An arm bone uncovered in an ancient river channel in Pennsylvania is helping researchers puzzle out the origins of walking in vertebrates. The rugged humerus, more than 350 million years old, belonged to an early four-limbed aquatic animal, paleontologists say, and was clearly designed to push up the animal's head and upper body.
Fossils from the Age of Fishes (more formally known as the Devonian Period), a time between 417 million and 354 million years ago, show that fish became extremely diverse. Their greatest change, of course, was to evolve into four-legged creatures called tetrapods. Paleontologists have long postulated that the ability to walk developed as animals left the water, but fossil evidence increasingly suggests another idea: Aquatic animals were the first walkers, scuttling along riverbeds, partially supported by their own buoyancy.
The new arm bone most likely belonged to an aquatic animal, argue paleontologists Neil Shubin and Michael Coates of the University of Chicago and Edward Daeschler of the Academy of Natural Sciences in Philadelphia. It was found near fossils of aquatic plants and animals. Its sturdy shape and hinge joint resemble the fin bones of lobe-finned fishes, believed to be the ancestors of terrestrial vertebrates. Yet, its pronounced L shape is a uniquely tetrapod feature, and the angle of a pronounced bone ridge where muscles attached means the bone was oriented more like a tetrapod limb than a fin. So the trio has tentatively characterized the bone as belonging to a "transitional" tetrapod.
What's most important is how it could have moved. Because the humerus has a hinge-like shoulder joint, the animal could only have swung it up and down, with little side-to-side maneuverability, the team concludes in the 2 April issue of Science . Thick and massive, the bone was clearly built to lift the whole front of the body, the researchers say, perhaps enabling the animal to more easily crawl along the bottom of shallow rivers or raise its head above the water. The researchers can't be completely certain the bone is from a tetrapod, because they didn't find any toes--a defining characteristic.
However the mystery animal is ultimately classified, the main point is that the humerus helps to “confirm that limbs and walking developed first in aquatic animals, and was not associated only with a move to terrestrial life,” says vertebrate paleontologist Robert Reisz of the University of Toronto at Mississauga.