The bone—a humerus, or upper arm or forelimb—is one of the earliest tetrapod limb bones ever found. (Tetrapods today include amphibians, mammals, reptiles, and birds, among others.)

The ancient bone shares features with primitive fish fins, but also has characteristics of a true limb bone. It bridges the gap between fish and amphibian.

"The transition wasn't all or nothing," said Ted Daeschler, a vertebrate zoologist with the Academy of Natural Sciences in Philadelphia "It's not that some animals were thrown on land. There were certainly other functions intermediate."

Daeschler and colleagues Neil Shubin and Michael Coates, paleontologists at the University of Chicago, say the fossil bone offers a window onto this intermediate stage.

The National Geographic Society Committee for Research and Exploration and the National Science Foundation supported the scientists' research. The trio describe the fossil in tomorrow's issue of the journal Science.

From Stream to Shore

"This new humerus shows some primitive features that are lost in later tetrapods but in this specimen have already begun to change their orientation into tetrapod-like configuration," said Jennifer Clack, a paleontologist at the University Museum of Zoology Cambridge, United Kingdom.

Clack, an expert on the fish-tetrapod transition, also wrote an essay on the discovery for Science.

To understand how tetrapods evolved limbs from fins, Shubin said it helps to imagine the environment that these creatures lived in. "Think of a shallow stream choked with plants, not of an open sea," he said. "At some level, these shallow streams approach a more terrestrial environment in the ways that animals would move around."

According to Daeschler, such an environment would require fish to have limblike fins to propel themselves along the surface of shallow waters, hold their position in a current, or lift their head to the water's surface to gulp air.

Push-Ups

The humerus—which the scientists say clearly belongs to a limbed amphibian and not a lobbed-fin fish—suggests that the tetrapod was an animal that had a powerful forelimb with a large area for the attachment of muscles at the shoulder. Such a muscle is associated with the ability to perform a push-up.

"The [newfound] humerus enables comparisons with fish that were not possible until now," Shubin said. "There is a large crest on the lower side of this humerus—it is where the pectoral muscle would attach. This same expanded crest is seen in fish."

The presence of a place to attach a pectoral muscle in both fish and tetrapods suggests that the ability to perform a push-up is ancient, evolving first in fish and not in terrestrial animals as was originally believed.

"The notion is that this movement is primitive," Shubin said. He believes the movement first arose in fish that required the appendages to move. That motion was akin to how creatures eventually used limbs to walk on land, Shubin said.

According to the researchers, however, fish with limblike fins had no intention of walking on land. Rather, they were adapting to their environment.

"When fish used their fins to prop themselves up on the bottom … it was a useful invention at that time" that helped fish "make a good living" feeding in their aquatic environment, Daeschler said.

Roadside Fossil Trove

The ancient tetrapod humerus is among several plant and animal fossils collected from an ancient streambed in north central Pennsylvania in 1993. Like many fossil sites in the region, the streambed was exposed as road crews cut through the bedrock in the course of constructing a highway.

"What we need are fresh exposures of bedrock. Pennsylvania has wonderful sequences of rock from this time period," Daeschler said. "But most of it is covered by forest or farmland."

So when road crews cut through bedrock, researchers make arrangements to study the fresh exposures. They often bring layers of rock back to their labs for painstaking analysis.

The research team has previously discovered fossil remains of two other tetrapods—Hynerpeton and Densignathus—from this same ancient streambed. But the significance of the new humerus went undetected for several years, since only a small portion of the bone was exposed.

In 2001 Fred Mullison, a staff scientist at the Academy of Natural Sciences in Philadelphia, excavated the bone from the rock. Only then did the fossil's import became apparent.

Daeschler and colleagues say they are unable to discern whether the humerus belongs to Hynerpeton, Densignathus, or an entirely new tetrapod species.

"We can't prove it's either or neither of them. So rather than throw another name on this thing … we're saying it is an early tetrapod humerus," Daeschler said.

The research team hopes further excavations will reveal more tetrapod fossils and provide more insights into a key evolutionary milestone—fins to limbs—of life on Earth.

For more fossil-find news, scroll down for related stories and links.