Paleontologists have long struggled to explain how vertebrates evolved legs. One popular theory is that ancient fish, stranded when ponds dried up, used muscular fins to drag themselves to new bodies of water. The fish that were able to cover the most ground were more likely to survive, causing true legs to evolve. In other words, this theory holds that fish began crossing dry land before they began evolving legs.

Newly discovered fossils of Acanthostega, an early air-breathing fishlike creature, however suggest that legs began evolving in vertebrates that were still completely aquatic. Acanthostega's limbs would have been unable to support its weight on land, looking something like paddles for swimming. And although it had lungs as well as gills, its ribs were too short to prevent the chest cavity's collapse once out of water.

Acanthostega's limbs may have evolved simply to allow the animal to raise its head above oxygen-poor water to breathe. A fossilized upper arm bone from a similar species bolsters this idea. The bone attached to the shoulder with a hingelike joint, as opposed to the ball-and-socket shoulder joint shared by modern land vertebrates. This arrangement would not have permitted walking, but it would have enabled a simple pushup for a gulp of air.



The passage states that Acanthostega


was an ancestor of land-dwelling mammals

had a chest cavity that would have collapsed on dry land

had a ball-and-socket shoulder joint

was the first vertebrate with true legs

used its limbs only as paddles for swimming

考题讲解

此讲解的内容由AI生成,还未经人工审阅,仅供参考。

正确答案是 B。阅读材料中提到,Acanthostega 拥有肺和鳃,但是它的肋骨太短,无法防止它在干燥的陆地上胸腔的崩塌。因此,B 选项是正确的。

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