Pocket three-dimensional model of fish life restoration in the border city (Xie Siyuan, Zhang Heming/production). Photo courtesy of Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
BEIJING, June 21 (Xinhua) -- Including humans, 99.8% of vertebrates on Earth have jaws (maxillaries and chins), collectively known as jaws, and their origin and evolution have always attracted the attention of the paleontological community.
Previously, there was a serious lack of fossil records spanning tens of millions of years throughout the Silurian period, and for a long time, the evolution of jaws in the Silurian period could only be speculated through scattered preserved scales and spines, and the discovery and research of Silurian ancient fish fossils was also quite eye-catching.
Early evolution of the mandible and teeth of jaw vertebrates: light blue is the primitive jaw of the endoskeleton, and light brown is the membranous jaw. Photo courtesy of Zhu You'an and Li Qiang
Provides new empirical evidence for the early evolution of jaws and teeth
The Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Paleovertebrates of the Chinese Academy of Sciences) said on the 21st that the Zhu Min research team of the Institute has insisted on exploring the Silurian strata nationwide for many years, and has carried out in-depth cooperation with geological survey departments around the world, and finally made a major breakthrough: they found a new and completely preserved Silurian fish "pocket border city fish" fossil of about 423 million years ago in Xiushan County, Chongqing, thus providing new empirical evidence for the early evolution of jaws and teeth.
The paper on the major discoveries and research results of this ancient fish fossil by Chinese scientists was recently published online by the internationally renowned academic journal "Contemporary Biology" beijing time. The research was done by the Chongqing Institute of Geology and Mineral Resources, the Chongqing Geological Survey, the Institute of Paleovertebrates of the Chinese Academy of Sciences, and the Qujing Normal University, and the research team was composed of all Chinese scholars and mainly young scholars. Li Qiang, postdoctoral fellow of Chongqing Institute of Geology and Mineral Resources, and Zhu You'an, associate researcher of the Institute of Paleovertebrates of the Chinese Academy of Sciences, are the co-first authors of the paper, and Zhu Min, researcher of the Institute of Paleovertebrates of the Chinese Academy of Sciences, and Zhu You'an are co-corresponding authors.
China is the only country in the world where more complete fossils of The Silurian jaws have been found, providing key empirical data for a comprehensive understanding of the early body structure of jaws, but the previous fossils are almost all from a fossil site of the Xiaoxiang fauna in Qujing, Yunnan. The discovery of the "pocket border city fish" in Xiushan, Chongqing, has also become the second fossil site in the world to find more complete Silurian jaws.
Ecological restoration map of the Rhodes Fauna in Xiushan Mountain, Chongqing (Zheng Qiuyang/Painting). Photo courtesy of Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
Accidental discovery of "pocket border city fish" 423 million years ago
According to the research team, in the late Silurian period 423 million years ago, the paleolurgy of South China drifted in the ocean near the equator, and the sea water invaded the shelf along the twisting and concave coastline, forming several huge bays or inland seas, and these shallow seas became the "shelter" of early aquatic life. At the turn of Xiangyu and Chongqing, the Xiushan area of Chongqing was located in an inland sea larger than today's Bohai Sea on the northern edge of the ancient land in South China, where rivers flowed into the inland sea, bringing a lot of nutrients and giving birth to abundant life, and a large number of primitive plants grew in the semi-salty water of the estuary and tidal flats, during which there were jawless armored fish, sea scorpions and the earliest jawed fish. Their remains were covered with constant sediment brought by rivers and fossilized over a long geological period of time.
At the end of 2019, Li Qiang explored the Shilurian fish-bearing formation along the "Chuanhe Gaitian Road" near The Hong'an Border City of Xiushan Andi, and accidentally found a complete and half-open fossil of jawed fish, which had never been reported intact in the past. He and Zhu You'an and others quickly began to study the fossil in detail. Although the fossil is very well preserved, many of the tiny structures are close to the upper limit of the accuracy of the particle size preservation of the siltstone, especially the bone fragment form buried inside the surrounding rock, which has been reconstructed by multiple high-precision CT scans through the tiny crevices along the bone fragments that are only a few microns wide.
The research team named the ancient fish fossil "Pocket Border City Fish" and found that its exoskeleton bone armor pattern was close to that of the unicorn fish previously found in the Qujing Xiaoxiang fauna, indicating that it belonged to the same full jaw shield skin fish as the unicorn fish, the whole jaw fish, and the Shiliu fish. The full-jawed shield fish 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, that is, teleost fish and cartilaginous fish, so it has attracted great attention from the academic community in recent years.
Rare fish fossil pectoral and ventral fins are intact preserved
According to the research team, the "pocket border city fish" preserves the mandibular and mandibular teeth, and its flanged mandibular margin jaw is very similar to the marginal jaw of the whole jaw, but it has a well-developed intraoral lobe on the inside, on which 5 larger conical teeth can be seen, and the growth and arrangement of these teeth are similar to the tooth process of other shield fish, especially arthropods.
Therefore, the jaws of the "pocket border city fish" may be more primitive than those of the whole jawfish and the unicorn fish, representing a transitional state between the jaws and teeth of modern fish and the traditional definition of shield fish such as arthropods, and providing important fossil evidence for the origin and evolution of modern jaws, including human jaws and teeth.
The pectoral and ventral fins of the "pocket border city fish" are rarely preserved intact, which is very rare in early fish fossils, further proving that most primitive fish have well-developed fleshy fins, while modern spokefin fish, especially crucian carp, perch and other eukeletal fish, have fan-shaped transparent fins that have been very specialized.
The research team believes that the fossil of the "pocket border city fish" is only 2 centimeters long, and the whole fish may be only about 4 centimeters long when it is alive. Although small, it may be a very fierce pocket predator from the jaw and teeth, feeding on other small animals such as the Mi's sea scorpion and The Xiushan shield fish in the living environment.
They also point out that although intact full-jawed shield fish are currently only 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 Indochina and South China plots had a very close paleogeographic connection in the Silurian period. (End)
Source: China News Network