"Border City Fish" deduces the evolution of teeth and jaws "Wordless Heavenly Book"

Restoration of the "Pocket Border City Fish". Courtesy of the research group

■ Reporter Han Yangmei

Ancient creatures can always provide "clues" for human beings to know themselves and understand themselves. For example, the formation of human teeth may be related to a fish hundreds of millions of years ago.

Recently, Zhu Min's team, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, discovered a completely preserved new Silurian jawed fish "Pocket Border City Fish" in Xiushan County, Chongqing, which provides new empirical evidence for the early evolution of jaws and teeth. The relevant research results were recently published in Contemporary Biology. Xiushan County has also become the second fossil site in the world to find more complete Silurian jaws.

Evolutionary "mysteries" spanning tens of millions of years

99.8% of vertebrates, including humans, have jaws, namely the upper jaw and chin, which are also collectively known as jaws.

"The appearance of jaws is one of the most important leaps in the evolutionary history of life, and the paleontological community is very concerned about the origin and early evolution of jawed fish." Zhu You'an, one of the corresponding authors of the paper and an associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, told China Science Daily.

The origins of jaws can be traced back to the Ordovicians, 444 million years ago. However, it was not until more than 20 million years later that the Devonian Period began to appear in large numbers of jaw fossils, and the Fossils of The Silurian jawed fish in the meantime were only scattered scales and spines, which made the jaws become a "wordless book" in this tens of millions of years of evolutionary history.

Zhu You'an pointed out that for a long time, it has not been clear what the earliest jawed forms are, nor how streamlined flexible fish such as teleost fish and cartilaginous fish evolved from the jawless armor fish that "wears armor" and the shield skin fish that has evolved from the jawless but the front half of the body is still covered with exoskeleton "armor".

Since 2007, Zhu Min led a team to discover the Silurian Xiaoxiang vertebrate fauna in Qujing, Yunnan, which includes a large number of well-preserved jawed fish. This makes China the only country where more complete fossils of The Semitic jaws have been found, providing key empirical data for a comprehensive understanding of the early body structure of the jaws. The team has identified the "initial whole jawfish" and "long-snouted unicorn" that occupy a key position in evolution.

"However, since only one place in the world has been found to have fully preserved Silurian jawfish, there is a view that the morphology of these fish is only the result of a very specialized nature in a limited area, so that the information of these fossils is less important." Zhu You'an said.

In order to find more Silurian jawed fish fossils and explore the truth of this problem, over the years, the research team has traveled almost all over the country in the Silurian strata, cooperated with local geological survey departments, and finally found it in Xiushan County.

Finding a Second "Home"

The late Silurian period, 423 million years ago, is also known as Rodrose. At that time, the ancient land of South China was drifting in the ocean near the equator, and the sea water invaded the shelf along the twisting and concave coastline, forming a number of huge bays or inland seas, and these shallow seas became the "shelters" of early aquatic life.

At that time, the Xiushan area, which was at the turn of Xiang and Chongqing, was located in an inland sea larger than the present-day Bohai Sea on the northern edge of the ancient land in South China. The rivers flow into the inland sea here, bringing with them a wealth of nutrients and giving birth to abundant life, with large numbers of primitive plants growing in the brackish waters of estuaries and tidal flats, inhabited by jawless armored fish, sea scorpions and the earliest jawed fish. Their remains are covered with sediment constantly brought by rivers and have been fossilized over a long geological period of time.

At the end of 2019, Team member Li Qiang, a postdoctoral fellow at the Chongqing Institute of Geology and Mineral Resources, accidentally found a complete and half-open jawed fish fossil in the Rhodrotong Creek Group near the Hong'an Border City of Xiushan Along the "Chuanhe Gaitian Road".

This is exactly the prototype of the pocket border city fish!

This formation has never been reported intact in the past. The team members were pleasantly surprised, and Li Qiang and Zhu You'an and others quickly began a detailed study of the fossil.

"Although the preservation is very complete, it is not easy to study this fossil, and many tiny structures have approached the upper limit of the accuracy of the grain size preservation of siltstone, especially the bone fragment form buried inside the surrounding rock, which has been reconstructed by multiple high-precision CT scans through tiny crevices that are only a few microns wide along the bone fragments." Zhu You'an said.

The exoskeleton bone carapace pattern of the Border City fish is close to that of the unicorn fish previously found in the Qujing Xiaoxiang fauna, indicating that it belongs to the same genus as the unicorn fish, the whole jaw fish, and the Shiliu fish. This proves that this taxon was widely distributed in southern China at that time, including the Indochina, so some of their key anatomical information was very important in clarifying the early evolution of jaws, especially the origin of bony fish and cartilaginous fish.

Important pocket "hunters"

The full-jawed shieldfish is a class of early jawed fish endemic to China, which is closely related to the origin of the common ancestor of modern jawed vertebrates, namely teleost fish and cartilaginous fish.

Through phylogenetic analysis, the research team found that the border city fish preserved the mandibular and mandibular teeth, and its mandible was very similar to the marginal jaw of the whole jaw, but it had a well-developed intraoral lobe on the inside, on which 5 larger conical teeth were visible. The growth and arrangement of these teeth is similar to the tooth process of other shield-skinned fish, especially arthropods.

As a result, the jaws of border city fish may be more primitive than those of whole jawfish and unicorns. This means that it is a transitional state of the jaws and teeth of shield fish traditionally defined by modern fish and arthropods, providing important fossil evidence for the origin and evolution of modern jaws, including our human jaws and teeth.

To the delight of the research team, the fossils of the border city fish are quite well preserved, for example, in the shield fish, insular fins are rarely found, and the border city fish has preserved this part of the structure. Its fins have a fleshy "stalk"-like part covered with well-developed scales, while the fin part has only a narrow outer circle. This is very different from the transparent, folded fan-shaped fins of common fish.

"Interestingly, the border city fish is 'very small', the fossil is only 2 centimeters long, and the whole fish may be only about 4 centimeters long when it is alive, but the teeth are large. Border city fish may be very fierce pocket 'hunters', feeding on other small animals in the living environment, such as the Mi's sea scorpion, Xiushan shield fish, tooth-shaped animals, etc. Zhu You'an said.

At present, although intact full-jawed shieldfish have only been found in China, the new findings provide comparative anatomical evidence that sporadic shield fish bone fragments found in the Silurian strata of Vietnam at the end of the last century should also belong to full-jawed shield fish, indicating that the Indochinese and South China plots have a very close paleogeographic connection in the Silurian period.

Zhu You'an said that the pocket border city fish is only the first jawed fish reported by the border city fauna. They have also found other fossils with and without jaws in the same layer, and important fossils have also been found in other sites of the Chongqing Silurian strata, which are currently being excavated and studied.

Source: China Science Daily