Another case solved! The third brother of Jiangxia fish appeared 4,000 kilometers away, and the "culprit" turned out to be plate drift!

Produced by: Science Popularization China

Author: Shan Xianren Gai Zhikun (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences)

Producer: China Science Expo

Recently, the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences has made new discoveries!

Researcher Gai Zhikun's team discovered the first true armored fish in the Tarim Basin in Xinjiang - Jiangxialepis rongi , the third of the "Three Brothers of Jiangxia Fish".

Ecological restoration map of Rong's Jiangxia fish

(Photo credit: Yang Dinghua)

Before this, Jiang Xiayu already had two "brothers".

In 2021, China University of Geosciences (Wuhan) discovered the first Jiangxia fish fossil in the Silurian strata in Wuhan, and named it Houspiny Jiangxia fish ; in May 2023, researcher Gai Zhikun led a team to discover another new species of Jiangxia fish in the Qingshui Formation of the lower red strata of the Silurian in Wuning, Jiujiang, Jiangxi, and named it Jiujiang Jiangxia fish . The two brothers were discovered less than 200 kilometers apart.

This time, the research team discovered the third species of Jiangxia fish - Rong's Jiangxia fish in the Tarim Basin 4,000 kilometers away!

So what kind of fish is Jiangxia fish, and why do researchers spend so much effort looking for these "three brothers"?

The story of the "three brothers" of Jiangxia fish

The genus name of Jiangxia fish comes from the place where the first species of the genus was discovered - Jiangxia District, Wuhan. It belongs to armored fish, with a triangular head armor on its head with a pointed tip facing forward. It is not big, and the longest is no more than 5 cm. There are two pairs of spinous horns on both sides of the head armor, and the middle part of the rear edge protrudes backwards to form a structure called the mid-dorsal spine.

The front of the Jiangxia fish's head armor has an elongated oval middorsal hole, which is also its nostril, followed by a pair of large eyes. There is also a small round hole in the middle of the eyes, called the pineal hole, which is mainly used for light perception and is known as the "third eye" of vertebrates. Like other armored fish, the mouth of the Jiangxia fish is located on the ventral side of the body, so the mouth is usually not visible on its head armor.

Fossil photos of Rong's Jiangxia fish and restoration of its head armor

(Image source: The upper picture was taken by Gai Zhikun, and the lower picture was drawn by Guifang)

The "three brothers" of Jiangxia fish have a common feature, that is, the front end of the middorsal foramen is not closed, making the snout of the head armor look as if it is split, that is, "split snout".

The mesodorsal foramen is a large hole at the front of the dorsal side of the head armor of armored fishes. It is a major identification feature and classification basis for armored fishes.

The 3D reconstruction of the skull of armored fish shows that the mesodorsal foramen is connected to the oral and nasal cavities and the pharyngeal cavity, indicating that it is also a water inlet or outlet with respiratory function, so it can be regarded as a nostril or nasopituitary foramen. In most armored fish, the front end of the mesodorsal foramen is closed, except in the genera of Schizorrhynchus and Jiangxias in the order Euarmorhynchus, where the front end of the mesodorsal foramen is not closed, so that the inner edge of the mesodorsal foramen is directly connected to the lateral edge of the head armor.

Although a split snout may sound unhealthy, it may be a good thing for armored fish.

The mesodorsal aperture of most armored fishes opens to the dorsal side of the head armor and can only inhale water from above the head armor. However, the "slit snout" adds an opening forward and downward to the mesodorsal aperture of armored fishes. During swimming, the mesodorsal aperture can simultaneously receive water from above, in front of, and below the head armor. Therefore, the "slit snout" may be able to greatly improve the water intake efficiency of armored fishes when breathing.

The newly discovered species, Rong's Jiangxia fish, is significantly different from the post-spined Jiangxia fish and Jiujiang Jiangxia fish in that its head armor edge and the inner edge of the dorsal pore are densely covered with sharp serrated spines . Studies have found that these structures are similar to the pharyngeal tooth whorls of early jawed fish and the forward-pointing tooth-like structures in the pharynx of some flower-scaled fishes. Their basic function is to filter and block external parasites when water flows through.

Pharyngeal teeth or pharyngeal tooth spirals located in the pharynx of early jawless fishes and early jawed fishes

(Image source: References [4,5,6,7])

Therefore, the new discovery of Rong's Jiangxia fish further confirms that the mid-dorsal pore of armored fish is functionally similar to the oropharyngeal tube of crocodile fish or the blowhole of cartilaginous fish, which is used to inhale water for breathing.

In addition, the size and decoration of the head armor of Rongshi Jiangxia fish are significantly different from those of Houji Jiangxia fish and Jiujiang Jiangxia fish, so it is considered to be the "third one".

Jiangxia fish, its fossil is the "ruler"

Compared with other armored fish, Jiangxia fish has a wider distribution and richer fossil materials, and the life period of all Jiangxia fish is limited to the early Telechian stage of the Silurian period 440 million years ago.

The wide paleogeographic distribution and short age limit have established the important biostratigraphic significance of Jiangxia fish, making it an iconic species in biostratigraphic comparison.

For example, for a long time, it was believed that there were no Silurian red beds in the Silurian strata in Wuhan, Hubei, but only the Fentou Formation that did not contain red beds. Therefore, when the Houjijiangxia fish was first discovered in Wuhan, Hubei, it was initially believed to be from the Fentou Formation, which was in the middle and late Trichian stage of the Llandovery stage of the Silurian Period (about 437 million years ago), but it was already the oldest true armored fish at that time. Later, researchers discovered the Jiujiang Jiangxia fish in the Qingshui Formation in Wuning County, Jiujiang, Jiangxi.

The two are very similar, which allows the biostratigraphy of Wuhan, Hubei and Wuning, Jiangxi to be accurately compared at the species level . However, the Silurian strata in Jiangxi are very continuous, with the upper and lower strata well preserved. The Qingshui Formation undoubtedly belongs to the lower red beds of the Silurian, which is older, dating from the early Trichy stage of the Llandovery stage of the Silurian (about 438 million years ago).

This prompted us to think, does the Hubei Wuhan Jiangxia fish really come from the Fentou Formation without red beds? Therefore, the researchers conducted a lot of field investigations in the Wuhan area and finally discovered the Silurian red beds in the Gumushan area of ​​Wuhan . Based on the Jiangxia fish fossils, they finally identified it as the Qingshui Formation of the lower red beds of the Silurian.

Jiangxia fish has a rich fossil record in the South China Plate, providing key fossil evidence for the comparison of Silurian shallow marine red beds in Hubei and Jiangxi. The Rong's Jiangxia fish discovered in the Tarim Basin in Xinjiang directly links the strata in South China and Tarim, which are 4,000 kilometers apart.

Systematic position and geographical distribution of the three brothers of Jiangxia fish

(Photo credit: Shi Aijuan, Liu Wenyu)

The culprit behind the brothers’ separation: plate drift

So why are the first two "brothers" of Jiangxia fish not far apart, but the "third brother" is 4,000 kilometers away from them?

This is because of continental drift.

Armored fishes mainly live in lagoons, deltas, coastal and shallow sea environments near the edge of the ancient continent, and mainly live a bottom-dwelling filter-feeding life. Most armored fishes have flat head armor and lack paired fins, which means that they have weak swimming ability and limited ability to spread and migrate. The vast land and ocean have become obstacles to their spread and migration. Therefore, armored fishes have a strong indigenous color, and their distribution can be used to divide paleobiogeographic regions.

Judging from the existing fossil records, in the early Silurian period 438 million years ago, the fossils of armored fish in my country were generally distributed along the edges of the Cathaysia paleoland and the Yunnan-Guizhou paleoland. For example, the Houjijiangxia fish and the Jiujiangjiangxia fish appeared in Wuhan, Hubei and Jiujiang, Jiangxi respectively. The straight-line distance between the two brothers was less than 200 kilometers. This shows that the vast area of ​​my country, mainly the Yangtze River Basin, was once a vast ocean at that time - the Yangtze Sea.

So why did Rong's Jiangxia fish suddenly appear in the Tarim Basin 4,000 kilometers away? This made the research team further think about the relationship between the South China and Tarim plates.

Paleogeographical location of the Tarim Plate and the South China Plate

(Photo credit: Shan Xianren, Liu Wenyu, Shi Aijuan)

The ancient plate pattern of the Silurian in China generally follows the characteristics of the Ordovician, with the North China, South China and Tarim plates constituting the main body of the ancient Chinese mainland . However, the ancient geographical locations of these three plates and their relationship have always been controversial.

According to paleomagnetic and paleofauna data, the South China Plate was located on the eastern edge of Gondwana during most of the Paleozoic Era, while the Tarim Plate was located approximately south of 15 degrees north latitude, near the equator, and slightly eastward. Due to the general absence of late Middle Paleozoic strata, the paleogeographic location of the North China Plate during the Silurian Period is even more difficult to determine. Based on Middle Paleozoic paleomagnetic data, South China and Tarim are considered to be two separate and far-flung plates.

Therefore, for a long time, geologists at home and abroad have always believed that the Tarim Plate is closely related to the North China Plate or the Sino-Korean Plate, and they are linked together to compile paleogeographic maps. Under the influence of the Caledonian Movement, the North China Plate and the Tarim Plate collided and completely spliced ​​into the northern continent.

However, this paleogeographic pattern makes it difficult to explain the distribution of armored fish, a group with strong regional characteristics, in the South China and Tarim plates .

Biogeographic studies of armored fish lineages have shown that there are five different armored fish lineages distributed on the South China and Tarim plates, namely, Xiushuiichthyidae, Hanyangichthyidae, Dayongichthyidae, Aurochrysidae and Palaeochrysidae. This shows that the armored fishes on the two plates did not evolve independently, and there was no biogeographic isolation between them, but at least five biota diffusion events occurred.

Jiangxia fish is originally an armored fish fossil unique to the South China plate. It was first discovered in the Tarim region, which fully demonstrates that biological exchange events between the two major plates had occurred as early as the early Telechian period 438 million years ago.

Judging from the nature and combination characteristics of the fish populations in the above-mentioned two places, the species of fish populations in the Tarim Plate and the South China Plate are basically the same. Therefore, the Tarim Plate and the South China Plate should belong to the same paleozoological zone in the Silurian, namely the Tarim-South China fish zoogeographical region. The two major plates had a very close paleogeographic connection in the early Silurian, and they probably coexisted in the same warm, organic-rich ancient sea basin environment.

As early as 2009, Zhao Wenjin and his colleagues proposed that the South China Plate and the Tarim Plate should be close enough in paleogeographic location in the early Silurian period, and were in the same shallow sea environment, and may even have formed the Tarim-South China Joint Plate. The first discovery of Rong's Jiangxia fish in the Tarim Basin in Xinjiang further supports the existence of the Tarim-South China Joint Plate.

There is no consensus on the time when the Tarim Plate separated from the United Plate. At present, based on the data of paleontological fossils, stratigraphic framework, lithofacies and paleomagnetic data, the Tarim Plate separated from the South China Plate in the Rhodosian or later.

The main reason for the disintegration of the joint plate may be that the Qiangtang and Qamdo blocks brought huge energy impact from south to north, thus knocking the joint plate apart. After the disintegration of the joint plate, the Tarim plate began to drift slowly to the northwest in a clockwise direction, and reached its current position around the late Permian.

It is precisely because of plate drift that the three Jiangxia fish brothers, who originally lived in the same sea area, are now thousands of miles apart.

Conclusion

The discovery of the three Jiangxia fish brothers shows the diversity of armored fish in the early Silurian period in Xinjiang, which helps us understand their radiation evolution, migration and diffusion process and paleogeographic distribution pattern. At the same time, this discovery also makes the truth of plate drift gradually clear.

A single leaf can tell the coming of autumn, and a single fossil can tell a piece of history for paleontologists. So, is paleontology research interesting?

References:

[1]Liu, WY et al. The first Eugaleaspiforme fish from the Silurian of the Tarim Basin reveals a close relationship between the Tarim and South China blocks at 438 mya

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[3]Shan, X., Zhao, W. & Gai, Z. A New Species of Jiangxialepis (Galeaspida) from the Lower Telychian (Silurian) of Jiangxi and its Biostratigraphic Significance. Acta Geologica Sinica - English Edition.

[4]Blais, SA, MacKenzie, LA & Wilson, MVH Tooth-like scales in Early Devonian eugnathotomes and the 'outside-in' hypothesis for the origins of teeth in vertebrates. J Vert Paleont 31, 1189-1199.

[5]Van der Brugghen, W. & Janvier, P. Denticles in thelodonts. Nature 364, 107 (1993).

[6]Donoghue, PC & Rucklin, M. The ins and outs of the evolutionary origin of teeth. Evolution & Development 18, 19-30.

[7]Huysseune, A., Cerny, R. & Witten, PE The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits. Biol Rev Camb Philos Soc. 97, 414-447.

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