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Tetrapod Fossil Found—First Ever in Asia

By John Roach
for National Geographic News
December 18, 2002
 
Tetrapods, Earth's oldest four-limbed creatures, walked along the
ancient Chinese coastline as early as 355 million years ago, according
to a fossilized jaw bone discovered and analyzed by an international
team of scientists.

The fossil is evidence that the first vertebrates with limbs—not fins—colonized most of the planet and evolved a diverse range of forms within a relatively short period of time, the team reports in the December 19 issue of the journal Nature.

"We are pretty certain that tetrapods originate no earlier than the beginning of the Late Devonian, about 370 million years ago," said Per Ahlberg, a co-author of the study and a paleontologist at the Natural History Museum in London, England.



All but one of the oldest tetrapods discovered until now were found on the Euramerican supercontinent, an ancient land mass that comprised Europe, North America, and Greenland.

The other species came from Australia, which suggests that tetrapods were already widespread in the Late Devonian. Researchers had thought this species might be a fluke—a single, short-lived dispersal of one type of tetrapod. The Chinese fossil erases any doubt about the creature's early colonization over a wide range.

"The new discovery shows they had become very widely distributed before the end of the Devonian, 354 million years ago," said Ahlberg, who studied the fossil along with researchers Min Zhu, Wenjin Zhao, and Lianto Jia of the Chinese Academy of Sciences in Beijing.

Jaw Bone

The Chinese tetrapod, named Sinostega pani, was discovered among fossilized tropical plants and lobe-finned fish in the red sandstone sediments of the Ningxia Hui Autonomous Region of northwest China.

Tetrapods are thought to have evolved from lobe-finned fish, such as lung fish and coelacanths. Researchers have been looking in Devonian deposits for evidence supporting the link between lobe-finned fish and the earliest four-legged creatures.

Zhu had made several visits since the mid-1980s to the Late Devonian sites in China in the hope of finding a tetrapod to make such a link.

In 2001, he and his colleagues Zhao and Jia found a lobe-finned fish that they believe is closely linked to basal (amphibious) tetrapods, but they did not find any four-legged creatures.

Their luck changed in July.

"We accompanied Dr. Per Ahlberg to visit the site where we had found the new lobe-finned fish. It is in this field trip that we have fortunately found a tetrapod lower jaw and an articulated lobe-finned fish," said Zhu.

Jenny Clack, a paleontologist at the University Museum of Zoology in Cambridge, England, who is familiar with the discovery but not a member of the team, said there is no reason to doubt the validity of the find.

Clack worked with Ahlberg studying the lower jaw bones of the tetrapod Acanthostega from Greenland. That analysis, she said, revealed several subtle but key features that distinguish tetrapods from fish.

"With Sinostega we have a jaw that is very similar to Acanthostega in overall appearance, though it is clearly a different animal," she said. "There are now several Devonian tetrapod genera known mainly or entirely from lower jaw material."

Widespread and Diverse

Ahlberg said Sinostega is most interesting because, although geographically closer to the Australian tetrapod Metaxygnathus, it is anatomically much more similar to Acanthostega.

"The easiest way to interpret this pattern is by arguing that tetrapods were both widespread and diverse in the Late Devonian, but that we have much poorer sampling in Australia and China than in Euramerica," he said.

Indeed, the researchers hope to confirm this theory with more tetrapod finds from the Devonian deposits in China. Until recently, scientists had not expressed much interest in these deposits, said Clack.

"My prediction is that that is about to change, and this find, I hope, could be the thin end of a very large wedge," she said.

If so, researchers may have to re-think how tetrapods were able to colonize the world so rapidly, said Alhberg. Previous studies have indicated that early tetrapods, like modern amphibians, were not able to navigate salt water.

"The fact that early tetrapods were able to colonize the globe so quickly suggests that they had a significant tolerance for salt water," he said. "If you don't have that, it is very difficult to move from river system to river system along the coast."

Ahlberg imagines that the early tetrapods, although not open ocean swimmers, dispersed around the world by swimming in the ocean from coastal delta to coastal delta, occasionally crossing narrow seas.
 

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