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Aetosaurs (''aetosaur'' ; order name ''Aetosauria'' ; from Greek, (aetos, "eagle") and (sauros, "lizard")) are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae. Two distinct subdivisions of aeotosaurs are currently recognized, Desmatosuchinae and Aetosaurinae, based primarily on differences in the morphology of the bony scutes of the two groups. Over 20 genera of aetosaurs have been described, and recently there has been controversy regarding the description of some of these genera. Aetosaur fossil remains are known from Europe, North and South America, parts of Africa and India. Since their armoured plates are often preserved and are abundant in certain localities, aetosaurs serve as important Late Triassic tetrapod index fossils. Many aetosaurs had wide geographic ranges, but their stratigraphic ranges were relatively short. Therefore, the presence of particular aetosaurs can accurately date a site that they are found in. Aetosaur remains have been found since the early 19th century, although the very first remains that were described were mistaken for fish scales. Aetosaurs were later recognized as crocodile relatives, with early paleontologists considering them to be semiaquatic scavengers. They are now considered to have been entirely terrestrial animals. Some forms have characteristics that may have been adaptations to digging for food. There is also evidence that some if not all aetosaurs made nests. ==Description== The head is small relative to the large body, and quite distinctive in shape, being flat and blunt at the front, like the snout of a pig. Some teeth are chisel-shaped, small and leaf-like, indicating a probable herbivorous diet, although peg-like teeth and a keratinous snout have been considered possible adaptations in some species for feeding on colonial insects. A study of the braincase of ''Stagonolepis robertsoni'' has shown that there are similarities between it and those of crocodylomorphs, which may indicate a close relationship.〔Archosaurian Anatomy and Palaeontology: Essays in Memory of Alick D. Walker, DB Norman & DJ Gower (eds.)〕 Aetosaurs had a "pillar-erect" erect limb posture similar to that seen in Rauisuchia, a related group of Triassic archosaurs.〔 A pillar-erect limb posture is one where the femur articulates vertically with the acetabulum of the hip, which is angled downward, so that the leg is positioned beneath the body and acts as a pillar bearing weight. While the limb posture is similar to rauisuchians, the feet resemble those of phytosaurs (crocodile-like semiaquatic crurotarsans) in the retention of primitive characteristics.〔 Although the forelimbs are much smaller than the hind limbs, all aetosaurs were quadrupeds. Aetosaurs were very heavily armored (most certainly as a defense against predators), with large quadrangular, interlocking bony plates, or osteoderms, protecting the back and sides, belly, and tail.〔 Most osteoderms are heavily pitted on their upper surfaces and smooth on their undersides. Their centers are made of cancellous or spongy bone (also called diploë) and their outer portions are made up of compact bone. In life, these plates were probably covered in horn. Dorsal osteoderms, which are found on the backs of aetosaurs, are often ornamented with radial grooves. Dorsal paramedians, those found along the midline of the animal, are often wide and quadrangular with a small boss called a dorsal eminence on the dorsal surface of each plate. In aetosaurs, paramedian plates often have raised or depressed anterior edges where the plates articulate with the ones in front of them. If the anterior edge is raised, the area is called an anterior bar, while if it is depressed, the area is called an anterior lamina. In lateral plates, which are positioned on either side of the paramedian plates, the dorsal eminence is often enlarged into a prominent spike.〔Bill Parker (August 12, 2009). "(Aetosaurs 101: Osteoderm Nomenclature )." ''Chinleana''. Accessed January 8, 2010.〕 This spike is especially noticeable in desmatosuchines such as ''Longosuchus'' and ''Desmatosuchus''. Osteoderms are useful in diagnosing aetosaur taxa, and aetosaur species can often be identified from individual scutes based on their ornamentation pattern. Primitive genera, like the widespread Norian genus ''Aetosaurus'' and the Carnian ''Coahomasuchus'', tended to be small, about a metre (3.2 ft) in length. However, more advanced forms were larger - about in length - with some taxa, such as ''Typothorax'' and ''Paratypothorax'', possessing broad turtle-like bodies, and others, like ''Desmatosuchus'', a narrow-bodied genus at least long, equipped with large spines over the shoulders, which added to the animal's defensive armament. 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「aetosaur」の詳細全文を読む スポンサード リンク
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