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The Crocodilia (or Crocodylia) are an order of mostly large, predatory, semiaquatic reptiles. They appeared 83.5 million years ago in the Late Cretaceous period (Campanian stage) and are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period, and diversified during the Mesozoic era. The order Crocodilia includes the true crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae), and the gharial and false gharial (family Gavialidae). Although the term 'crocodiles' is sometimes used to refer to all of these, a less ambiguous vernacular term for this group is crocodilians. Large, solidly built, lizard-like reptiles, crocodilians have long flattened snouts, laterally compressed tails, and eyes, ears, and nostrils at the top of the head. They swim well and can move on land in a "high walk" and a "low walk", while smaller species are even capable of galloping. Their skin is thick and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite. They have a four-chambered heart and, somewhat like birds, a unidirectional looping system of airflow within the lungs, but like other reptiles they are ectotherms. Crocodilians are found mainly in lowlands in the tropics, but alligators also live in the southeastern United States and the Yangtze River in China. Florida, in the United States, is the only place that crocodiles and alligators live side by side. They are largely carnivorous, the various species feeding on animals such as fish, crustaceans, molluscs, birds, and mammals; some species like the Indian gharial are specialised feeders, while others like the saltwater crocodile have generalised diets. Crocodilians are typically solitary and territorial, though cooperative feeding does occur. During breeding, dominant males try to monopolise available females. Females lay eggs in holes or in mounds and, unlike most other reptiles, care for their hatched young. Eight species of crocodilians are known to have attacked humans. The largest number of attacks comes from the Nile crocodile. Humans are the greatest threat to crocodilian populations through activities that include hunting and habitat destruction, but farming of crocodilians has greatly reduced unlawful trading in wild skins. Artistic and literary representations of crocodilians have appeared in human cultures around the world since at least Ancient Egypt. The earliest known mention of the story that crocodiles weep for their victims was in the 9th century; it was later spread by Sir John Mandeville in 1400 and then by William Shakespeare in the late 16th century and early 17th century. ==Spelling and etymology== Crocodilia and Crocodylia have been used interchangeably for decades starting with Schmidt's redescription of the group from the formerly defunct term Loricata.〔Schmidt, K.P. 1953. A Checklist of North American Amphibians and Reptiles. Sixth edition. Amer. Soc. Ichthy. Herp. Chicago, University of Chicago Press.〕 Schmidt used the older term Crocodilia, based on Owen's original name for the group.〔Owen, R. 1842. Report on British Fossil Reptiles. Part II. Report British Association Adv. Sci. Plymouth Meeting. 1841:60–240.〕 Shortly after, Wermuth opted for Crocodylia as the proper name for this redescribed group,〔Wermuth, H. 1953. Systematik der Rezenten Krokodile. Mitt. Mus. Berlin. Vol. 29(2):275–514.〕 basing it on the type genus ''Crocodylus'' (Laurenti, 1768).〔Laurenti, J.N. 1768. Specimen Medicum, Exhibens Synopsin Reptilium Emendatum cum Experimentis Circa Venena et Antidota Reptilium Austriacorum. Joan. Thom. Nob. de Trattern, Vienna.〕 Dundee—in a revision of many reptilian and amphibian names—argued strongly for Crocodylia to be the spelling for the group.〔Dundee, H.A. 1989. Higher Category Name Usage forAmphibians and Reptiles. Syst. Zool. Vol. 37:410–419.〕 However, it was not until the advent of cladistics and phylogenetic nomenclature that a more solid justification for assuming one spelling over the other was proposed.〔Brochu, C.A. 2003. Phylogenetic Approaches Toward Crocodylian History. Ann. Rev. Earth Planet. Sci. Vol. 31:357–397.〕 Prior to 2003, Crocodilia/Crocodylia was a group that encompassed the modern-day animals (the crown group) as well as their more distant relatives now in the larger groups called Crocodylomorpha and Pseudosuchia.〔 Under its current definition Crocodylia is restricted to only the most recent ancestor of today's modern-day crocodilians (alligators, crocodiles, and gharials).〔 This distinction is more important for paleontologists studying crocodilian evolution. As such, the alternate spellings Crocodilia and Crocodylia are still used interchangeably in the neontological literature. Crocodilia〔 appears to be a Latinizing of the Greek κροκόδειλος (''crocodeilos''), which means both lizard and Nile crocodile. Crocodylia, as coined by Wermuth,〔 in regards to the genus ''Crocodylus'' appears to be derived from the ancient Greek κρόκε (''kroke'')—meaning shingle or pebble—and δρîλος or δρεîλος (''dr(e)ilos'') for "worm". The name may refer to the animal's habit of basking on the pebbled shores of the Nile.〔Kelly, 2006. p. xiii.〕 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「crocodilia」の詳細全文を読む スポンサード リンク
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