Spectacular fossil fish reveal a critical period of evolution

Spectacular fossil fish reveal a critical period of evolution

Published September 28, 2022

14 min read

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 include animals with spinal columns. The evolution of the jaw is perhaps the most important step in vertebrate evolution.

From vocalizing to biting food, the jaw is essential to the survival of 99.8 percent of living vertebrates. Only a few vertebrates with spines, like lampreys or hagfish, are able to survive without 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. Now, however, rocks in western China have revealed some amazing fossils that reveal some of the story’s earliest characters. These include jawed fish that have the oldest known skeletons. Across four papers published in the journal Nature, a team of Chinese paleontologists 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. The fossils found in the new studies are remarkable. Remains found in China’s Chongqing municipality include a new inch-long close cousin to sharks, as well as a newfound type of early armored 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 millimeters across.

Taken together, the material “shows us for the first time a chunk of our own evolutionary history,” says Per Ahlberg, a paleontologist 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. These lineages produced the once-dominant jawless vertebrates that lived in ancient oceans. Later branches produced vertebrates with jaws. These eventually diversified into bony fish, which eventually evolved into amphibians and reptiles as well as mammals.

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, paleontologists 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. Researchers could not infer much about the lives and anatomies of these early jawed vertebrates without complete skeletons.

“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 paleontologist at China’s Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), writes in the Chinese edition of National Geographic.

Ancient fossils by the roadway

Many of the new discoveries came after Chinese paleontologists searched along a picturesque roadway recently cut into Chongqing’s steep mountains that leads to the village of Chuanhegai. A team organized in 2019 by paleontologists Min Zhu and Qiang Li examined the exposed rock where 45 hairpin turns were cut through the landscape, hunting for ancient fossils. The team discovered a small fish fossil embedded in a stone that had been thrown from an overhead rock layer at one of the roads. It was the first known fossil of Bianchengichthys, an early armored fish unveiled in 2021. The fossil’s discovery showed that the area’s rocks could contain remarkable remains of ancient fish. So in 2020, the paleontologists began searching in even older layers of rock nearby–and they started to find a remarkably diverse fossil fauna.

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. Modern observers find the bottom-feeding fish to be strange. It has a boomerang-shaped bony skull, three dorsal fins and two long, stubby tail fins that run along its belly. These early fins reveal the structures that predated fish’s pelvic or pectoral fins, which later became 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 armored fish Xiushanosteus mirabilis followed–named after the “miracle” discovery of complete skeletons of its age. Although these fish measured only an inch from head to tail, they still retained a lot of 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. Carole Burrow, a Queensland Museum paleontologist who was not involved in the study, stated in an email that “The combination of features found in the two jawed fish species in Chongqing… blur the distinction among different lineages within these groups.”

Other remains found within the same fossil sites probably represent even more newfound species that the scientists are still working to describe. Burrow says that the two new species are just two of the potential ten or more jawed fish species found in the assemblage. “No doubt, more exciting times lie ahead in the field 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 paleontologists 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 paleontologists 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.

The team also found 23 spiraling structures–each just a few millimeters across–called tooth whorls. These fossils are a good indicator that jawed vertebrates have been found. The spiraling structures formed when new teeth grew alongside older teeth. The whorls are asymmetrical which strongly suggests that each one formed along one of the jaws 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 paleontologist 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.”

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 believes that the Chongqing site is similar to other Ordovician or Silurian fish sites. It is most likely a tidal area located close to shore. How did this small body of water suddenly become the scene for a fish slaughter? The sudden burial of the fish could have been caused by sediments washed in rivers or the ocean, or even earthquake rumblings.

The researchers have already made plans to continue studying China’s fossil treasures. Ahlberg, the Swedish coauthor, said that the team had made X-ray scans at high resolution of several fossils and are currently working to analyze the images. It will take many years to unravel the fossils’ secrets. For the researchers, the quest for understanding where humans and jawed cousins came is just beginning.

“Like archaeologists getting inscriptions from ruins weathered by wind and sand for thousands of years to restore the history of ancient civilizations,” 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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