PHILADELPHIA --- Scientists from the University of Pennsylvania and the Academy of Natural Sciences of Philadelphia have discovered dramatic new evidence of how arms and legs developed from the fins of ancient fish. The evidence was discovered in a rock found in a pile of boulders lying along a busy highway in north-central Pennsylvania.

The scientists reported their findings in the Jan. 8 issue of Nature.

The rock contains a 370 million-year-old fossil fish fin. Edward B. Daeschler, Academy of Natural Sciences paleontologist and Penn doctoral student in geology, said, "The structure of this fin is so limb-like that we're tempted to call it a fish with fingers."

The fish fin also changes our understanding of how and when limbs developed in vertebrate animals. The fin dates from a period when fish were abundant and came in many forms that would be considered bizarre today. While primitive plants and invertebrate animals had moved from sea to shore, no backboned animals had yet made the transition to land.

"This new fin shows us that fingers and other limb bones could have evolved in fish for use in water, instead of strictly for use on land, which has been the common assumption," said Neil H. Shubin, associate professor of biology at Penn, who discovered the fossil along with Daeschler and who studies the development of limbs from fossil evidence and modern animal embryos.

One of the most amazing and unexpected features of the new fin is the fan-shaped array of rectangular bones in the positions where fingers would be. "These bones were clearly part of a stiff web," Daeschler said. "They were sandwiched on each side by very long, narrow rods which would have formed the outer part of a large paddle.

"The finger-like bones may have provided areas for muscle attachment so that these fins had more power and mobility than your average fish," he said. The fossil was contained in the Catskill Formation rock layer, which runs through much of northern Pennsylvania.

Through special arrangements, the Pennsylvania Department of Transportation allows scientists to investigate layers of rock newly exposed as a result of road work, and it was at just such a project -- a widening of U.S. 15 in Lycoming County -- that Daeschler and Shubin made their find.

Besides PennDOT, the scientists also credit their find to persistence in the often-tedious task of fossil exploration and to sheer luck.

"We almost walked right past the rock with the fin in it," Shubin said. "On the surface, it looked like the fish scales that we had seen many times before at the site."

Daeschler and Shubin's research project is supported by the National Geographic Society and the National Science Foundation.