Body proportions are also similar to "Dimetrodon", with a very large head, short neck, robust trunk, relatively short front and hind limbs, and a tapering tail that makes up about half the animal's entire length. However, the tops of the neural spines along the back bone are strikingly different in each genus. In "Dimetrodon", the neural spines develop into long, narrow, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail. In "Sphenacodon", the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, and form a crest rather than a tall sail. (The sphenacodontid genus "Ctenospondylus" also has blade-like neural spines, but its dorsal crest is taller than in "Sphenacodon", although not as tall as the sail in "Dimetrodon".)