JOHN SIBBICK / SCIENCE PHOTO LIBRARY JOHN SIBBICK / SCIENCE PHOTO LIBRARY
Acanthostega and Ichthyostega tetrapods, illustration. Acanthostega (bottom two animals) was a primitive tetrapod, one of the first vertebrates to evolve limbs and be capable of moving around on land. It first appeared in the Upper Devonian period (around 365 million years ago). Ichthyostega (top) was another early tetrapod, and also among the first terrestrial vertebrates. It existed from 365 to 360 million years ago. It was a descendent of lobe-finned fishes and an ancestor of amphibians. Fossils of both tetrapods are found in Greenland.
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