The 365 million-year-old fossil, discovered in the mountains of Pennsylvania, is unique in another way: it’s far more massive than the arm bones of other tetrapods. What this means is that evolution wasn’t a simple ladder of progress, with a single creature plopping somehow onto the mud and evolving ashore. The “fish-tetrapod transition” was characterized by a panoply of competing adaptations, each struggling to outdo the others. The 7 million-year period from 365 million to 358 million years ago, during which creatures like the Pennsylvania tetrapod lived, supported a wide variety of amphibians with limbs, including more delicate arms and flippers–all with hands and feet and fingers and toes. University of Chicago biologist and study coauthor Neil Shubin believes the arm bone belonged to a creature with six to eight fingers, one that had lungs and needed to put its head above water to breathe. The finding “shows us very clearly that the ability to do push-ups was important,” he says. “Fish did push-ups on the floor of streams.”
Shubin plans to keep looking in the area for fossils of legs, which may or may not be similarly huge. Surprisingly enough, scientists can’t say which of the arm-and-leg variations in this period appears to be the clear winner in the evolutionary competition. Today’s salamander is a youngster in evolutionary terms–not a direct relative of these primitive tetrapods. But the giant-armed head-bobber from Pennsylvania is clearly a friend to our ancestor, the amphibian that led to the mammal that eventually led to human beings.
