Chinese scientists have unearthed a skull fossil of a fish-like stem-tetrapod that could extend the earliest record of tetrapods by some 10 million years, according to a research paper published Tuesday in the online journal "Nature Communications."

The stem-tetrapod, discovered in southwest China, will increase scientists' understanding of the fin-to-limb evolutionary transition, the paper said.

Living tetrapods, such as frogs, turtles, birds and mammals, comprise just one subgroup of the tetrapod superclass, which also includes finned and limbed stem-tetrapods.

Some stem-tetrapod species are believed to have moved onto land about 370 million years ago and gradually evolved into the earliest terrestrial vertebrates and eventually humans.

Previously, there was a gap of at least 16 million years from the oldest fossil record of the lungfish lineage to the earliest known stem-tetrapod Kenichthys, a finned stem-tetrapod.

The report said the new discovery pushes the fossil record of tetrapods back by some 10 million years and, as a result, the first appearance of the tetrapod superclass has been drawn far closer to the estimated time of the lungfish-tetrapod split.

The study further fills in the morphological gap between tetrapods and lungfish and unveils the evolutionary pattern of character changes during the initial diversification of stem-tetrapods, the report said.

In addition, an X-ray tomography study of the skull has provided new fossil evidence on the origin of the tetrapod brain.

A simulation of its brain conditions indicated that some important brain modifications related to terrestrial life occurred at the beginning of tetrapod evolution, much earlier than previously thought, the report said.

The newly discovered stem-tetrapod was named "Tungsenia paradoxa," a tribute to renowned Chinese geologist Liu Dongsheng (1917-2008), who won China's top science award in 2004.

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