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Oviraptorosauria
Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, with or without bony crests atop the head. They ranged in size from ''Caudipteryx'', which was the size of a turkey, to the 8 metre long, 1.4 ton ''Gigantoraptor''.〔Xu, X., Tan, Q., Wang, J., Zhao, X., and Tan, L. (2007). "A gigantic bird-like dinosaur from the Late Cretaceous of China." ''Nature'', 447: 844-847.〕 The group (along with all maniraptoran dinosaurs) is close to the ancestry of birds. Analyses like those of Maryanska ''et al'' (2002) and Osmólska ''et al.'' (2004) suggest that they may represent primitive flightless birds.〔〔Osmólska, Halszka, Currie, Philip J., Brasbold, Rinchen (2004) "The Dinosauria" Weishampel, Dodson, Osmólska. "Chapter 8 Oviraptorosauria" University of California Press.〕 The most complete oviraptorosaur specimens have been found in Asia.〔 The North American oviraptorosaur record is sparse.〔Varricchio, D. J. 2001. Late Cretaceous oviraptorosaur (Theropoda) dinosaurs from Montana. pp. 42–57 in D. H. Tanke and K. Carpenter (eds.), Mesozoic Vertebrate Life. Indiana University Press, Indianapolis, Indiana.〕
The earliest and most basal ("primitive") known oviraptorosaurs are ''Protarchaeopteryx robusta'' and ''Incisivosaurus gauthieri'', both from the lower Yixian Formation of China, dating to about 125 million years ago during the Aptian age of the early Cretaceous period. Some fragmentary species, such as ''Calamospondylus oweni'' and ''Thecocoelurus daviesi'', may have been even earlier members of the Oviraptorosauria. A tiny neck vertebra reported from the Wadhurst Clay Formation of England shares some features in common with oviraptorosaurs, and may represent an earlier occurrence of this group (at about 140 million years ago).〔Naish, D. and Sweetman, S.C. (2011). "A tiny maniraptoran dinosaur in the Lower Cretaceous Hastings Group: evidence from a new vertebrate-bearing locality in south-east England." ''Cretaceous Research'', 32: 464-471. 〕
==Description==
Oviraptorosaurians are different from most other maniraptorans in the form of their skulls. They have shortened snouts, beak-like jaws with few or no teeth, and a large opening in the lower jaw bone. Some have bony crests atop the skull. The most primitive members have a few teeth in the front of the mouth; in ''Incisivosaurus'', they are enlarged and form bizarrely prominent "bucktoothed" incisors. The arms and hands are generally long (though very reduced in some advanced species) and the shoulder girdle is large and massive, with flexed coracoid bones and prominent attachments for strong arm muscles.
Their tails are very short compared to other maniraptorans. In ''Nomingia'' and ''Similicaudipteryx'', the tail ends in four fused vertebrae which Osmólska, He, and others have referred to as a "pygostyle", but which Witmer found was anatomically different and evolved separately from the pygostyle of birds (a bone which serves as the attachment point for a fan of tail feathers).〔He, T., Wang, X.-L., and Zhou, Z.-H. (2008). "A new genus and species of caudipterid dinosaur from the Lower Cretaceous Jiufotang Formation of western Liaoning, China." ''Vertebrata PalAsiatica'', 46(3): 178-189.〕〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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