DIMETRODON

genus of extinct primitive predatory reptiles found as fossils in Permian deposits of North America (the Permian Period occurred from 286 to 245 million years ago). Dimetrodon, which grew to be more than 3.5 m (11 feet) long, had a large dorsal sail formed by elongated dorsal vertebral spines connected by a membrane containing many blood vessels. The sail probably functioned as a temperature-regulating device. The skull was high and narrow; the region in front of the eyes was long. The many teeth were differentiated into types suited to grasping and holding, to piercing, and to dividing food material into smaller pieces. The body was slim, with a long tail and sprawling limbs not as awkwardly set as in other primitive reptiles; Dimetrodon probably could move more efficiently than could its contemporaries, obviously an advantage to a predator. It is probable that the therapsids, the mammallike reptiles that eventually gave rise to the mammals, were descended from Dimetrodon or a similar ancestor.