Acanthostega

这种生物已经演化出前后肢，但每肢都有8趾。这种生物是从鱼类进化到两栖类的过渡产物。而且，它具有更多的鱼类特征，如有鳃、鳍和只能在水中起作用的感官。伦敦自然历史博物馆的古生物学家珀·阿尔博格说，Acanthostega也许一生很少会离开水。但是化石研究表明、棘螈可以用前肢撑起头部进行呼吸，就像现在的娃娃鱼[大鲵]。

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.