Triumph of the Titans

竜脚類 1億5000万年の栄華

By Kristina A. Curry Rogers /Michael D. D’Emic K. A. カリー=ロジャーズ /M. D. デミック
English 日本語 日本語
Ever since fossils of the behemoth, long-necked dinosaurs known as sauropods surfaced in England nearly 170 years ago, they have awed and confused scientists. Even when the great English anatomist Sir Richard Owen recognized in 1842 that dinosaurs constituted a group of their own, apart from reptiles, he excluded the gigantic bones later classified as sauropods. Instead he interpreted them as belonging to a type of aquatic crocodile, which he had named Cetiosaurus, or “whale lizard,” for the enormous size of its bones. Nearly 30 years later, in 1871, University of Oxford geologist John Phillips would report the discovery of a Cetiosaurus skeleton sufficiently complete to reveal that, far from being an aquatic crocodile, the animal spent at least some of its time on land.  約170年前,首の長い巨大な恐竜「竜脚類」の化石が英国で発見されて以来,研究者は感銘を受けつつも困惑してきた。英国の著名な解剖学者オーウェン卿(Sir Richard Owen)は1842年に,恐竜が爬虫類とは異なる独自のグループを形成することに気付いたが,後に竜脚類に分類されるその巨大な骨の化石については,恐竜のものではないとみていた。彼はそれを水生ワニの一種のものと考え,骨の巨大さから“クジラトカゲ”を意味するケティオサウルスと命名した。
 それから約30年後の1871年,オックスフォード大学の地質学者フィリップス(John Phillips)がほぼ完全なケティオサウルスの骨格を発見し,この動物は水生ワニなどではなく,多少なりとも陸上で過ごしていたと報告した。
Phillips’s assessment caused considerable consternation among paleontologists for decades—they just could not conceive how such a massive animal could support its weight on land. Because sauropods were perceived as animals without a place, unsuited for land or sea, they came to be seen as unwieldy, overgrown, archaic herbivores fated for rapid extinction or, at least, marginalization by more “advanced” dinosaurs. As recently as 1991, scientists argued that sauropods were far from the apex of dinosaur success and only flourished in the absence of more specialized plant-eating dinosaurs. In this view, these giants of the Jurassic period, between about 200 million and 145 million years ago, gave way to bigger-brained, better-adapted herbivores in the Cretaceous, between some 145 million and 65.5 million years ago, such as the duckbilled hadrosaurs and horned ceratopsians, which outcompeted the sauropods and pushed them to the fringe.  このフィリップスの判断に,古生物学者たちは何十年もの間かなり困惑していた。こんな巨大な動物が陸上でどのように体重を支えていたか,想像もつかなかったからだ。竜脚類は陸にも海にも適応できず居場所のない動物だったと考えられていた。そのため,図体が大きくなりすぎた原始的な植物食動物で,すぐに絶滅したか,少なくとも,より“進化した”恐竜によって駆逐される運命にあったと見なされるようになった。
Relegated to the Southern Hemisphere, so the story goes, only a handful of sauropod stragglers held out to the end of the Cretaceous, when an asteroid impact brought the dinosaur era to a close.  こうして散り散りになった竜脚類のうち,南半球に移り住んだほんの一部が白亜紀末期まで細々と生き残り,小惑星衝突によって他の恐竜ともども絶滅,恐竜の時代が幕を閉じたという筋書きだ。
This view of maladapted sauropods plodding along to obscurity was itself destined for extinction. Recent discoveries of sauropods from locales around the world—more than 60 new species in the past 10 years alone—have revealed an extraordinarily resilient group that flourished for millions of years at the observed limits of terrestrial body size. Thanks to such finds and new analytical tools for evaluating them, scientists have begun to answer key questions about how sauropods reproduced, how they grew, what they ate and how they adapted to dramatic environmental change. These revelations have changed almost everything that we thought we knew about this iconic group.  ところが,環境に適応できなかった竜脚類がひっそりと姿を消していったというこの説の方が,消えゆくことになった。近年,世界中で発見されている竜脚類の化石(ここ10年だけでも60種以上の新種が見つかっている)から,竜脚類は陸上生物としては最大サイズでありながら何千万年もの間繁栄した,並外れた適応力を持つグループだったことが明らかになった。