U.S. scientists have discovered a fossil of what may be the world's oldest known arm bone. It is from a swimming creature called a tetrapod, so named because it had four limbs. The discovery offers more evidence that limbs first evolved for use in the water, not on land as a means to survive.

The artist's conception of the animal to which the fossil bone belongs looks like a fat salamander about two-thirds of a meter long with short muscular arms.

University of Chicago evolutionary biologist Neil Shubin and colleagues dug the fossil from rocks along a highway in the northeastern U.S. state of Pennsylvania. Mr. Shubin says it is from a tetrapod that lived in a freshwater stream about 370 million years ago before animals crawled on land . Its limbs were at a stage of development between fish fins and the arms and legs of an amphibian, with features of both. Mr. Shubin says the fossil's shape indicates that the animal's arms were strong enough to be able to push itself up.

"We're seeing in this animal some of the first stages of where limbs are being used to hold the body up and push it up," he said. "It would have used its arms to do pushups to push its head above the water so that it could breathe. It could use its limb in this way to move around, even to walk on the bottom of the water."

The researchers report in the journal Science that the ability to prop up the body had been thought to have evolved on land. This is the first sign it is far more ancient.

The arm was flat with a hinge at the shoulder to allow the animal to flap it. It would be many millions of years before a ball-and-socket joint would allow arms to rotate.

The first such tetrapod was discovered in 1987 in Greenland by British paleontologist Jennifer Clack of Cambridge University. It was a landmark discovery, the first to show that some fish-like creatures had an early form of limbs rather than fins. Ms. Clack says that by the time the upper limb had developed in this newly discovered tetrapod from Pennsylvania, it was pointing in a different direction than a fish fin.

"It's a fishy feature and it has been twisted around so that the arm, instead of pointing backwards like a fin, points horizontally out to the side. That was one of the first anatomical features to change," she explained.

At the time of the anatomical transition, the Earth was changing drastically. Complex plant systems were forming on land, the first woody plants appeared, and the water's edge was becoming a new kind of environment. Ms. Clack says fish may have evolved into four-legged walking creatures to take advantage of the new food supplies available on land. The transition might also have had to do with the evolution of the atmosphere and its increasing oxygen, supplied in part by the increasing vegetation.

"In this period, there would have been quite a lot more oxygen in the air than there was in the water," she said. "In fact, the water they were living in was probably quite oxygen poor. So they would have wanted to come close to the surface to get the oxygen. That would have led them into shallower water."

The fossil arm that Neil Shubin discovered in Pennsylvania was only the upper bone. The rest was missing, including the fingers it presumably had. They were probably stubby and most likely numbered more than five. He says the different kinds of tetrapod that lived nearly 400 million years ago had many features that have since disappeared, like more than five digits at the end of a limb.

"What it tells us is that the route from life in water to life on land was not a direct one, that nature too many detours," he explained. "What's fascinating about this bone is that it shows us that instead of evolution at this stage proceeding like a ladder of progress from fish to amphibians, the earliest known creatures with limbs were very diverse. At this time period, there is a great menagerie of forms, and many of them look different."

Mr. Shubin adds that the unique form of the new tetrapod fossil arm - flat and wide - might explain the mysterious fossil footprints seen in some river beds.