Dimetrodon

Dimetrodon (/daɪˈmiːtrədɒn/ (listen) or /daɪˈmɛtrədɒn/, meaning 'two measures of teeth') is an extinct genus of synapsids that lived during the Cisuralian (Early Permian), around 295–272 million years ago (Ma). It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, fossils have been found in Germany. Over a dozen species have been named since the genus was first described in 1878. Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs. Reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than to modern reptiles, though it is not a direct ancestor of mammals. Dimetrodon is assigned to the 'non-mammalian synapsids', a group traditionally called 'mammal-like reptiles'. This groups Dimetrodon together with mammals in a clade (evolutionary group) called Synapsida, while placing dinosaurs, reptiles and birds in a separate clade, Sauropsida. Single openings in the skull behind each eye, known as temporal fenestrae, and other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids. Dimetrodon was probably one of the apex predators of the Cisuralian ecosystems, feeding on fish and tetrapods, including reptiles and amphibians. Smaller Dimetrodon species may have had different ecological roles. The sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation. Some recent studies argue that the sail would have been ineffective at removing heat from the body due to large species being discovered with small sails and small species being discovered with large sails, essentially ruling out heat regulation as its main purpose. The sail was most likely used in courtship display with methods such as threatening rivals or showing off to potential mates. Dimetrodon was a quadrupedal, sail-backed synapsid. Most Dimetrodon species ranged in length from 1.7 to 4.6 metres (6 to 15 ft) and are estimated to have weighed between 28 and 250 kilograms (60 and 550 lb). The largest known species of Dimetrodon is D. angelensis at 4.6 metres (15 ft) and the smallest is D. teutonis at 60 centimetres (24 in). The larger species of Dimetrodon were among the largest predators of the Early Permian, although the closely related Tappenosaurus, known from skeletal fragments in slightly younger rocks, may have been even larger at an estimated 5.5 metres (18 ft) in total body length. Although some Dimetrodon species could grow very large, many juvenile specimens are known. A single large opening on either side of the back of the skull links Dimetrodon with mammals and distinguishes it from most of the earliest sauropsids, which either lack openings or have two openings. Features such as ridges on the inside of the nasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early four-limbed land-dwelling vertebrates to mammals. The skull of Dimetrodon is tall and compressed laterally, or side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket is a single hole called an infratemporal fenestra. An additional hole in the skull, the supratemporal fenestra, can be seen when viewed from above. The back of the skull (the occiput) is oriented at a slight upward angle, a feature that it shares with all other early synapsids. The upper margin of the skull slopes downward in a convex arc to the tip of the snout. The tip of the upper jaw, formed by the premaxilla bone, is raised above the part of the jaw formed by the maxilla bone to form a maxillary 'step.' Within this step is a diastema, or gap in the tooth row. Its skull was more heavily built than a dinosaur's. The size of the teeth varies greatly along the length of the jaws, lending Dimetrodon its name, which means 'two measures of tooth' in reference to sets of small and large teeth. One or two pairs of caniniforms (large pointed canine-like teeth) extend from the maxilla. Large incisor teeth are also present at the tips of the upper and lower jaws, rooted in the premaxillae and dentary bones. Small teeth are present around the maxillary 'step' and behind the caniniforms, becoming smaller further back in the jaw. Many teeth are widest at their midsections and narrow closer to the jaws, giving them the appearance of a teardrop. Teardrop-shaped teeth are unique to Dimetrodon and other closely related sphenacodontids, and help distinguish them from other early synapsids. As in many other early synapsids, the teeth of most Dimetrodon species are serrated at their edges. The serrations of Dimetrodon teeth were so fine that they resembled tiny cracks. The dinosaur Albertosaurus had similarly crack-like serrations, but, at the base of each serration was a round void, which would have functioned to distribute force over a larger surface area and prevent the stresses of feeding from causing the crack to spread through the tooth. Unlike Albertosaurus, Dimetrodon teeth lacked adaptations that would stop cracks from forming at their serrations. The teeth of D. teutonis lack serrations, but still have sharp edges.