英語聽力文摘 棘螈

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Acanthostega

這種生物已經(jīng)演化出前后肢，但每肢都有8趾。這種生物是從魚類進化到兩棲類的過渡產(chǎn)物。而且，它具有更多的魚類特征，如有鰓、鰭和只能在水中起作用的感官。倫敦自然歷史博物館的古生物學家珀·阿爾博格說，Acanthostega也許一生很少會離開水。但是化石研究表明、棘螈可以用前肢撐起頭部進行呼吸，就像現(xiàn)在的娃娃魚[大鯢]。

It happened in the Devonian Period, about 365 million years ago, what’s known as the Age of Fishes. There were no land vertebrates, only aquatic vertebrates. Then some fish called lobe-finned fish, creatures with bony limb-like fins, slowly evolved into amphibians, the first vertebrates to roam on land.

One of the best pieces of evidence is a fossil called Acanthostega, discovered in Greenland. What’s so remarkable about it is that it’s a transition fossil. It defies categories. The creature isn’t a fish exactly, nor an amphibian, but somewhere in between.

It has both lungs and internal gills. It has a tail like a fish, but limbs something like later land creatures. Before the discovery of fossils like Acanthostega, scientists hypothesized that the evolution of limbs was a result of the transition from water to land.

However, Acanthostega’s limbs weren’t capable of supporting the creature’s weight. Although it had hands and feet, it appears that its limbs evolved for use in water in swamps, and only later in evolution were adapted for walking on land.