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 sail's evolutionary history, thermoregulation could not have served an important function.