In southern China, paleontologists have found two rich fossil sites that contain large numbers of well-preserved fish fossils. The finds date back to the early Silurian, around 439 to 436 million years ago, and include the oldest known specimens of pine mouths (Gnathostomata) – the group of vertebrates to which we humans also belong. The discovery provides new insights into the early evolution of vertebrates and opens a rich treasure trove for further research.
With a few exceptions like lampreys and hagfish, almost all modern-day vertebrates have a jaw. It allows them to grab, hold, and shred their food—a distinct evolutionary advantage over animals that rely on sucking food. Scientists believe that the jawmouths, also known as gnathostomata, evolved around 450 million years ago. However, fossil evidence from this period has been sparse, making it difficult to reconstruct the early evolutionary history of this group of vertebrates. The earliest fossils of moving-jaw fish identified to date date from about 425 million years ago.
© Heming Zhang
Two new fossil sites
A team led by You-an Zhu from the Chinese Academy of Sciences in Beijing has now uncovered a large number of early Silurian fish fossils, helping to fill this research gap. At Chongqing and Guizhou in southern China, they found two previously unknown fossil sites spanning the period from about 439 to 436 million years ago and containing exceptionally well-preserved fossils of animals living at the time. "Until now, we could only dream of such extraordinary and early fossils," says co-author Per Ahlberg of Uppsala University in Sweden. “They are more than just curiosities, however; they are primarily important data for testing – and either confirming or disproving – our long-held hypotheses about the genesis of our family tree.”
Among numerous other fossils, Zhu and his colleagues discovered two new species, which they studied and described in detail: the armored fish Xiushanosteus mirabilis and the primeval shark relative Shenacanthus vermiformis. About three centimeters long, Xiushanosteus mirabilis is a member of the placoderms, an extinct group of armored prehistoric fish that were the earliest known jawed vertebrates. With its flat, semi-circular head, it is reminiscent of its jawless ancestors, but also shows characteristics of later jawed fish. He thus provides a hitherto unique insight into the early evolution of the skull shape in jawed vertebrates. Apparently it was widespread in the time and region from which the deposit originates: the researchers found 20 specimens.
Paleontologists have only discovered one individual of the second newly described species, Shenacanthus vermiformis. This has a body length of 22 millimeters and, as an early cartilaginous fish, is a relative of the sharks. However, while today's sharks only have tiny scales, Shenacanthus vermiformis wore a kind of shoulder armor made of several large plates that completely surrounded its body - a feature that was previously considered a typical of placoderms and suggests that the first cartilaginous fish were armored. "For a long time it was believed that other jawfish evolved from a shark-like archetype," says Zhu. "Now, with the discovery of Shenacanthus, we can finally be sure that the opposite is the case."
Forerunners of the limbs in the jawless
In separate papers, also published in the journal Nature, other research teams describe other well-preserved fish fossils also from the newly discovered deposit. The analysis of a 436-million-year-old jawless fish from the galeaspid group sheds light on the early development of fins and limbs for the first time. Since only the head was preserved from almost all previously known fossils of this genus, the rest of the body remained a mystery for a long time.
"The new fossils are spectacular because they show the entire body for the first time and reveal that these animals had paired fins that extended continuously from the back of the head to the tip of the tail," says lead author Zhikun Gai from the Chinese Academy of Sciences. The new fossils provide the first evidence for the hypothesis that differentiated pectoral and pelvic fins and later arms and legs evolved from a continuous fin precursor.
Early diversification of shark ancestors
Other research teams analyzed fossil teeth of a previously unknown shark relative called Qianodus duplicis and described a spiny, shark-like fish called Fanjingshania renovata - each with an age of around 439 million years. This pushes back the oldest known examples of vertebrate teeth by 14 million years.
"These remarkable finds bring a once nebulous interval into focus," writes Matt Friedman of the University of Michigan in a commentary accompanying the paper. "They clearly demonstrate an ancient origin of the jawed vertebrates and clarify how some of their distinctive features may have evolved." In his view, it is likely that the new fossils will spark a scientific debate about their distinctive features and the intricacies of their classification. "But their collective message is unmistakable," Friedman said. "The diversification of jawed vertebrates was in full swing as early as the earliest stages of the Silurian."
Sources: You-an Zhu (Chinese Academy of Sciences, Beijing, China) et al., doi: 10.1038/s41586-022-05136-8; Zhikun Gai (Chinese Academy of Sciences, Beijing, China) et al., doi: 10.1038/s41586-022-04897-6; Plamen Andreev (Chinese Academy of Sciences, Beijing, China) et al., doi: 10.1038/s41586-022-05166-2; Plamen Andreev (Chinese Academy of Sciences, Beijing, China) et al., doi: 10.1038/s41586-022-05233-8