China News Service, Beijing, June 21 (Reporter Sun Zifa) Including humans, 99.8% of vertebrates on Earth have jaws (upper jaw and chin), which are collectively referred to as jaws, and their origin and evolution have been greatly affected. The paleontology community is concerned.

　　Previously, there was a serious lack of fossil records spanning the entire Silurian for tens of millions of years. For a long time, it was only possible to speculate on the evolution of jaws in the Silurian through the scattered scales and spines, and the discovery of Silurian ancient fish fossils. The research has therefore attracted much attention.

The evolution of mandibles and teeth in early jawed vertebrates: light blue is the primitive endoskeletal jaw, and light brown is the membranous jaw.

Photo courtesy of Zhu Youan and Li Qiang

Provide new evidence for the early evolution of jaws and teeth

　　On the 21st, the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Paleospine, Chinese Academy of Sciences) reported on the 21st that the Zhu Min research team of the institute insisted on surveying the Silurian strata nationwide for many years, and carried out in-depth cooperation with the geological survey departments of various regions. A major breakthrough: They discovered a completely new and completely preserved fossil of the jawed fish "pocket border fish" from the Silurian period about 423 million years ago in Xiushan County, Chongqing, providing new evidence for the early evolution of jaws and teeth.

　　The important discovery 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 completed in cooperation with Chongqing Institute of Geology and Mineral Resources, Chongqing Institute of Geological Survey, Institute of Paleospine, Chinese Academy of Sciences, and Qujing Normal University. The research team is all composed of Chinese scholars and mainly young scholars.

Postdoctoral researcher Li Qiang of Chongqing Institute of Geology and Mineral Resources and associate researcher Zhu Youan of Institute of Paleospine of Chinese Academy of Sciences are the co-first authors of the paper. Researcher Zhu Min and Zhu Youan of Institute of Paleospine of Chinese Academy of Sciences are co-corresponding authors.

　　China is currently the only country in the world where more complete Silurian jaws have been discovered, providing key empirical data for a comprehensive understanding of the early body structure of jaws, but almost all of the previous fossils came from a fossil site in the Xiaoxiang fauna in Qujing, Yunnan .

The discovery of the "pocket side town fish" in Xiushan, Chongqing, has also become the second place in the world where more complete Silurian jawed fossils have been discovered.

Ecological restoration map of Roderos fauna in Xiushan, Chongqing (Zheng Qiuyang/painted).

Photo courtesy of Institute of Ancient Spine, Chinese Academy of Sciences

Unexpected discovery of "pocket side town fish" 423 million years ago

　　According to the research team, in the late Silurian period 423 million years ago, the ancient South China land drifted in the ocean near the equator. The seawater invaded the continental shelf along the tortuous and uneven coastline, forming a number of huge bays or inland seas. These shallow seas became early aquatic life. "Sanctuary".

The Xiushan area of ​​Chongqing at the turn of Hunan and Chongqing was located in an inland sea larger than the Bohai Sea on the northern edge of the ancient South China land at that time. The rivers merged into the inland sea here, bringing a lot of nutrients and nurturing rich life. The brackish water of estuaries and tidal flats A large number of primitive plants grow, among which jawless armored fishes, sea scorpions and the earliest jawed fishes live.

Their remains are covered by continuous mud and sand brought by the river and turned into fossils over a long geological time.

　　At the end of 2019, Li Qiang surveyed the Silurian fish-bearing strata along the "Chuanhe Gaitian Road" near Hong'an Border Town in Xiushan, and accidentally discovered a complete half-open jawed fish fossil, which has never been reported in the past. Complete jawed fish.

He and Zhu Youan quickly began a detailed study of this fossil.

Although the fossil is very intact, many tiny structures are close to the upper limit of the precision of siltstone particle size preservation, especially the bone fragments buried in the surrounding rock. After many high-precision CT scans, they passed through the fractures along the bone fragments. Only a few micrometers wide tiny gaps are reconstructed.

　　The research team named the ancient fish fossil "Pocket Biancheng Fish" and found that its exoskeleton bone armor pattern is similar to the unicornfish previously found in the Xiaoxiang fauna of Qujing, showing that it belongs to the same genus as unicornfish, full jaw fish, and silurian fish. Full jaw shield skin fish.

Pangnathus is a kind of early jawed fishes unique to China. It is closely related to the origin of modern jawed vertebrates, namely the common ancestor of bony fishes and cartilaginous fishes, and has attracted great attention in recent years.

Rare fish fossil pectoral and pelvic fins are preserved intact

　　According to the research team, the “pocket edge city fish” preserves the mandible and mandibular teeth. Its flanged mandibular marginal jaw is very similar to the marginal jaw of a full-jaw fish, but there are developed intraoral lobes on the inner side of which 5 can be seen. A large conical tooth, the growth and arrangement of these teeth are similar to those of other scutellum fishes, especially the beetles.

　　Therefore, the jaws of the “pocket side town fish” may be more primitive than the full jaw fish and unicornfish, representing the gap between the jaw bones and teeth of modern fish and the traditional definition of scutellum fish such as arthropods. The newly discovered A transitional state provides important fossil evidence for the origin and evolution of modern jaws, including human jaws and teeth.

　　The pectoral fins and pelvic fins of the "pocket side town fish" are rarely preserved intact, which are very rare in early fish fossils, which further proves that the even fins of most primitive fish have developed fleshy parts, while modern radiated fish, especially crucian carp The fan-shaped transparent fins of bony fishes such as perch have been very specialized.

　　The research team believes that the fossil of the "Pocket Biancheng Fish" is only 2 cm long, and the whole fish may be only about 4 cm long when it is alive.

Although it is small in size, judging from its jaws and teeth, it may be a very fierce pocket predator. It feeds on other small animals such as Miche's sea scorpion and Xiushan shield fish in its living environment.

　　They also pointed out that although intact scutellum fish are currently only found in China, the comparative anatomical evidence provided by the new findings shows that the sporadic scutellum fish bone fragments found in the Silurian formations in Vietnam at the end of the last century are also found. It should belong to the full-mandibular shield skin fish, which indicates that the Indosinian block and the South China block have a very close palaeogeographical connection in the Silurian.

(Finish)