Dimetrodon - Dimetrodon (English spelling)

It is one of the early mammal-like reptiles that lived in the early to middle Permian period of the Paleozoic Era, about 299 to 260.4 million years ago, and is a genus in the order Pelycosauria. It was distributed in North America, was carnivorous, and had a total length of about 3 meters. The spines at the top of the spine became extremely long, and were the longest in the center of the back, giving the animal a strange appearance like a sail hanging vertically on its back. This sail was supplied with a large amount of blood, and it is said that it was responsible for regulating body temperature like a radiator, and that it helped to increase the surface area of ​​the body to warm and cool the body. The teeth at the front of the skull (about 45 cm) which was relatively large compared to the body length were large and dagger-shaped. The narrow width and tall height of the skull indicate that strong jaw muscles were developed to open and close the mouth widely. It appears that the animal lived near water, as its bones were found among fish bones in sediment that had accumulated in a shallow river.

[Ikuo Obata]

"The Vanished Dragon, by J.C. McLoughlin, translated by Ikuo Obata and Hiromichi Hirano (1982, Iwanami Shoten)" ▽ "Mammalian-like Reptiles: The Unknown Ancestors of Humans, by Ryuichi Kaneko (1998, Asahi Sensho)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

古生代ペルム紀（二畳紀）前期ないし中期の、約2億9900万年～2億6040万年前にいた初期の哺乳類型爬虫類(ほにゅうるいがたはちゅうるい)の一つで、盤竜目のなかの1属。北アメリカ大陸に分布し、肉食性で、全長約3メートル。背骨の上方の棘(きょく)突起が極端に長くなり、背中の中央でもっとも長く、この動物の背に縦に帆をかけたような異様な姿をしていた。この帆は大量の血液が供給されるようになっていて、ラジエーターのように体温の調節をつかさどり、体の表面積を大きくして体を温めたり冷やしたりするのに役だてたという説がある。体長に比べて比較的大きな頭骨（約45センチメートル）の前部に生えていた歯は大きく短剣状であった。頭骨の幅が狭く丈が高いことは、口を大きく開けたり閉じたりするための強い顎筋(がくきん)が発達していたことを示す。水辺にいたらしく、骨は浅い川にたまった堆積(たいせき)物から、魚の骨に混じって発見された。

［小畠郁生］

『J・C・マクローリン著、小畠郁生・平野弘道訳『消えた竜』（1982・岩波書店）』▽『金子隆一著『哺乳類型爬虫類――ヒトの知られざる祖先』（1998・朝日選書）』

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