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New Fossil: Link Between Fish and Land Animals?

What creature first crawled out of the prehistoric swamps to conquer the land? The question has long puzzled paleontologists because the transitional species seems to have lived during a mysterious 30-million-year gap in the fossil record called Romer's Gap.

Now a researcher in Britain has found a very rare fossil of a short, squat crocodile-like creature that she believes provides a stepping stone between our aquatic ancestors and the first four-legged land dwellers.

"This really is one of a find," said Jennifer Clack, a paleontologist at the University Museum of Zoology Cambridge, United Kingdom. "It may even be the first five-toed foot," she added, "but I can't swear to that yet."

Shallow Lifestyle

Pederpes probably came from a shallow-water environment—a lagoon or coastal flat—that was vulnerable to sudden increases in salinity as water levels rose and fell. Romer believed such an environment would have favored the evolution of limbs enabling land travel, to allow the animal a broader area to feed. The discovery is published in the July 4 issue of the journal Nature.

Clack feels that the early evolution of hands and feet occurred in relationship to strictly aquatic locomotion, somewhat in the manner of seals.

fossil tetrapod

Pederpes finneyae

Photograph by S. M. Finney, Courtesy of University Museum of Zoology, Cambridge; Specimen GLAHM (Hunterian Museum,Glasgow, UK) 100815

The fossil was originally discovered in 1971 in a stream valley that cuts through moorland about three kilometers from Dumbarton, Scotland, and lay incorrectly classified in the Hunterian Museum, in Glasgow. The skeleton is almost complete, just missing a tail, and is thought to date back to 348 to 344 million years ago—the heart of Romer's Gap. The Gap is named for Alfred Sherwood Romer, an American paleontologist and a prolific writer of textbooks in the 1950s and 1960s, who first recognized the lack of fossils from this 30-million-year period.

New Feet

The hint that Clack's new species, called Pederpes finneyae, might provide a missing link between the swimmers and the landlubbers lies in the bone structure of the hind leg.

During the late Devonian period, about 365 million years ago, tetrapods had paddle-like feet for swimming that pointed back or to the side, said Clack.

But Pederpes feet are different.

"Pederpes looks as if its feet have been reoriented to point forward—perfect for locomotion on land," said Clack. That is, the middle toe on each foot points straight ahead just as is does in modern tetrapods like dogs, mice, and humans.

The late Devonian period has is a rich fossil history of lobed fishes. The earliest four-legged specimens like Acanthostega and Ichthyostega lived about 363 million years ago. Acanthostega had limbs and eight digits on each hand and foot, and also had fish characteristics like gills, fins, and sensory organs that only worked underwater. But these fishy animals probably rarely left the water, said Per Ahlberg, a paleontologist of fossil fish and amphibians at the Natural History Museum in London.

After the Devonian the fossil record disappears, at least for a while—20-30 million years. Only three informative fossils dating back to this time have been found.

After Romer's Gap

When the fossil record resumes roughly 25 million years later, there was already a tremendous variety of tetrapod landforms. Ancestors of modern mammals, amphibians, reptiles, and birds had already evolved and were diverging along distinct branches.

And that left questions.

"We lack a focus from which all modern tetrapods evolved," said Robert Carroll, professor of zoology, curator of vertebrate paleontology at Montreal's McGill University. "Romer's Gap is a 30-million-year black box that, frankly, keeps me up at night."

Clack's latest find may help scientists sleep better.

Pederpes will be particularly useful for the purpose of "reciprocal illumination," said John Bolt, curator of fossil amphibians and reptiles at The Field Museum in Chicago. "Seeing a new complete skeleton adjusts our mindsets to see new features in fossils that have already been examined. Otherwise we often see only what we expect to see."

The Pederpes fossil was originally misclassified as a rhizodont—an extinct type of lobed fish with the equivalent of upper arm and leg bones. In the mid-1990s, Clack's graduate student perusing the collection at the Hunterian Museum noticed the fossil—basically a lump of rock with a few teeth protruding—and brought it back to Cambridge for further study.

"When I saw the rock I became excited. I could see some scales covered the belly which were quite unlike fish scales," Clack said. "I also noticed another protruding bone called the ischium and I became very excited because I knew that this was not a fish but a tetrapod."

Clack sent spore samples from the rock to a laboratory and found that the fossil dated back to approximately 350 million years ago. Working for four years, painstakingly chipping away at the rock under a microscope, Clack uncovered the well-preserved skeleton of Pederpes.

Clack says she has returned to the region where the fossil was found to scour the area for more specimens, but none have been found. Pederpes may have been short-lived, or we just may not be looking in the right places to find more. Who knows what other creatures may crawl out of Romer's Gap?

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