Spectacular fossil fish reveal a critical period of evolution

Before animals crawled out of the sea and spread onto land, the appearance of jaws marked a significant time in the development of nearly all living vertebrates, including humans.

By Michael Greshko
Published 28 Sept 2022, 17:09 BST
A scientist is shown holding one of the fossils found in Chongqing in his hands. Most fish fossils sealed in siltstone still preserve their living posture with the back on top and the abdomen at bottom. Some fish bodies are twisted as if still in a final struggle.
Photograph by LI HAO

Over hundreds of millions of years, chance and survival have sculpted an extraordinary menagerie of vertebrate life that thrives on land, in the air, and in the water. Vertebrates are animals with spinal columns, including humans, and arguably the single biggest step in vertebrate evolutionary history—even more significant than our distant aquatic forebears’ first waddles onto land—is something we may take for granted: the evolution of the jaw.

From vocalising to biting food, the jaw is essential to the survival of 99.8 percent of living vertebrates. Only a precious few animals with spines, such as lampreys and hagfish, have made it to modern times without these hinged mouth structures.

The rich story of how jawed vertebrates spread to all corners of the globe—a saga spanning some 450 million years—has long been missing the first few pages. But now rocks in western China have yielded spectacular fossils that show us some of this story’s earliest characters: jawed fish that are the oldest skeletons of their kind ever found. Across four papers published in the journal Nature, a team of Chinese palaeontologists and international collaborators describes sites that preserve astoundingly complete fossils of these earliest known jawed vertebrates, including bones and teeth from fish estimated to have lived between 439 million and 436 million years ago, tens of millions of years before animals moved onto land.

Many of the record-breaking fossils hail from Xiushan Country in the Chinese municipality of Chongqing. The road up to the village of Chuanhegai cuts through Silurian rock formations like a knife slicing through layers of cake. Side slopes opened up by road construction exposed fresh rocks convenient for fossil hunting.

The new studies’ fossils are remarkably complete. Remains found in China’s Chongqing municipality include a new inch-long close cousin to sharks, as well as a newfound type of early armoured fish. In addition, fossils found farther south in the province of Guizhou include the spines of an ancient shark cousin and the oldest known teeth of a jawed vertebrate—tiny semicircular arcs of pointy teeth, barely a few millimetres across.

Taken together, the material “shows us for the first time a chunk of our own evolutionary history,” says Per Ahlberg, a palaeontologist at Uppsala University in Sweden who co-authored one of the four studies. “We’ve known it’s existed, we’ve known it’s really important, but we haven’t had any direct evidence for it basically at all—and then suddenly, boom, here it comes.” 

Our fishy ancestors

The new specimens help to narrow a nagging gap between genetic evidence and the fossil record. The DNA of living vertebrates strongly suggests that the earliest branches of the vertebrate family tree began to emerge by about 450 million years ago. The earliest of these lineages yielded the once-dominant jawless vertebrates that lived in the ancient oceans. A later branch yielded the vertebrates with jaws, which eventually diversified into the bony fish—a subset of which left the oceans and eventually evolved into amphibians, reptiles, birds, and mammals, including humans—and the cartilaginous fish that include today’s sharks and rays.

But until these new discoveries, complete skeletons of the earliest jawed vertebrates hadn’t been found in rocks more than 425 million years old. In recent years, palaeontologists have found scales and other bits that suggest the presence of jawed fish during the Silurian period, which lasted from 443.8 million to 419.2 million years ago. But without complete skeletons to refer to, researchers could infer little about these early jawed vertebrates’ lives and anatomies.

“Due to the gaps in fossil records, the jawed fish were always a bunch of wandering ‘spirits’ in the first tens of millions of years after their birth,” lead study author You-an Zhu, a palaeontologist at China’s Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP), writes in the Chinese edition of National Geographic.

Ancient fossils by the roadway

Many of the new discoveries came after Chinese palaeontologists searched along a picturesque roadway recently cut into Chongqing’s steep mountains that leads to the village of Chuanhegai. A team organised in 2019 by palaeontologists Min Zhu and Qiang Li examined the exposed rock where 45 hairpin turns were cut through the landscape, hunting for ancient fossils. At one turn where the road cut through the rocks, the team spotted a stone with a small fish fossil on it, part of a boulder that had tumbled from an overhead rock layer. It was the first known fossil of Bianchengichthys, an early armoured fish unveiled in 2021.

The fossil’s discovery proved that the area’s rocks could preserve astounding remains of ancient fish. So in 2020, the palaeontologists began searching in even older layers of rock nearby—and they started to find a remarkably diverse fossil fauna.

The early armored fish Xiushanosteus mirabilis, excavated at Chongqing, was only about 1.2 inches (3 centimeters) long.
This digital render of one of Chongqing's fossil blocks, generated from many 2D images of the stone, showcases just how jam-packed the rock is with fossils. Though these fish are preserved intact, they are usually very small, no more than 1.6 inches (4 centimeters) in length.
Based on the spiraling tooth whorls of Qianodus duplicis, researchers think the teeth most likely sat near the front of the jaw of a shark-like cartilaginous fish.
The roughly inch-long Fanjingshania renovata, a spiny cartilaginous fish found in Guizhou Province, shedded its scales by reabsorbing hard tissues, a trait normally seen in bony fish.

The first creature to emerge was Tujiaaspis vividus, a member of a group of ancient jawless fish known as the galeaspids that’s named for China’s Tujia ethnic population. To modern observers, the bottom-feeding fish looks somewhat alien, with a boomerang-shaped bony head, three dorsal fins, and two long, stubby fins running along either side of its belly. These early fins show the structures that preceded fish’s pelvic and pectoral fins—which later gave rise to our own limbs.

Next was the shark cousin Shenacanthus vermiformis, which was named after the celebrated Chinese author Congwen Shen, whose 1934 novel Border Town was set near the fossil site. The armoured fish Xiushanosteus mirabilis followed—named after the “miracle” discovery of complete skeletons of its age. These fish were each only about an inch long, head to tail, but they nonetheless preserve rich anatomical detail.

While the two fish represent different branches of the jawed vertebrates’ family tree, they still resemble each other in certain ways, which is to be expected among animals so close to the origin of all jawed vertebrates. “The combination of features in the two new jawed fish species from Chongqing … blur the distinction between different lineages within these groups,” Carole Burrow, a palaeontologist at Australia’s Queensland Museum who wasn’t involved with the studies, said in an email. 

This team of scientists and modelers created the life reconstructions of the ancient creatures seen above. Thanks to their work, fish that died hundreds of millions years ago reappear before us as if alive.
Photograph by LI HAO

Other remains found within the same fossil sites probably represent even more newfound species that the scientists are still working to describe. “Given that the new species described are only two of potentially ten or more jawed fish species in the assemblage, no doubt more exciting times are ahead in the field of early vertebrates,” Burrow continues.

Whorls of little teeth

As important as Xiushanosteus and Shencanthus are to understanding the origins of jawed vertebrates, they aren’t the oldest fossils unveiled today. Led by Min Zhu, many of the Chinese palaeontologists who worked in Chongqing extended their work farther south into the village of Leijiatun, in China’s Guizhou Province, to even older rocks dating to about 439 million years ago. The team excavated some 8,800 pounds of rock from Guizhou and transported it to a laboratory in the city of Quijing, where they carefully dissolved the rock with weak acid and then sieved through the half a ton of sand-like material that remained. 

Five palaeontologists spent a year and a half sifting through the material, You-an Zhu writes in the Chinese edition of National Geographic, and they found scales, hard spines that supported fish fins, and other remains of a cartilaginous fish—a fish with a spinal column of cartilage instead of bone, like modern sharks. The remains suggest that the creature shed its scales like bony fish rather than cartilaginous fish, which shows that even 439 million years ago, early jawed vertebrates were already diversifying. Researchers have named the creature Fanjingshania renovata, a nod to its unusual scale-shedding abilities as well as Mount Fanjingshan to the northeast of the fossil site.

Flanked by microscopes, a scientist carefully blows dust and other debris off of one of the fish fossils. Small though these remains may be, the Chinese team studying the fossils have gleaned a great deal of information from them through statistical analyses, high-powered microscopes, precision X-ray scans, and 3D modeling.
Photograph by LI HAO

The team also found 23 spiralling structures—each just a few millimetres across—called tooth whorls. These fossils are a dead giveaway for jawed vertebrates. The spiralling structures formed as new teeth continually grew next to older teeth, and the whorls are asymmetrical, which strongly implies that each one formed along either the left or right jaw of the creature. Based on the whorls’ features, the teeth’s most likely bearer was a new type of cartilaginous fish, which the study names Qianodus duplicis.

These teeth were so rare in the rock, “the chances of hitting [them] are somewhat minimal” without an industrial-size operation, says Ivan Sansom, a palaeontologist at the United Kingdom’s University of Birmingham and a co-author of two of the new studies. “In some ways, it’s luck, but also, you’ve gotta be persistent in looking—and that’s certainly one thing that [Min Zhu’s] group has done.”

At the Beijing campus of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, a huge warehouse preserves many unprepared fossils excavated from around China. Though worn and torn by time, these rocks hold the keys to unlock many mysteries of ancient life.
Photograph by LI HAO

The ancient world of the first jawed fish

For all the detail that the new studies reveal, mysteries still remain—such as how these fossils formed in the first place. Sansom says that the Chongqing site probably resembles other Ordovician and Silurian fish sites: most likely a tidal zone fairly close to shore. So how did this shallow body of water suddenly become the setting for a fish massacre? Sediments washed in by rivers or the sea, or even the rumblings of an earthquake, could have suddenly buried the fish all at once.

The researchers already have plans to further study China’s new fossil treasures. Ahlberg, the Swedish co-author, says that the team has made X-ray scans of several of the fossils at high resolution and are working to analyse the images. It will take years to decipher the fossils’ full secrets, and for the researchers, the quest to understand where we humans and our jawed cousins came from is only just beginning. 

“Like archaeologists getting inscriptions from ruins weathered by wind and sand for thousands of years to restore the history of ancient civilisations,” You-an Zhu writes in the Chinese edition of National Geographic, “we are also working hard to decipher the wordless [scriptures] in fossils billions of years old, to authenticate the true whole picture of the evolution of life.”

This story uses material from a feature originally published by National Geographic magazine’s Chinese edition.


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