved--which means that, technically speaking, Dimetrodon was closer to being a mammal than it was to being a dinosaur!
2. ...and it lived 50 million years before the first dinosaurs appeared on earth.
Even people who know that Dimetrodon wasn't technically a dinosaur mistakenly assume that it lived alongside its more famous cousins. In fact, Dimetrodon prospered during the middle Permian period, between 280 and 265 million years ago, while the first dinosaurs (according to our current state of knowledge) evolved in South America during the middle Triassic period, about 50 million years later.
3. Dimetrodon was a close relative of Edaphosaurus...
To the untrained eye, Edaphosaurus may look like nothing more than a scaled-down version of Dimetrodon, complete with a miniaturized sail. However, this ancient pelycosaur subsisted mostly on plants and mollusks, whereas Dimetrodon was a devoted meat eater. (In fact, the 200-pound Edaphosaurus was one of the first substantially sized plant eaters on the planet.)
4. ..and the strangely named Casea...
Yet another obscure pelycosaur of the Permian period, Casea (Greek for "cheese"), was halfway in heft between Dimetrodon and Edaphosaurus. What distinguished this reptile from its more famous cousins was its lack of a sail, which drew attention to its grotesquely swollen belly (the plant-eating Casea needed plenty of intestines to process tough vegetable matter) and slug-like trunk.
5. ...which it may have eaten for lunch and dinner.
Edaphosaurus lived slightly before the golden age of Dimetrodon (during the late Carboniferous and early Permian periods), while Casea appeared on the scene slightly afterward, at the cusp of the Triassic period. However, it's possible that these three genera briefly overlapped, so it may well have been the case that Dimetrodon occasionally preyed on its smaller pelycosaur cousins.
6. Dimetrodon may have used its sail to warm up in the sun...
The most distinctive feature of Dimetrodon was this pelycosaur's giant sail, the like of which wasn't seen again until the late Cretaceous Spinosaurus. Since this slow-moving reptile almost certainly had a cold-blooded metabolism, it may have evolved this sail as a temperature-regulation device, using it to soak up valuable sunlight during the daytime and dissipate excess heat at night.
7. ...or to attract members of the opposite sex.
Not everyone subscribes to the sail-as-solar-panel theory. Dimetrodon's sail may have been secondarily (or even strictly) a sexually selected characteristic, meaning that males with bigger sails established dominance within the herd and had the opportunity to mate with more females. (It's also possible that females, rather than males, were endowed with bigger sails; we don't have enough fossil evidence to say for sure!)
8. The name Dimetrodon means "two measures of teeth."
Given its prominent sail, it's an odd fact that Dimetrodon was named after one of its more obscure features, the two different kinds of teeth embedded in its jaws. The dental arsenal of Dimetrodon included sharp canines in the front of its snout, ideal for digging into quivering, freshly killed prey, and shearing teeth in the back for grinding up tough muscle and bits of bone.
9. Dimetrodon walked with a splay-legged posture.
One of the primary features that distinguished true dinosaurs from the archosaurs, pelcyosaurs and therapsids that preceded them was the upright orientation of their limbs. That's why we can be sure that Dimetrodon wasn't a dinosaur: this reptile walked with an ambling, splay-footed, crocodilian gait, rather than the vertical posture of comparably sized quadrupedal dinosaurs.
10. Dimetrodon fossils have been discovered in Europe and North America.
There are no less than 15 named species of Dimetrodon, all of which have been discovered in North America and western Europe. A full one-third of these species were named by the famous American dinosaur hunter Edward Drinker Cope, which may help to explain why Dimetrodon is so often identified as a a dinosaur rather than a pelycosaur, even by people who should know better!