page_title: Dimetrodon
section_title: Thermoregulation
hierarchical_section_title: Dimetrodon / Paleobiology / Function of neural spines / Thermoregulation
caption_attribution_description: English: Skeleton of Sphenacodon ferox in the Field Museum of Natural History.
context_page_description: Dimetrodon is an extinct genus of non-mammalian synapsids that lived during the Cisuralian, around 295–272 million years ago. It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, fossils have been found in Germany. Over a dozen species have been named since the genus was first erected in 1878.
  Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs. Reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than to modern reptiles, though it is not a direct ancestor of mammals. Dimetrodon is assigned to the "non-mammalian synapsids", a group traditionally called "mammal-like reptiles".
context_section_description: In 1940, Alfred Romer and Llewellyn Ivor Price proposed that the sail served a thermoregulatory function, allowing individuals to warm their bodies with the sun's heat. In the following years, many models were created to estimate the effectiveness of thermoregulation in Dimetrodon. For example, in a 1973 article in the journal Nature, paleontologists C. D. Bramwell and P. B. Fellgett estimated that it took a 200 kilograms (440 lb) individual about one and a half hours for its body temperature to rise from 26 to 32 °C (79 to 90 °F). In 1986, Steven C. Haack concluded that the warming was slower than previously thought and that the process probably took four hours. Using a model based on a variety of environmental factors and hypothesized physiological aspects of Dimetrodon, Haack found that the sail allowed Dimetrodon to warm faster in the morning and reach a slightly higher body temperature during the day, but that it was ineffective in releasing excess heat and did not allow Dimetrodon to retain a higher body temperature at night. In 1999, a group of mechanical engineers created a computer model to analyze the ability of the sail to regulate body temperature during different seasons, and concluded that the sail was beneficial for capturing and releasing heat at all times in the year.
  Most of these studies give two thermoregulatory roles for the sail of Dimetrodon: one as a means of warming quickly in the morning, and another as a way to cool down when body temperature becomes high. Dimetrodon and all other Early Permian land vertebrates are assumed to have been cold-blooded or poikilothermic, relying on the sun to maintain a high body temperature. Because of its large size, Dimetrodon had high thermal inertia, meaning that changes in body temperature occurred more slowly in it than in smaller-bodied animals. As temperatures rose in the mornings, the small-bodied prey of Dimetrodon could warm their bodies much faster than could something the size of Dimetrodon. Many paleontologists including Haack have proposed that the sail of Dimetrodon may have allowed it to warm quickly in the morning in order to keep pace with its prey. The sail's large surface area also meant heat could dissipate quickly into the surroundings, useful if the animal needed to release excess heat produced by metabolism or absorbed from the sun. Dimetrodon may have angled its sail away from the sun to cool off or restricted blood flow to the sail to maintain heat at night.
  In 1986, J. Scott Turner and C. Richard Tracy proposed that the evolution of a sail in Dimetrodon was related to the evolution of warm-bloodedness in mammal ancestors. They thought that the sail of Dimetrodon enabled it to be homeothermic, maintaining a constant, albeit low, body temperature. Mammals are also homeothermic, although they differ from Dimetrodon in being endothermic, controlling their body temperature internally through heightened metabolism. Turner and Tracy noted that early therapsids, a more advanced group of synapsids closely related to mammals, had long limbs which can release heat in a manner similar to that of the sail of Dimetrodon. The homeothermy that developed in animals like Dimetrodon may have carried over to therapsids through a modification of body shape, which would eventually develop into the warm-bloodedness of mammals.
  Recent studies on the sail of Dimetrodon and other sphenacodontids support Haack's 1986 contention that the sail was poorly adapted to releasing heat and maintaining a stable body temperature. The presence of sails in small-bodied species of Dimetrodon such as D. milleri and D. teutonis does not fit the idea that the sail's purpose was thermoregulation because smaller sails are less able to transfer heat and because small bodies can absorb and release heat easily on their own. Moreover, close relatives of Dimetrodon such as Sphenacodon have very low crests that would have been useless as thermoregulatory devices. The large sail of Dimetrodon is thought to have developed gradually from these smaller crests, meaning that over most of the 
caption: Sphenacodon had a low crest along its back
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