for National Geographic News
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.
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