Brachiosaurus - Brachiosaurus (English spelling)

A herbivorous dinosaur found in the strata of the Late Jurassic Period in North America and East Africa, dating from approximately 155.7 to 145.5 million years ago. It is classified as a member of the Saurischian order, Sauropoda (suborder), Titanosauriformes, Brachiosauridae family. Its name means "arm reptile" and comes from the fact that its front legs are longer than its back legs. It is a gigantic quadrupedal sauropod, the heaviest of all dinosaurs, with a total length of approximately 25 meters, a height of 13 meters, and a weight of 78 tons. It has a giraffe-like body shape and is said to have once preferred swamps and walked deep underwater, allowing it to look around with only its face above the water, and its nostrils were surrounded by the raised ridges of its skull. However, in the early 1950s, a theory was proposed that dinosaurs were adapted to life on land, eating new shoots at the top of tall trees, and since the 1960s, it has been assumed that their legs were strong like those of an elephant, and suited to walking on land. In order to reduce weight while strengthening their limbs, depressions called lateral cavities developed in the spine, and the sides of the vertebrae were hollowed out and made of thin bone plates and columns. The ribs were also hollow. There is also a theory that air sacs like those found in birds were developed to reduce weight and improve respiratory function. On the other hand, another theory has been proposed that the air sac system of dinosaurs emerged due to the harsh environment of abnormally low oxygen concentrations on Earth for 100 million years from 250 million years ago to the Jurassic period, and that this helped improve the survival rate of dinosaurs.

[Ikuo Obata]

Brachiosaurus, written by Heather Amery, illustrated by Tony Gibbons, translated by Kyoichi Tomita (1994, Holp Publishing) ▽ Sato Satoshi, edited and written by the Nature Pro editorial office, Dinosaur Behavior and Life 3: Brachiosaurus - How did dinosaurs get so large? (1995, Kaiseisha) ▽ NHK "Earth's Great Evolution" project, NHK Special: Earth's Great Evolution 4.6 Billion Years: A Journey to Humanity 4, Mass Extinction (2004, Japan Broadcasting Publishing Association) ▽ Hirayama Ren, The Latest in Dinosaur Studies (Heibonsha Shinsho)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

北アメリカや東アフリカのジュラ紀後期、約1億5570万年～1億4550万年前の地層から産出した草食恐竜。分類学上は竜盤目竜脚形類（亜目）竜脚類（下目）ティタノサウルス形類Titanosauriformesブラキオサウルス科Brachiosauridaeに属する。その名は「腕の爬虫(はちゅう)類」という意味で、前肢が後肢よりも長いことに由来する。全長約25メートル、体高13メートル、体重78トンと恐竜のなかでももっとも重い四肢歩行の巨大な竜脚類。キリンのような体形で、かつては沼地を好み水中深くを歩いたという説があり、水面に顔だけ出して周囲を見渡すことができ、鼻孔は頭骨の隆起した稜(りょう)に取り囲まれていたといわれていた。しかし、1950年代初めには陸上生活に適応し、高い樹木の上のほうにある新芽を食べたとする説が主張され、1960年代以降は肢(あし)がゾウのように頑丈で陸上歩行に向いていたと想像されている。四肢がじょうぶになる一方で、体重を減らすために、背骨には側腔(そくこう)とよばれるくぼみが発達し、椎骨(ついこつ)の側面はえぐられたようになり、薄い骨の板と柱でつくられている。肋骨(ろっこつ)も中空であった。鳥類にみられるような気嚢(きのう)が発達し、軽量化と呼吸機能を高めたという説もある。一方、2億5000万年前からジュラ紀にかけての1億年間、地球上の酸素濃度が異常に低いという厳しい環境のゆえに、恐竜の気嚢システムが出現し、恐竜の生存率向上に役だったという説も提唱されている。

［小畠郁生］

『ヘザー・アメリー文、トニー・ギボンズ絵、富田京一訳『ブラキオサウルス』（1994・ほるぷ出版）』▽『佐藤哲、ネイチャー・プロ編集室構成・文『恐竜の行動とくらし3　ブラキオサウルス――恐竜はどうしてここまで巨大になれたのか』（1995・偕成社）』▽『NHK「地球大進化」プロジェクト編『NHKスペシャル　地球大進化　46億年・人類への旅4　大量絶滅』（2004・日本放送出版協会）』▽『平山廉著『最新恐竜学』（平凡社新書）』

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