Dimetrodon (Greek for "two measures of teeth") is often mistaken for a dinosaur, but it lived at least 50 million years before the first dinosaurs evolved--280 million years ago, during the Permian period.

Dimetrodon's most spectacular feature was its large sail, which ran along the entire length of its back. Paleontologists speculate that the main purpose of this sail was temperature regulation: the flap of skin soaked up sunshine during the day (much like a solar panel), and also helped dissipate energy at night. (Of course, this theory presumes that Dimetrodon possessed an ectothermic, or cold-blooded, metabolism.) Dimetrodon's sail may also have served as a mating display, and it also helped its owner look a lot bigger in the eyes of any hungry carnivores lurking nearby.

At the Museum of Ancient Life, Thanksgiving Point, Utah.