Fossils of 439 million years ago were revealed aga

99.8% of vertebrates on the earth have jaws (upper jaw and chin), which are collectively referred to as jawed vertebrates or jawed animals. The emergence and rise of jaw is one of the most critical leaps in the evolution history of vertebrates from fish to human. When did this jump happen? How did it happen?

Recently, Academician Zhu Min of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences found "Chongqing Special Buried Fossil Pool" and "Guizhou Shiqian Fossil Pool" in Chongqing, Guizhou and other geological strata about 440 million years ago in the early Silurian, filling the gap in the global record of early Silurian jaw fossils, and providing accurate evidence for the rise and earliest radiation differentiation of jaw for the first time.

Fossils in Chongqing biota. The Beijing News reporter Zhang Lu Photograph

team used new technologies and methods to carry out detailed research on fish fossils, and made new progress in the exploration of important scientific issues such as the earliest differentiation of jawed species, important organs and body configuration evolution, which refreshed traditional cognition. The British journal Nature published four academic papers of the team at 23:00 on September 28, Beijing time, focusing on these research results.

The absence of fossils once made the origin of "jawed" a mystery

the emergence of "jaw". What emerged at the historic moment was the "rise" of vertebrates. Without jaw, man eating sharks and dinosaurs would not be able to prey. Many important organs and body configurations of human beings can be traced back to the beginning of the evolution of jawed species.

The origin and rise of the jawed is one of the most critical leaps in the evolutionary history of vertebrates. However, jawed fossils did not appear in large numbers until the beginning of the Devonian (419 million years ago), and molecular biology data prove that the origin of the jawed should be earlier than the late Ordovician (about 450 million years ago). Therefore, there is a huge record gap in the early evolution of the maxilla, which lasts at least 30 million years and spans the late Ordovician and Silurian periods (about 440 million years ago - 420 million years ago).

Alfred· Romer once called it "a significant and stubborn blank in the history of paleontology". Due to the existence of this blank, although it is known that jawed fishes already existed in the Silurian Period, people do not know what they look like, how big they are, what niche they are in, and how they evolve, or even whether the sporadic spines and scales found in the Silurian strata belong to jawed fishes.

The lack of fossil evidence has shrouded the "origin and rise of the jaw" in a heavy fog. The jaw has been a "ghost branch" for 30 million years before its appearance. For a long time, the academic community can only speculate the body configuration of the earliest jaw like "blind man feeling elephant" through scattered preserved scales and thorns.

Chongqing biota brings "the dawn of fish"

in order to fill this gap, Zhu Min, an academician of the CAS Member, led his team to visit more than 200 places of Silurian strata in China in the past decade, and finally found "Chongqing special buried fossil bank" and "Guizhou Shiqian fossil bank" in the Early Silurian strata of South China, and found the key to solving the puzzle.

Among them, the age of "Chongqing Special Buried Fossil Pool" is about 436 million years ago, which is the only special buried fossil pool in the world to preserve early Silurian complete jaw fossils, and can be called "the dawn of fish".

Jawed fish and invertebrate sea scorpion fossils in Chongqing biota. The figure

provided by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences

"Fossils have been preserved under special conditions, and the world's peers did not expect to find such ancient and complete fossils. Fossils of ancient fish are not only numerous and complete in number, but also well preserved and exquisite. They present a high diversity and provide unprecedented anatomical information." Academician Zhu Min said that this is another world-class specially buried fossil pool discovered in China after Chengjiang Biota and Jehol Biota, which provides a lot of key evidence for exploring important nodes in the evolution of the tree of life.

How were these fossils discovered and brought back for research? Zhu You'an, an associate researcher of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, displayed a picture on his mobile phone. Inside was a stone pile as high as a hill, which was the working environment for researchers. Fossils are like a small black stain on stones, and researchers should distinguish them with "insight" and experience. "We use a geological hammer to break the stones one by one. If we use other machines or big hammers, they will be broken in one blow." Once fish fossils are found, researchers should take them back to the laboratory for repair and "pick out" the fish bit by bit with a thin steel needle under a microscope. "You can't scratch or file in this process. The surrounding rock cut by one needle is at most as big as a grain of sand." Professionals repair fossils very carefully, ranging from one week to one or two years.

Field excavation photos. The picture

provided by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences also tells a story. Zhu You'an said that researchers must first know where the early Silurian strata are located, which depends on the fact that over the past hundred years, geologists have fully understood the upper and lower strata of each era step by step. In 2018, Chongqing opened a new sky road. Usually, fresh stones will be dug up on the new road. Li Qiang, a postdoctoral fellow jointly trained by the Institute of Ancient Spine of the Chinese Academy of Sciences and Qujing Normal University, began to look around. When it was raining, he did not find it for a whole day. He was a little frustrated, and suddenly found a complete piece of late Silurian jawed fossils from the rockfall of the landslide. In the following three years, researchers kept searching and making new discoveries, and finally found a large number of precious fossils in the early Silurian.

Pushing forward the fossil record of complete jaw species by 11 million years

Although the fish fossils of Chongqing biota are well preserved, the individuals of the fish are usually very small, 3 or 4 cm long. Their bones are very thin and there is no density difference between them and the rocks, because it brings great challenges to the research, and even the basic photographic work is very difficult.

To this end, the team has spent nearly three years carrying out careful research and repeated exploration by using high-precision CT scanning, fine three-dimensional reconstruction, scanning electron microscope element analysis, full optical image, multivariate statistical analysis and other means, making these fossils "reveal" a lot of precious anatomical information.

Lu Jing, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, said that this research is the first time to apply the full light technology to the study of vertebrates paleontology. More fine textures and structural details of specimens can be observed, and the virtual light source can be manipulated to change, and the three-dimensional images of fossils can be presented repeatedly and verifiably.

"In the past, this technology was usually used in forensic identification and archaeological research to identify fingerprints, handwriting, inscriptions and other fine traces. Researchers also used it to study insect veins." She said that the fossils were very small and compressed very flat, so it was necessary to adjust the light source to constantly observe the fine structure. The machine is like a hood, with a circle of round lights inside, which will light up one by one during the shooting process. Researchers will get a virtual ambient light, and better observe the detailed structure by changing the direction of the light. She mentioned that in previous papers, researchers usually only provided one photo under one lighting condition, and the images presented may have problems of repeatability and verifiability. This time, through all optical technology, we can share digital images with subtle 3D surface information for other researchers, and let others manipulate and change the direction and intensity of the light source, so as to check whether the morphological information and interpretation are accurate.

Only in the late Silurian period (425 million years ago) did a relatively complete fossil record appear for the jaw. This study pushed forward the complete fossil record of jawed animals by 11 million years, and traced the origin of several human body structures back to the fossil fish 436 million years ago.

Among these fossils, the jawless armored fish, the smart Tujia fish, provides key fossil evidence for the origin of paired appendages of vertebrates; The dentate wormlike Shen's ratchet is the earliest well preserved cartilaginous fish known so far, which confirms that sharks evolved from the ancestors of "wearing armor". The other jawed fish, Miracle Xiushan Fish, combines the characteristics of several large groups of scutellaria, providing precious data for exploring the origin of main groups at the root of jawed life tree and the evolution of vertebrate skulls.

Miracle Xiushan Fish Restoration Map. Figure

439 million years ago fish fossils provided by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences

pushed forward the earliest fossil record of teeth 14 million years

Over the past ten years, Zhu Min's team has also carried out field work in the early Silurian strata of Guizhou. In 2019, they made a breakthrough in Shiqian. The Shiqian Fossil Pool in Guizhou is 439 million years old and contains a large number of well preserved jaw microfossils. The researchers found 23 jaw teeth samples from nearly 4 tons of fish microfossils collected in the field.

These teeth are only 2.5 mm long. Through high-precision CT, 3D reconstruction, histological sectioning and other technical means, researchers have carried out a detailed study on the Qianodon bimaculatus 439 million years ago, revealing the growth structure and development characteristics of the earliest jaw teeth. It is very rare to find tooth fossils in the early Silurian, which is the earliest and most intuitive evidence for the emergence of jaw so far, pushing forward the earliest fossil record of teeth by 14 million years.

Restoration of Qianodon. The

phylogenetic analysis of the figure provided by the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences shows that Qianodon belongs to the whole group of cartilaginous fishes, which supports the view that jaw vertebrates have appeared as early as the Ordovician biological radiation period (about 485 million to 450 million years ago).

440 million years ago, the jaw has been thriving in South China

Academician Zhu Min said that the discovery of "Chongqing Special Buried Fossil Pool" and "Guizhou Shiqian Fossil Pool" showed the appearance of Silurian fishes, especially the jaw, for the first time in the history of paleontology, and revealed the process of the rise of early jaw. 440 million years ago, all major groups of the maxilla had flourished in South China; By the late Silurian, more diverse and larger jawed species emerged and began to spread to the world, opening the process of fish landing and eventually evolving into humans.

The fossil research of "Chongqing Special Buried Fossil Pool" and "Guizhou Shiqian Fossil Pool" traces many anatomical structures related to human beings back to ancient fish 440 million years ago, filling the initial link missing in the evolutionary history of "from fish to man", updating the traditional understanding of the origin and rise of jaw, and further consolidating the evolutionary path of "from fish to man".

"Chongqing Special Buried Fossil Pool" and "Guizhou Shiqian Fossil Pool" will continue to make contributions to solving many mysteries surrounding the origin of the jaw.

New Beijing News reporter Zhang Lu

editor Liu Xixian proofread Yang Xuli

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