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Fossils indicate earlier arrival of 'the Age of Fish' in southwest China

(Xinhua)    20:28, March 17, 2017

KUNMING, March 17 -- Scientists have described a new primitive fish that evolved some 423 million years ago, before the period known as the "Age of Fish," after studying fossils unearthed in Qujing in southwest China's Yunnan Province.

Recent discoveries found at the Kuanti Formation led to descriptions of the new fish species, which is believed to have been 20 cm long and covered in thick rectangular scales, according to Zhu Min, research professor and overall coordinator of the Silurian-Devonian fish research at the Institute of Vertebrate Paleontology and Paleoanthropology under the Chinese Academy of Sciences.

Brian Choo from Flinders University, Australia, has worked with Zhu since 2009. He explained that the fossil was initially thought to be a new species of Psarolepis, a fish discovered in slightly more recent sediment.

"When we started looking at this beautifully preserved specimen in detail in 2010, we immediately noticed it was a quite different animal," said Choo in an e-mail to Xinhua.

The new fish, Sparalepis tingi, is a close relative of Guiyu and Psarolepis from the same region, but displays unique characteristics, with distinctive bone ornamentation and extremely tall, thick scales, he noted.

Specifically, it has unique interlocking structures on the scales at the front of its body, which help to illustrate that, even at this early stage, bony fish were displaying considerable diversity and anatomical advances, Choo explained.

Zhu and Choo view the Kuanti Formation as unique window into what is still a largely unknown chapter in the history of vertebrate life.

Only tiny fragments of Silurian jawed fish were known up until the end of the 20th century, from sites scattered around the globe. Almost all understanding of the early evolution of jawed vertebrates was based on more complete discoveries from the later Devonian Period (419.2-358.9 million years ago).

Thus, the Devonian Period is often called "The Age of Fish," due to the apparent increase in vertebrate size and variety over the preceding Silurian Period.

In the past few years, however, there have been some major discoveries at Qujing, forcing scientists to re-evaluate what they thought they knew about vertebrate evolution, Choo said.

For instance before Kuanti, little evidence of large jawed vertebrates from before the Devonian were available for scientists to study. Silurian fish were thought to be generally small and occupy low positions on the food chain, with the top marine predators being invertebrates such as sea scorpions.

The discovery of Megamastax, which Choo named in 2014, however, revealed that large predatory fishes, well over a meter in length, prowled the Silurian seas of south China.

The excitement surrounding Kuanti comes from the unparalleled quantity and quality of Silurian fossilized fish available for study.

In other Silurian sites around the world, jawed fish are generally only represented by a handful of small forms. Whereas at Qujing, which back then was situated near an equatorial coast, researchers are learning of a pre-Devonian fauna with dozens of bizarre species, some growing quite large.

"It seems that the "Age of Fish" arrived early in south China, although we still don't have enough information to know exactly why this was the case," said Choo.

"The ongoing discoveries at Qujing have drastically enhanced our understanding. This is the only place on Earth to have yielded articulated remains of jawed fishes from before the Devonian, beginning with Guiyu described in 2009," said Choo.

"Scientists were shocked and exhilarated by these fossils finds, revealing that Silurian fishes were larger, far more diverse and much weirder than expected," he said.

Zhu said that his research team have identified a growing number of bizarre fish living with Sparalepis tingi, including Guiyu oneiros, the first completely preserved Silurian bony fish fossil.

Sparalepis tingi was the second such well-preserved fossil from this group and provided significant new evidence for the understanding of the early diversification of jawed vertebrates and the origin of bony fish, Zhu said.

Another major Kuanti discovery, announced in 2013, was the description of Entelognathus, a fish combining a placoderm-like trunk armor with the jawbones of a bony fish, which demonstrated that the bones in our human face originated among the placoderms, Choo explained.

Before that finding, placoderms (an extinct group of jawed fish with bony armor) were considered an evolutionary dead end. The skulls of modern bony vertebrates, including humans, were thought to have evolved independently.

"With great quantities of Kuanti fossils awaiting description, representing multiple species of previously unknown fish, we can expect more surprises in the future," said Choo.

A detailed research article written by Choo and Zhu was published March 8 in the international journal "PLOS ONE."

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