|
''Sphenacodon'' (meaning "wedge point tooth") is an extinct genus of synapsid that lived from about 300 to about 280 million years ago (Ma) during the Late Carboniferous and Early Permian periods. Like the closely related ''Dimetrodon'', ''Sphenacodon'' was a carnivorous member of the Eupelycosauria family Sphenacodontidae. However, ''Sphenacodon'' had a low crest along its back, formed from blade-like bones on its vertebrae (neural spines) instead of the tall dorsal sail found in ''Dimetrodon''. Fossils of ''Sphenacodon'' are known from New Mexico and the Utah-Arizona border region in North America. Researchers currently recognize two species: ''Sphenacodon ferox'' (the type species) and ''Sphenacodon ferocior''. ''Sphenacodon ferocior'' can be up to 40% larger in overall size (at about 3 m (10 ft) long) compared to ''Sphenacodon ferox'' (at about 2 m (6 ft long)). In addition, the dorsal spines in ''Sphenacodon ferocior'' are proportionately 45% taller than in ''Sphenacodon ferox''. The recent discovery of a nearly complete skull of ''Sphenacodon ferox'' has helped clarify other distinctions between the two species, including the number of teeth in certain parts of the jaws and the size of the indented notch between the maxillary and premaxillary bones in the upper jaw. The two species occur together in some formations, but ''Sphenacodon ferox'' apparently survived later into the Early Permian. ''Sphenacodon'' and ''Dimetrodon'' typically have been found in different geographical areas that were separated by the ancient Hueco Seaway that penetrated equatorial Pangaea during the Early Permian and "covered much of southern New Mexico and parts of West Texas". ''Sphenacodon'' is known from the west in New Mexico, Arizona, and Utah, and ''Dimetrodon'' is known mainly from the east in Texas and Oklahoma in more deltaic environments. However, the species ''Dimetrodon occidentalis'' is found in New Mexico. Each genus would have been an apex land predator in its region and likely preyed on amphibians, diadectids, and early synapsids and diapsids. ''Sphenacodon'' appears to have died out before about 280 million years ago during the Wolfcampian. The genus ''Dimetrodon'' survived until about 270 million years ago. Such large sphenacodontid predators were later replaced by therapsids, the group of synapsids that includes the direct ancestors of mammals. ==Description== The skull of ''Sphenacodon'' is very similar to that of ''Dimetrodon''. It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into sharp pointed "incisors" (), large stabbing "canines" (), and smaller slicing back teeth (). The orbit is set high and far back with a single opening (temporal fenestra) behind and partly below the eye, a characteristic of synapsids. 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''.) There is evidence for strong epaxial muscles along the base of the raised neural spines in both ''Sphenacodon'' and ''Dimetrodon'', likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion. A recent study of the structure of the neural spines on ''Sphenacodon'' confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest. Finds of sphenacodontid specimens in which postmortem distortion of the body caused the dorsal spines to overlap suggests that the spines were not connected by hard or particularly tough tissue. The possible function of a low, skin-covered crest in ''Sphenacodon'' is debated. A thermoregulatory role seems unlikely, although the taller crest in ''Sphenacodon ferocior'' is allometrically larger than in ''S. ferox''. Recent research has favored a display role for the tall sails in ''Dimetrodon'' and ''Edaphosaurus''. Both ''Sphenacodon'' and ''Dimetrodon'' have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail (and even belly) dragging on the ground, similar to modern lizards and crocodiles. A sprawling stance is also typical for ''Sphenacodon'' and ''Dimetrodon'' skeletons as currently mounted in museums. However, trackways called ''Dimetropus'' (“''Dimetrodon'' foot”) that match the foot configuration of large sphenacodontids show animals walking with their limbs brought under the body for a narrow, semi-erect gait without tail or belly drag marks. Such clear evidence for a more efficient upright posture suggests that important details about the anatomy and locomotion of ''Sphenacodon'' and ''Dimetrodon'' may not be fully understood. Some well preserved narrow ''Dimetropus'' tracks found in parts of the Prehistoric Trackways National Monument in New Mexico match the smaller size of ''Sphenacodon'', a genus known from skeletal fossils in the state, but could also come from a small ''Dimetrodon''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sphenacodon」の詳細全文を読む スポンサード リンク
|