Edaphosaurus cruciger (Cope 1882) Early Permian ~280 mya, was derived from a sister to Haptodus and represents the last of its lineage. Sister taxa include Dimetrodon and Sphenacodon.
Distinct from Haptodus, the skull of Edaphosaurus was extremely reduced. The lateral temporal fenestra was enlarged for larger jaw muscles. Rather than sloping down posteriorly, the skull reached new heights. The coronoid process also shifted anteriorly, providing more room for jaw muscles. Batteries of teeth were present on tooth pads indicating a diet of plants. Over the orbits the skull was laterally extended to produce large "brows." The mandible was deeper than the skull.
The cervicals were shortened. The neural spines were extremely elongated, creating a sail that included lateral protuberances. This created a display structure and helped regulate body temperature as a solar heat collector. The caudal spines were also taller.
The ilium anterior process was more strongly developed.
Edaphosaurus was the first known plant-eater among archosauromorphs and synapsids.