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''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 123.2 and 100 million years ago. It is notable for being the most species-rich dinosaur genus. Up to 11 species are known, from across Mongolia, Siberia, China, and possibly Thailand. The species of ''Psittacosaurus'' were obligate bipeds at adulthood, with a high skull and a robust beak. One individual was found preserved with long quills on the tail, similar to those of ''Tianyulong'', yet scales of varying sizes and shapes across the rest of the animal. ''Psittacosaurus'' probably had complex behaviours, based on the proportions and relative size of the brain. It may have been active for short periods of time during the day and night, and had good smell and well developed vision. ''Psittacosaurus'' was one of the earliest ceratopsians, but closer to ''Triceratops'' than ''Yinlong''. Once in its own family, Psittacosauridae, with other genera like ''Hongshanosaurus'', it is now considered to be senior synonym of the latter and an early offshoot of the branch that led to more derived forms. The genera closely related to ''Psittacosaurus'' are all from Asia, with the exception of ''Aquilops'', from North America. The first species was either ''P. lujiatunensis'' or closely related, and it may have given rise to later forms of ''Psittacosaurus''. ''Psittacosaurus'' is one of the most completely known dinosaur genera. Fossils of over 400 individuals have been collected so far, including many complete skeletons. Most different age classes are represented, from hatchling through to adult, which has allowed several detailed studies of ''Psittacosaurus'' growth rates and reproductive biology. The abundance of this dinosaur in the fossil record has led to the labelling of Lower Cretaceous sediments of east Asia the ''Psittacosaurus'' biochron. ==Description== Different species of ''Psittacosaurus'' varied in size and specific features of the skull and skeleton, but shared the same overall body shape. The best-known species, ''P. mongoliensis'', reached 2 meters (6.5 ft) in length. The maximum adult body weight was most likely over 20 kilograms (44 lb) in ''P. mongoliensis''. Several species approached ''P. mongoliensis'' in size (''P. lujiatunensis'', ''P. neimongoliensis'', ''P. xinjiangensis''), while others were somewhat smaller (''P. sinensis'', ''P. meileyingensis''). ''P. ordosensis'' was the smallest known species, 30% smaller than ''P. mongoliensis''.〔 The largest were ''P. lujiatunensis'' and ''P. sibiricus'', although neither was significantly larger than ''P. mongoliensis''. The skull of ''Psittacosaurus'' was highly modified compared to other ornithischian dinosaurs of its time. The skull was extremely tall in height and short in length, with an almost round profile in some species. The portion in front of the orbit (eye socket) was only 40% of total skull length, shorter than any other known ornithischian. The lower jaws of psittacosaurs are characterized by a bulbous vertical ridge down the center of each tooth. Both upper and lower jaws sported a pronounced beak, formed from the ''rostral'' and ''predentary'' bones, respectively. The bony core of the beak may have been sheathed in keratin to provide a sharp cutting surface for cropping plant material. As the generic name suggests, the short skull and beak superficially resembled those of modern parrots. ''Psittacosaurus'' skulls shared several adaptations with more derived ceratopsians, such as the unique rostral bone at the tip of the upper jaw, and the flared jugal (cheek) bones. There was still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians.〔You Hailu & Dodson, Peter. (2004). Basal Ceratopsia. In: Weishampel, David B., Dodson, Peter, & Osmolska, Halszka (Eds.). ''The Dinosauria'' (2nd Edition). Berkeley: University of California Press. Pp. 478–493.〕 Bony horns did protrude from the skull of ''P. sibiricus'', but these are thought to be an example of convergent evolution.〔 ''Psittacosaurus'' postcranial skeletons were more typical of a 'generic' bipedal ornithischian. In ''P. mongoliensis'', similarly to other species, the forelimbs were only 58% as long as the hindlimbs and their range of motion indicates that the hands could neither be rotated at the forearm (''pronated'') nor used in scrambling fashion, to generate propulsive force, suggesting that these animals were totally bipedal in life. There were only four digits on the manus ('hand'), as opposed to the five found in most other ornithischians (including all other ceratopsians). Overall, the four-toed hindfoot was very similar to many other small ornithischians.〔 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「Psittacosaurus」の詳細全文を読む スポンサード リンク
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