Dimetrodon: The Oft-Misidentifed Permian Synapsid.

Greetings. Dimetrodon. An extinct genus of non-mammalian synapsid that lived during the Early Permian Period, approximately 295 to 272 million years ago. With the genus' species measuring between 1.7 and 4.6 meters in length as adults, and weighing between 28 and 250 kilograms, Dimetrodon's most prominent feature was the large neural spine sail formed by elongated spines extending from the animal's vertebrae.

Named in 1878 by Edward Drinker Cope, Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but this belief is without merit, for the genus went extinct some 40 million years before the appearance of the first dinosaurs during the subsequent Triassic Period. Although reptilian in outward appearance and internal physiology, Dimetrodon was much more closely related to mammals than to reptiles, despite the fact that it is not a direct ancestor of modern-day mammals. Dimetrodon is classified as a non-mammalian synapsid, a group traditionally called the "mammal-like reptiles," but now correctly referred to as "stem mammals." This scientific assignment groups Dimetrodon together with all mammals in the clade Synapsida, while reptiles are placed in a separate clade, the Sauropsida.

Dimetrodon was a powerful apex predator, feeding on fish and tetrapods, including other reptiles and amphibians. Some smaller species of Dimetrodon may have had slightly different ecological roles. All species of dimetrodon had a semi-sprawling posture between that of a mammal and a lizard and also could walk in a more upright stance with its body and the majority or all of its tail held completely off the ground. The largest species of Dimetrodon were among the most successful predators of the Early Permian. A single large opening on either side of the back of the skull links Dimetrodon to mammals and distinguishes it from most of the earliest synapsids, which either lack cranial openings or have two openings. Physical features such as ridges on the inside of the animal's nasal cavity and a ridge at the back of the lower jaw are now thought to be part of an evolutionary progression from early four-limbed land-dwelling reptilian vertebrates to the earliest terrestrial mammals. 

There is some evidence suggesting that some species of Dimetrodon were endothermic, warm-blooded like true mammals, dinosaurs, and the birds (avian dinosaurs.) If so, Dimetrodon would be the earliest example of an endothermic animal known to modern science. 

The sail of Dimetrodon was formed by elongated neural spines projecting from the vertebrae. Each spine varied in cross-sectional shape from its base to its tip in what is now known as dimetrodont differentiation. Near the vertebrae, the spine cross-section was laterally compressed into a rectangular shape and, closer to the tip, it took on a figure-eight shape as a groove ran along either side of the spine, reinforcing the spine, preventing bending and/or fractures. Fossilized remains have demonstrated that the shape of the spines changed as each individual animal progressed through its life cycle. The lower or proximal portion of the spine had a rough surface that would have served as an anchoring point for the back muscles and also had a network of strong but flexible connective tissues. Higher up on the outer or distal portion of the spine, the bone surface was substantially smoother. The periosteum, a layer of tissue surrounding the bone, was covered in small grooves that presumably supported the blood vessels that vascularized the entire sail structure. The large groove that ran the length of the spinal column was once thought by paleontologists to be a channel for blood vessels, but since the bone does not contain any vascular canals, the sail of Dimetrodon is now thought to have not been as highly vascularized as once thought. Interestingly, some fossilized 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. 

As with other sail-backed species in the fossil record, the physiological function of the neural sail has not been firmly determined by paleontologists. 

Fossilized remains of the genus are known from the United States; Texas, Oklahoma, New Mexico, Arizona, Utah and Ohio, from Canada; Prince Edward Island, and interestingly, from Germany, locations that were all part of the supercontinent Pangaea during the Early Permian. 

Thank you for your time and consideration.