Web edition: January 5, 2012
You may not be familiar with the word tetrapod, but you know one when you see it. All tetrapods are vertebrates — animals with backbones — and most move on land. They also have four limbs — or their ancestors did, as in the case of snakes and whales, for example. Reptiles, birds and amphibians all count as tetrapods, as do mammals. You’re a tetrapod.
By studying fossils, scientists know that tetrapods haven’t always roamed Earth. Now, biologists have found evidence that animals were preparing to walk while still living underwater.