99.8% of the existing vertebrates on the earth have jaws (maxilla and chin), which are collectively called jawed vertebrates or jawed animals. The appearance and rise of jaws is one of the most critical leaps in the evolutionary history of vertebrates from fish to human. When exactly did this jump happen? How did it happen?

Recently, the team of Academician Zhu Min from the Institute of vertebrate paleontology and Paleoanthropology of Chinese Academy of Sciences discovered the "Chongqing Special Buried Fossil Bank" and the "Guizhou Shiqian Fossil Bank" in the strata about 440 million years ago in the Early Silurian of Chongqing and Guizhou, which filled the gap in the global early Silurian fossil record and provided definitive evidence for the rise of jaws and the earliest radiation differentiation for the first time.

Chongqing biota fossils. Beijing News reporter Zhang Lu photo

The team studied fish fossils in detail by using new technologies and methods, and made new progress in exploring important scientific issues such as the earliest differentiation of jaws, the evolution of important organs and body configurations, which refreshed traditional cognition. The British magazine Nature published four academic papers of the team at 23: 00 Beijing time on September 28th, focusing on these research results.

The lack of fossils once made the origin of "great white shark" a mystery.

The appearance of "jaw" led to the "rise" of vertebrates. Without jaws, man-eating sharks and dinosaurs would not be able to hunt. Many important human organs and body structures can be traced back to the beginning of jaw evolution.

The origin and rise of jaw is one of the most critical leaps in the evolutionary history of vertebrates. However, jaw fossils did not appear in large numbers until Devonian (419 million years ago), and molecular biological data proved that the origin of jaw should be earlier than the Late Ordovician (about 450 million years ago). Therefore, there is a huge record gap in the early evolution of jaws, which lasted for at least 30 million years, spanning the late Ordovician and Silurian (about 440 million years ago-420 million years ago).

Alfred Rommel, the giant of vertebrate paleontology, once called it "a big blank in the history of paleontology". Because of this blank, although people know that the jawed class has existed in Silurian, they know nothing about their appearance, size, niche and evolution mode, and even can't determine whether the scattered spines and scales found in Silurian strata belong to jawed fish.

The lack of fossil evidence has shrouded the "origin and rise of the great white shark" in heavy fog, and the great white shark has been a "ghost branch" for the first 30 million years. For a long time, the academic community can only guess the earliest jaw shape by scattered scales and thorns.

Chongqing biota brings "the dawn of fish"

In order to fill this gap, Zhu Min, an academician of China Academy of Sciences, led a team to inspect more than 200 sites in the Silurian strata in China in the past decade, and finally found the "Chongqing Special Buried Fossil Bank" and "Guizhou Shiqian Fossil Bank" in the Early Silurian strata in South China, and found the key to the mystery.

Among them, the "Chongqing Special Buried Fossil Bank" is about 436 million years old. It is the only special buried fossil bank in the world that preserves complete early Silurian jaw fossils, and it can be called "the dawn of fish".

Jaw fish and invertebrate sea scorpion fossils in Chongqing biota. Photo courtesy of vertebrate paleontology Institute, Institute of Paleoanthropology, China Academy of Sciences

"Fossils are preserved under special conditions, and colleagues all over the world did not expect to find such ancient and complete fossils. Ancient fish fossils are not only numerous and full of species, but also well-preserved and exquisite. They present a high degree of diversity and provide unprecedented anatomical information. " Academician Zhu Min said that this is another world-class special buried fossil bank 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 to study? Zhu Youan, an associate researcher at the Institute of vertebrate paleontology and Paleoanthropology, Chinese Academy of Sciences, used his mobile phone to show a picture of a stone pile as high as a hill, which is the working environment of researchers. Fossils are like a small black dot on a stone, which researchers should distinguish with "insight" and experience. "We use geological hammers to knock stones one by one. If we use other machinery or a sledgehammer, one hammer will be smashed. " Once fish fossils are found, researchers should take them back to the laboratory for repair and "pick out" the fish with a fine steel needle under a microscope. "In this process, you can't row or file. The surrounding rock removed by a needle is at most as big as a grain of sand." Professionals repair fossils very carefully, ranging from a week to a year or two.

Digging for photos in the field. Photo courtesy of vertebrate paleontology Institute, Institute of Paleoanthropology, China Academy of Sciences

There is also a story about finding a specially buried fossil bank in Chongqing. Zhu Youan said that researchers should first know where there are early Silurian strata, which depends on geologists' step-by-step understanding of the upper and lower strata of each era for hundreds of years. On 20 18, a new Tianlu Road was opened in Chongqing. Usually, new stones will be dug up on newly opened roads. Li Qiang, a postdoctoral fellow jointly trained by vertebrate paleontology Institute of Chinese Academy of Sciences and qujing normal College, began to search nearby. It was raining that day, and he couldn't find it all day. He just breathed a little sigh of relief and suddenly found a complete Late Silurian jawbone fossil from the falling rocks 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.

Advance the complete fossil record of jaws 1 1 billion years.

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

To this end, the team spent nearly three years, using high-precision CT scanning, fine three-dimensional reconstruction, scanning electron microscope element analysis, all-optical images, multivariate statistical analysis and other means to conduct detailed research and repeated exploration, so that these fossils "confide" a lot of precious anatomical information.

Lu Jing, a researcher at the Institute of vertebrate paleontology and Paleoanthropology, Chinese Academy of Sciences, said that all-optical positioning technology was applied to vertebrate paleontology for the first time in this study, so that finer texture and structural details of specimens could be observed, and virtual light sources could be manipulated to present three-dimensional images of fossils in a repeatable and verifiable way.

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

It was not until the late Silurian (425 million years ago) that there was a relatively complete record of jaw fossils. In this study, the complete fossil record of the jawbone was advanced by 1 1 100 million years, and the origin of some human structures was traced back to fossil fish 436 million years ago.

Among these fossils, the smart Tujia fish of the jawless turtle provides key fossil evidence for the origin of paired appendages of vertebrates; Shen Shi spiny fish with jaws is the earliest preserved cartilaginous fish so far, which proves that sharks evolved from ancestors wearing helmets and armor. Another jaw fish, Miracle Xiushan Fish, combines the characteristics of several shield fish, providing valuable information for exploring the origin of the main groups at the root of the living jaw tree and the evolution of vertebrate skulls.

Miracle Xiushan fish restoration map. Photo courtesy of vertebrate paleontology Institute, Institute of Paleoanthropology, China Academy of Sciences

Fish fossils 439 million years ago pushed the earliest tooth fossil record forward by 654.38+04 million years.

For more than ten years, Zhu Min's team has also carried out field work in the Early Silurian strata in Guizhou. 20 19 They made a breakthrough in Shiqian. "Guizhou Shiqian Fossil Bank" is 439 million years ago and contains a large number of well-preserved jaw microfossils. From nearly 4 tons of fish microfossils collected in the field, the researchers found 23 jaw-shaped tooth specimens.

These teeth are only 2.5 mm long. Through high-precision CT, three-dimensional reconstruction and tissue section, the researchers studied the double-row Guizhou toothfish 439 million years ago in detail, revealing the growth structure and development characteristics of the earliest jaw teeth. The discovery of early Silurian tooth fossils is very rare, which is the earliest and most intuitive evidence of the appearance of jaws so far, pushing forward the earliest tooth fossil record by 6.5438+0.4 million years.

Restoration map of Guizhou toothfish. Photo courtesy of vertebrate paleontology Institute, Institute of Paleoanthropology, China Academy of Sciences

Phylogenetic analysis shows that Guizhou toothfish belongs to the whole group of cartilaginous fish, which supports the view that jaw vertebrates appeared as early as the Ordovician biological radiation period (about 485 million to 450 million years ago).

440 million years ago, great white sharks flourished in southern China.

Academician Zhu Min said that the discovery of "Chongqing Special Buried Fossil Bank" and "Guizhou Shiqian Fossil Bank" showed the characteristics of Silurian fish, especially jaws, on a large scale for the first time in the history of paleontology, and revealed the early rise of jaws. 440 million years ago, the Jaw ethnic group in southern China was very prosperous. In the late Silurian, more diverse and larger jaw species appeared and began to spread to the whole world, thus starting the process of fish landing and eventually evolving into humans.

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

"Chongqing Special Buried Fossil Bank" and "Guizhou Shiqian Fossil Bank" will continue to contribute to solving many mysteries surrounding the origin of jaws in the future.

Beijing News reporter Zhang Lu

Editor Liu Qianxian proofreads Yang.