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''Utatsusaurus'' is the earliest-known form of an ichthyopterygian (the group of marine reptiles that includes the ichthyosaurs), which lived in the early Triassic period (c. 245–250 millions years ago).It is nearly 3m (about 9 feet) long with a slender body.〔Motani, R., N. Minoura, and T. Ando, ''Ichthyosaurian relationships illuminated by new primitive skeletons from Japan''. Nature, 1998. 393(6682): p. 255-257. 〕 The first specimen was found in Utatsu-cho (now part of Minamisanriku-cho), Miyagi Prefecture, Japan. The name ''Utatsusaurus'' was given after the city.〔Shikama, T., T. Kamei, and M. Murata, ''Early Triassic Ichthyosaurus, Utatsusaurus hataii Gen. et Sp. Nov., from the Kitakami Massif, Northeast Japan''. Science Reports of the Tohoku University Second Series (Geology), 1977. 48(1–2): p. 77-97.〕 The fossils have been found from the Lower Triassic of Miyagi Prefecture, Japan and British Columbia, Canada.〔Motani, R., ''Phylogeny of the Ichthyopterygia''. Journal of Vertebrate Paleontology, 1999. 19(3): p. 473-496.〕 ''Utatsusaurus'' is one of the most primitive grades of ichthyosaurs, a basal ichthyosaur.〔 〔Motani, R., ''Evolution of fish-shaped reptiles (Reptilia : Ichthyopterygia) in their physical environments and constraints''. Annual Review of Earth and Planetary Sciences, 2005. 33: p. 395-420. 〕 ==Description== Unlike the more advanced ichthyosaurs, ''Utatsusaurus'' has no dorsal fin and has a broad skull. The snout gently tapers, compared to the more rounded one of more derived ichthyopterygians.〔Cuthbertson, R.S., A.P. Russell, and J.S. Anderson, ''Reinterpretation of the cranial morphology of Utatsusaurus hataii (Ichthyopterygia) (Osawa Formation, Lower Triassic, Miyagi, Japan) and its systematic implications''. Journal of Vertebrate Paleontology, 2013. 33(4): p. 817-830. 〕The postorbital underlaps the elongate posterior process of the postfrontal. This is an evident plesiomorphic condition for ichthyopterygians.〔 For the size of the skull, the teeth are rather small, and arranged in a primitive groove. They have longitudinal grooves and were first thought to be longer and more acute than'' Grippia'', which is a closely related ichthyosaur.〔 But, after that, it was reported that they were rather bluntly pointed and robust by reexamining the holotype.〔Motani, R., ''Redescription of the dental features of an early Triassic ichthyosaur, Utatsusaurus hataii''. Journal of Vertebrate Paleontology, 1996. 16(3): p. 396-402.〕 ''Utatsusaurus'' had small fins, with five digits.〔Motani, R., ''New information on the forefin of Utatsusaurus hataii (Ichthyosauria)''. Journal of Paleontology, 1997. 71(3): p. 475-479.〕 In addition, those digits have up to five extra finger bones, which is referred to as hyperphalangy.〔 〔The tail had a long low fin, suggesting that the animal swam by undulation, rather than using its paddles and tail. ''Utatsusaurus'' has transitional features between ancestral terrestrial amniotes and the more derived ichthyosaurs. First, the attachment of the pelvic girdle to the vertebral column was probably not robust enough to support the body on land unlike terrestrial amniotes. The pelvic girdle is attached to the vertebral column by the sacral ribs probably articulating with the ilium, but the ribs are not fused to the sacral vertebrae. Second, the humerus and femur of ''Utatsusaurus'' has the equal length. While all other ichthyosaurs have the longer humerus, terrestrial amniotes have the longer femur. Furthermore, the hindlimb of ''Utatsusaurus'' seems to be larger than the forelimb.〔 They also used phylogenetic analyses and concluded that ichthyosaurs were a member of the Diapsida and the sister group of the Sauria. 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「Utatsusaurus」の詳細全文を読む スポンサード リンク
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