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}} ''Lystrosaurus'' (; "shovel lizard") is a genus of Late Permian and Early Triassic Period dicynodont therapsids, which lived around 250 million years ago in what is now Antarctica, India, and South Africa. Four to six species are currently recognized, although from the 1930s to 1970s the number of species was thought to be much higher. One specimen unearthed in Karoo measured 2.5 meters long.〔() 〕 Being a dicynodont, ''Lystrosaurus'' had only two teeth, a pair of tusk-like canines, and is thought to have had a horny beak that was used for biting off pieces of vegetation. ''Lystrosaurus'' was a heavily built, herbivorous animal, approximately the size of a pig. The structure of its shoulders and hip joints suggests that ''Lystrosaurus'' moved with a semi-sprawling gait. The forelimbs were even more robust than the hindlimbs, and the animal is thought to have been a powerful digger that nested in burrows. ''Lystrosaurus'' was by far the most common terrestrial vertebrate of the Early Triassic, accounting for as many as 95% of the total individuals in some fossil beds. It has often been suggested that it had anatomical features that enabled it to adapt better than most animals to the atmospheric conditions that were created by the Permian–Triassic extinction event and which persisted through the Early Triassic—low concentrations of oxygen and high concentrations of carbon dioxide. However, recent research suggests that these features were no more pronounced in ''Lystrosaurus'' than in genera that perished in the extinction or in genera that survived but were much less abundant than ''Lystrosaurus''. ==Description== ''Lystrosaurus'' was a dicynodont therapsid, between long with an average of about depending upon the species.〔(【引用サイトリンク】url=http://www.prehistoric-wildlife.com/species/l/lystrosaurus.html )〕 Unlike other therapsids, dicynodonts had very short snouts and no teeth except for the tusk-like upper canines. Dicynodonts are generally thought to have had horny beaks like those of turtles, for shearing off pieces of vegetation which were then ground on a horny secondary palate when the mouth was closed. The jaw joint was weak and moved backwards and forwards with a shearing action, instead of the more common sideways or up and down movements. It is thought that the jaw muscles were attached unusually far forward on the skull and took up a lot of space on the top and back of the skull. As a result, the eyes were set high and well forward on the skull, and the face was short. Features of the skeleton indicate that ''Lystrosaurus'' moved with a semi-sprawling gait. The lower rear corner of the scapula (shoulder blade) was strongly ossified (built of strong bone), which suggests that movement of the scapula contributed to the stride length of the forelimbs and reduced the sideways flexing of the body.〔 The five sacral vertebrae were massive but not fused to each other and to the pelvis, making the back more rigid and reducing sideways flexing while the animal was walking. Therapsids with fewer than five sacral vertebrae are thought to have had sprawling limbs, like those of modern lizards.〔 In dinosaurs and mammals, which have erect limbs, the sacral vertebrae are fused to each other and to the pelvis. A buttress above each acetabulum (hip socket) is thought to have prevented dislocation of the femur (thigh bone) while ''Lystrosaurus'' was walking with a semi-sprawling gait.〔 The forelimbs of ''Lystrosaurus'' were massive,〔 and ''Lystrosaurus'' is thought to have been a powerful burrower.〔 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「Lystrosaurus」の詳細全文を読む スポンサード リンク
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