A summary of vertebrate diversity helps to understand the significance of Guiyu in the evolutionary history. Of the 51,000 or more living species of vertebrates, 99.9 percent have jaws: these are the gnathostomes. Gnathostomes include the bony Osteichthyes and the cartilaginous Chondrichthyes. Chondrichthyes (sharks, rays and chimaeras) account for only 2 percent of gnathostome species. Osteichthyes account for the other 98 percent.

European scientists began to carry out studies on bony fish in the Silurian period (443 to 416 million years ago) four decades ago, but were only able to find fragmentary fossils such as scales or fin-spines. Those fossils were not adequate to piece together the comprehensive features of the bony fish ancestors.

At the beginning of this century, European scientists intensified their efforts in finding bony fish fossils and Nature reported the discovery of two incomplete fish jaw fossils in 2007.

Still, there are many doubts and questions on the Silurian bony fish which remain unsolved. A complete ancient fish, recovered from the depositions which were 400 to 350 million years old, could help answer those questions, Zhu said.

The discovery of Guiyu further proved that the evolutionary history of osteichthyans (bony fish plus tetrapods) extends back to the Silurian period.

"On the whole, early fossils are thought to be unreliable as minimum-date markers of evolutionary branching events, because they are less complete or lack the full anatomical signature of the group to which they are assigned," said Coates.

"Guiyu might be an exception that proves the rule, for it provides a new and exceptionally reliable earliest fossil marker for a major split in vertebrate evolution," he said.

Coates predicted that Chinese scientists' discovery would provoke a rash of new fieldwork in the Silurian depositions and a fresh look at existing collections of pre-Devonian fossils.

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Editor:Yang Jie