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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