The fish almost got away.

But Biology Professor Neil Shubin and doctoral student Ted Daeschler, a paleontologist at the Academy of Natural Sciences, decided to take a closer look at the fossil they collected from a highway-widening project and found something dramatic: evidence that fingers and other limbs may have developed in fish before the first amphibians migrated onto dry land.

The fossil contains the fin bones of a fish that lived about 370 million years ago, when north-central Pennsylvania was part of a vast sea. According to Daeschler, "The structure of the fin is so limb-like that we're tempted to call it a fish with fingers." Daeschler and Shubin's report on the fish fin was published in the Jan. 8 issue of Nature.

Neil Shubin, Ted Daeschler and the fossil that challenges traditional ideas of how finger and limb bones developed.

Photo by Candace diCarlo

"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," Shubin said.

The fin contains a fan-shaped array of bones in the positions where fingers would be. Daeschler and Shubin speculate that the fish may have used this hand-like fin to help it move along the muddy sea floor.

"The finger-like bones may have provided areas for muscle attachment so that these fins had more power and mobility than your average fish," Daeschler said.

During the period when the fish was alive, primitive plants and invertebrate animals had migrated from sea to shore, but no backboned animals had yet done so. Fish, on the other hand, were abundant and came in many different forms, some of which would be considered bizarre today.

The fossil was contained in the Catskill Formation rock layer, which runs through much of northern Pennsylvania. The Pennsylvania Department of Transportation often exposes new layers of this formation when it widens roads and allows scientists to examine the exposed rock for fossils. It was at such a project, along U.S. 15 in Lycoming County, that Daeschler and Shubin made their find.

But they didn't think they had anything significant at first. "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 so many times before at the site." It was only after they had taken the rock home and examined it more closely that they realized the importance of their find.