Dimetrodon incisivus (Cope 1878, D. grandis Case 1907) Early Permian ~270 mya, up to 4 m in length, was derived from a sister to Haptodus and was a sister to Sphenacodon (see below), Biarmosuchus and Edaphosaurus. Dimetrodon left no descendants.
Distinct from Haptodus saxonicus, the skull of Dimetrodon had a more highly curved dorsal profile and jawline. The naris was higher on the premaxilla. The anterior teeth were longer. The maxilla rose higher on the skull. The orbit was smaller and the jugal was deeper. The top and back of the skull formed a smooth curve. The reflected lamina of the mandible was more distinct and narrow. The teeth were larger.
The neural spines were extremely elongated, creating a sail that did not include lateral protuberances as seen in Edaphosaurus. The caudal neural spines were not enlarged as in Edaphosaurus.