Dimetrodon

Dimetrodon (meaning "two-measures tooth", in reference to the fact that unlike reptiles, the teeth differ in size.) is an extinct genus of terrestrial predatory synapsid ('mammal-like reptile') genus of pelycosaur that flourished during the Permian Period, living between 280–265 million years ago. Dimetrodon was the largest and most specialized pelycosaur, and remained the dominant carnivore of its environment. It was superseded by the eotitanosuchians and dinocephalian theraspids during the Roadian age. Dimetrodon first appeared in the late Paleozoic Era, and died out following the Permian-Triassic extinction event, 245 million years ago, which immediately preceded the Mesozoic Era. It was more closely related to mammals than to true reptiles such as lizards. When fully grown, Dimetrodon grew up to three meters (9.8 ft) or more and perhaps 150 kg (330 lb), with species varying in weight and size. Dimetrodon is an advanced member of the Sphenacodontid family, and may have evolved from an early Sphenacodon-like form (such as Sphenacodon ferox) during the Late Carboniferous or Early Permian (Gzhelian-Asselian) time.

Iconic and well-known for its immediately recognizable "fin" or "sail" made up of elongate vertebral spines running along its back, up to a meter in length, and in life covered with a layer of skin and blood vessels and serving as a thermoregulatory structure for a heat-exchange mechanism. The sail may have also been used for mating and dominance rituals, and/or for making it look much larger than it was to predators. Despite popular belief, Dimetrodon was not a dinosaur. Rather, it is classified as a pelycosaur, and more closely related to mammals than to reptiles. Dimetrodon was the dominant predator in its environment for nearly twenty-five million years, during which time it evolved into about a dozen species, becoming steadily larger as time progressed.

Dimetrodon fossils have been found in Early to Middle Permian deposits of a localized area in western equatorial Euramerica, now Oklahoma (Wellington Formation) and Texas. A significant find by Jerry MacDonald was the discovery of Dimetrodon footprints in southern New Mexico. Fragmentary remains have also been found in Europe.

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