Early fish-like tetrapod discovered

Chinese scientists have found a new 409-million-year-old fishlike tetrapod (four-legged animals) from Zhaotong, Yunnan in southwestern China, a discovery that extends the history of this group by 10 million years and provides novel fossil evidence on the origin of the tetrapod brain.

In an Oct 23 article in Nature Communications, Zhu Min along with his research team from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, published the findings.

The new species is named Tungsenia paradoxa in memory of Liu Dongsheng, a renowned Chinese geologist died in 2008.

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

Living and extinct tetrapods comprise the first four-limbed vertebrates, and their descendents include amphibians, reptiles, birds and mammals.

But a subgroup of the tetrapod lineage, known as stem-tetrapod from early Devonian (about from 359 to 416 million years ago), includes the half-fish, half-tetrapod animals that walked from water to land.

Previously, the undisputed earliest known stem-tetrapod was Kenichthys, a finned member from the late Emsian (399 million years ago) of Yunnan, China. Its advanced fish-like stem-tetrapods have greatly improved our knowledge of the fin-to-limb transition.

But a scarcity of fossil data from primitive finned tetrapods have prevented researched from obtaining a profound understanding of the acquisition sequence of tetrapod characters, according to the journal.

By comparison, the oldest fossil record of the lungfish lineage, which forms the immediate sister group of tetrapod total-group, can be traced back 415 million yeas ago.

The new discovery by Dr Zhu and his team pushes back the fossil record of tetrapods by some 10 million years. As a result, the first appearance of the stem tetrapod has now been drawn far closer to the estimated time of the lungfish?Ctetrapod split.

More remarkably, the X-Ray tomography study of this fish-like tetrapod depicts the plesiomorphic condition of the tetrapod brain, which is crucial for revealing the acquisition sequence of tetrapod brain features.

The enlargement of the cerebral hemispheres and the possible presence of the pars tuberalis in this tetrapod indicate that some important brain modifications related to terrestrial life had occurred at the beginning of the tetrapod evolution much earlier than previously thought.

Zhu and his team have been researching the origin and evolution of primitive fishes and early vertebrates. His previous key findings include the earliest known coelacanth skull and sinostega pani, the earliest tetrapod from Asia.

Their research is being supported by grants from the Chinese Academy of Sciences, the Major Basic Research Projects of Ministry of Science and Technology of China and the National Nature Science Foundation of China.