 The sail of Dimetrodon is formed by elongated neural spines projecting from the vertebrae . Each spine varies in cross @-@ sectional shape from its base to its tip in what is known as " <unk> " differentiation . Near the vertebra body , the spine cross section is laterally compressed into a rectangular shape , and closer to the tip , it takes on a figure @-@ eight shape as a groove runs along either side of the spine . The figure @-@ eight shape is thought to reinforce the spine , preventing bending and fractures . A cross section of the spine of one specimen of Dimetrodon <unk> is rectangular in shape but preserves figure @-@ eight shaped rings close to its center , indicating that the shape of spines may change as individuals age . The microscopic anatomy of each spine varies from base to tip , indicating where it was embedded in the muscles of the back and where it was exposed as part of a sail . The lower or proximal portion of the spine has a rough surface that would have served as an anchoring point for the <unk> muscles of the back , and also has a network of connective tissues called <unk> 's fibers that indicate it was embedded within the body . Higher up on the distal ( outer ) portion of the spine , the bone surface is smoother . The <unk> , a layer of tissue surrounding the bone , is covered in small grooves that presumably supported the blood vessels that vascularized the sail . The large groove that runs the length of the spine was once thought to be a channel for blood vessels , but since the bone does not contain vascular canals , the sail is not thought to have been as highly vascularized as once thought . Some specimens of Dimetrodon preserve deformed areas of the neural spines that appear to be healed @-@ over fractures . The cortical bone that grew over these breaks is highly vascularized , suggesting that soft tissue must have been present on the sail to supply the site with blood vessels . Layered lamellar bone makes up most of the neural spine 's cross @-@ sectional area , and contains lines of arrested growth that can be used to determine the age of each individual at death . In many specimens of D. <unk> the distal portions of spines bend sharply , indicating that the sail would have had an irregular profile in life . Their <unk> suggests that soft tissue may not have extended all the way to the tips of the spines , meaning that the sail 's webbing may not have been as extensive as it is commonly imagined . 
