‘Exquisite’ Chinese fish fossils fill gaps in understanding the evolution of jaws in vertebrates
- Chinese team led by Zhu Min spent a decade hunting across China before discovering a crucial fossil depository in southern China’s Chongqing in late 2020
- Vertebrate palaeontologist John Long said the team’s discoveries rewrite ‘almost everything we know about the early history of jawed animal evolution’
New fish fossils from southwestern China have provided long sought-after evidence about the origin of jaws, one of the greatest innovations in the history of vertebrates.
For hundreds of millions of years, our planet’s most successful predators have used the same equipment – a set of powerful jaws – to capture and kill prey.
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Today, about 99.8 per cent of vertebrates, humans included, have jaws. A crocodile’s bite can be twice as powerful as a nail gun, enabling it to feed on large animals such as water buffalo.
However, how this innovation came about was unclear. While molecular studies led scientists to believe that jawed vertebrates first appeared 450 million years ago, an abundance of fossil records dates only as far back as 420 million years ago, leaving a critical 30-million-year gap in direct evidence showing what early jawed animals were like.
After a decade hunting at numerous sites across China, Zhu Min and his team from the Institute of Vertebrate Palaeontology and Paleoanthropology in Beijing discovered a fossil depository in Xiushan county in Chongqing in late 2020. It was dated back to the early Silurian period (439-436 million years ago).
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“When we uncovered the complete jawed fishes in the laboratory from the rock matrix we brought back from Chongqing, our jaws were on the floor,” Zhu said. “We had never dreamed of finding such diversified, complete and exquisite jawed vertebrates from the early Silurian.”
The discovery was almost miraculous, he said, and detailed anatomy of the fish led to quite a few “now I know” moments of his research career.
“We are closer to the origin of jaws than ever before.”
In their first paper, Zhu’s team reported on the dominant species (with more than 20 specimens) among the exceptionally well-preserved fish fossils.
The 3cm-long Xiushanosteus mirabilis, with a flat head and trunk armour, belonged to an extinct group of armoured prehistoric fish and seemed to be a good swimmer thanks to its paired fins and powerful tail.
The second paper described Tujiaaspis vividus, a jawless fish dating back to some 436 million years ago.
The ancient species surprised researchers with its paired fins extending all the way from the back of its head to the tip of the tail, offering rare evidence of the evolution hypothesis of fins – which would later develop into chest and belly fins, and then arms and legs.
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In the third paper, scientists pieced together thousands of tiny skeleton fragments to reconstruct Fanjingshania renovata, a sharklike fish they identified as humans’ oldest jawed ancestor.
The fish was found in sediment about 439 million years old at a remote site in Shiqian county in Guizhou province in southern China.
The fourth paper revealed 439-million-year-old teeth which belonged to a previously unknown shark relative named Qianodus duplicis.
The teeth, about 1.5mm to 2.5mm long, were arranged in a spiral-like structure with multiple generations of teeth added throughout the fish’s lifetime.
The discovery pushed back the minimum age for the origin of vertebrate jaws and dentitions about 14 million years.
“This is really an awesome, game-changing set of fossil discoveries. It rewrites almost everything we know about the early history of jawed animal evolution,” wrote John Long from Flinders University in Adelaide, Australia, who is former president of the Society of Vertebrate Palaeontology, in a congratulatory message to the team.
Researchers of the four papers used a range of techniques, including high-resolution CT scanning and computational fluid dynamics modelling, to date and analyse the samples. Some of the specimens were studied at the National Synchrotron Radiation Research Centre in Hsinchu, Taiwan.
The international team will continue to study specimens from the Chongqing and Guizhou sites, hoping they will yield other major findings and “provide key evidence for how the extraordinary diversity of jawed vertebrates we see today arose”, Zhu said.
