Ancient arm bone shows fish used limbs

Discovery of 365-million-year-old fossil of salamander-like creature bridges major gap in evolution

Friday, April 2, 2004 - Page A3


American scientists have unearthed the world's oldest arm bone, a 365-million-year-old fossil that provides key evidence that fish used limbs in water well before animals used them to climb up on land.

In today's issue of the journal Science, researchers describe an aquatic, salamander-like creature that would have pushed its arms downward to move through shallow rivers, and used them to prop itself up while waiting for prey or to get air.

Its upper arm bone or humerus, which was discovered along a road cut in Pennsylvania, bridges a major gap in our grasp of the progression from fins to limbs, say authors Neil Shubin and Michael Coates of the University of Chicago and Ted Daeschler of Philadelphia's Academy of Natural Sciences.

"It immediately became evident that, wow, this really helps us understand the evolution of the limb," Dr. Daeschler said. "The story of the emergence of animals with limbs from their fish ancestors is the sexiest part of what we do."

Little is known about that topic.

The Devonian period in which this creature lived is thought to have been a hotbed of evolutionary activity among plants, invertebrates and vertebrates.

There were three main continental masses; what is now North America and Europe sat together near the equator with much of the land largely under water.

The first land plants developed, into so many types of vegetation that the phenomenon has been called the "Devonian explosion."

Animals in these shallow, freshwater environments experimented with all sorts of designs of fins, and eventually limbs, to exploit different ecosystems, Dr. Daeschler said.

Some creatures had both gills and lungs to survive above and below water. Others were starting to develop limbs, but there were no vertebrates living on land even in the late Devonian period of 370 to 360 million years ago.

Permanent land-living animals took another 30 million years to develop into reptiles, birds and mammals, but what happened during that transition is unclear.

"We're learning now what the animals, during that 20- or 30-million-year gap, were doing with their limbs," Dr. Daeschler said.

Sometimes that happens by accident.

The fossil in Pennsylvania was found in 1993 along a highway. It was among layers of rock formed by an ancient stream system that deposited sediment as it flowed through what is now known as Red Hill, a region rich in fossils from the period.

The importance of this fossil went unnoticed until 2001, when the red sandstone that encased it was picked away to expose a humerus from an unidentified metre-long tetrapod, or four-limbed creature.

Analysis of the arm bone, which is small enough to fit in the palm of a human hand, suggests a shoulder joint that would not allow a great degree of motion.

However, it would anchor a large muscle across the chest, enabling the tetrapod to produce a pushup or bench-press motion in a low, wide stance.

Fish also possess this kind of structure, but in a much less sophisticated design.

"This function represents the intermediate condition between primitive steering and braking functions in fins and the derived aquatic or terrestrial walking gait," the study noted.

What's also interesting is that this humerus bone looks different from those of other early limbed vertebrates.

It suggests some variety among the first sets of arms and legs, which in turn may be important in explaining strange sequences of early footprints in river beds, the scientists say.

Dr. Daeschler said this creature probably would have had more than five digits on each limb and a flattened newt-like tail, and was likely a competent swimmer in open water.

"These early tetrapods are essentially entrepreneurs of the Devonian," Dr. Daeschler said.

"Evolution took advantage of the niches in these shallow freshwater environments."

The researchers also noted punctures on the surfaces of the fossil, suggesting it could have been bitten, perhaps killed, by another creature. Some aggressive carnivores lived at this time, including a "spiny shark" that grew longer than nine metres.

In an accompanying article, From Fins to Fingers, Jennifer Clack of Cambridge University describes the discovery of what could be a previously unknown tetrapod as another in a series of important finds from this region.

"These are not the conservative, clumsy creatures envisaged by popularists," Prof. Clack wrote. ". . .These animals probably did not walk efficiently, but their modes of locomotion certainly varied, as they adapted skeletons and sensory organs for the challenges posed by emergence from the water."

This research was funded by the National Geographic Society and the U.S. National Science Foundation.

The scientists are to return to the site next month.