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''Pachyrhinosaurus'' (meaning "thick-nosed lizard") is an extinct genus of centrosaurine ceratopsid dinosaurs from the Late Cretaceous period of North America. The first examples were discovered by Charles M. Sternberg in Alberta, Canada, in 1946, and named in 1950. Over a dozen partial skulls and a large assortment of other fossils from various species have been found in Alberta and Alaska. A great number were not available for study until the 1980s, resulting in a relatively recent increase of interest in the ''Pachyrhinosaurus''. Three species have been identified. ''P. lakustai'', from the Wapiti Formation, the bonebed horizon of which is roughly equivalent age to the upper Bearpaw and lower Horseshoe Canyon Formations, is known to have existed from about 73.5-72.5 million years ago. ''P. canadensis'' is younger, known only from the lower Horseshoe Canyon Formation, about 71.5-71 Ma ago. Fossils of the youngest species, ''P. perotorum'', have been recovered from the Prince Creek Formation of Alaska, and date to 70-69 million years ago. The presence of three known species makes this genus the most speciose among the centrosaurines. ==Description== The largest ''Pachyrhinosaurus'' species were long.〔Holtz, Thomas R. Jr. (2008) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages'' (Supplementary Information ) 〕 It weighed about four tonnes. They were herbivorous and possessed strong cheek teeth to help them chew tough, fibrous plants. Instead of horns, their skulls bore massive, flattened bosses; a large boss over the nose and a smaller one over the eyes. A prominent pair of horns grew from the frill and extended upwards. The skull also bore several smaller horns or ornaments that varied between individuals and between species. In ''P. canadensis'' and ''P. perotorum'', the bosses over the nose and eyes nearly grew together, and were separated only by a narrow groove. In ''P. lakustai'', the two bosses were separated by a wide gap. In ''P. canadensis'' and ''P. lakustai'', the frill bore two additional small, curved, backward-pointed horns. These were not present in ''P. perotorum'', and in fact some specimens of ''P. lakustai'' also lack them, which may indicate that the presence of these horns varied by age or sex.〔 Various ornaments of the nasal boss have also been used to distinguish between different species of ''Pachyrhinosaurus''. Both ''P. lakustai'' and ''P. perotorum'' bore a jagged, comb-like extension at the tip of the boss which was missing in ''P. canadensis''. ''P. perotorum'' was unique in having a narrow dome in the middle of the back portion of the nasal boss, and ''P. lakustai'' had a pommel-like structure projecting from the front of the boss (the boss of ''P. canadensis'' was mainly flat on top and rounded). ''P. perotorum'' bore two unique, flattened horns which projected forward and down from the top edge of the frill, and ''P. lakustai'' bore another comb-like horn arising from the middle of the frill behind the eyes.〔 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「Pachyrhinosaurus」の詳細全文を読む スポンサード リンク
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