Swamp creature's fossil raises theories

Arm fossil raises theories

Tetrapod: An ancient arm bone from a salamander-like animal could help answer a longstanding question: Did limbs develop on land or in water?

April 02, 2004|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

Researchers digging along a rural Pennsylvania highway have unearthed what they say is the world's oldest known arm bone, once used by a slithery creature to raise itself up out of a prehistoric swamp.

"We're looking at our very distant ancestor," said Neil Shubin, a professor of organismal biology at the University of Chicago who worked on the discovery.

The bone formed the upper arm of an animal about 3 feet long that looked like a flat-headed salamander and lurked in swamps and shallow waters 365 million years ago.

Lodged in a geological formation exposed by highway construction a decade ago, the bone will help scientists determine what kinds of aquatic creatures first ventured out of the primordial ooze to form the roots of our family tree.

The discovery, reported in today's issue of Science, might also help determine when amphibians first emerged from the swamps and when they formed limbs.

The size and shape of the thumb-sized bone indicate that the animal was a four-limbed creature - a tetrapod - and used its arms to raise the front end of its body, push-up style.

"I guess it's like the Rosetta stone. It helps us understand a little more about the fin in the fish and the limb in the land-based creatures that came after them," said Edward B. Daeschler, an assistant curator at the Academy of Natural Sciences in Philadelphia and a co-author of the report.

Daeschler and Shubin dug up the bone about 10 years ago when they unearthed a slab of red sandstone at a geological site known as Red Hill, a treasure-trove of fossils uncovered in the 1980s when Pennsylvania highway crews widened Route 120 near Hyner, about 90 miles northeast of Altoona. But the bone remained boxed and undetected at the Academy for about eight years before Daeschler began a serious examination.

"Like everybody, everywhere, you have a list of priorities and new things are always coming in, so we didn't get to it right away," Daeschler said. "Believe it or not, eight years isn't so bad."

Experts say such delays aren't unusual among fossil hunters.

"Some of these collections are very large and you may have things that sit on shelves for decades, even a century or so," said Robert Carroll, a paleontologist at McGill University and a curator at the school's museum.

The Pennsylvania creature thrived in the Devonian period, an era about 400 million years ago when swimming animals emerged from the water. The transition to land was gradual, taking 15 million years, researchers say.

Many experts agree that fins on fish evolved into legs as primitive amphibians began to move from one waterway to another during dry seasons. But researchers say this discovery could help resolve a longstanding question among evolutionary biologists: Did limbs develop on land or in the water.

"These debates rage back and forth in the biological community: How did animals develop the ability to move on land?" said Duncan Irschik, an evolutionary biologist at Tulane University.

The discovery of an aquatic creature that could lift itself with arms bolsters the water view. "They provide very solid evidence that these changes occurred in water and not on land," said David Skelly, a Yale University expert on amphibians who reviewed the findings.

Support for the aquatic limb theory developed in 1990, when English researchers showed that a skeleton found in Greenland belonged to one of the earliest known tetrapods.

"It was an aquatic beast with paddle-like limbs," said Jennifer Clack, a paleontologist at Cambridge University who made the discovery. Its limbs could not support all its weight, but it did use them to navigate swamps.

Clack believes the skeleton she found is older than the bone from Pennsylvania. But she acknowledged that dating such finds is difficult, and she credited the American team for shedding light into how amphibians evolved.

"It gives us a clue into what sort of sequence these creatures went through," she said.

Other experts cautioned that much is still unknown about the Pennsylvania creature and the role it played in the emergence of terrestrial life. "On the basis of one bone, you can't even say this was a tetrapod," said McGill's Carroll, who also reviewed the findings.

Speculation based on fossil finds has led to mistakes. For example, researchers once used fossil evidence to conclude that Tyrannosaurus rex was a swift runner - a theory that has since been discounted. "One has to be cautious when you're talking about limb functions. It's all speculation," Irschik said. "We don't know what these animals actually did."

But the study's authors say the bone's shape and size say a lot about the creature.

The bone shows a heavy muscle pattern where it meets the shoulder, evidence that the arm could support considerable weight, researchers say.

They note that the way the bone apparently fit with adjoining bones shows a downward thrust to the limb, evidence the creature had the same broad stance as a crocodile.

"Though this is a single bone, it's loaded with information," said Michael I. Coates, a University of Chicago researcher and another co-author.

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