[Creative Cultivation Program] Fish are the evidence that the Tarim Basin actually "drifted" from the equator

Author: Gai Zhikun Shan Xianren

Reviewer: Zhao Wenjin

In 1910, German meteorologist and geophysicist Alfred Wegener discovered that the shapes of the two sides of the Atlantic Ocean complement each other, which gave rise to the idea of ​​continental drift. He believed that the landmass of the world was connected to each other as a whole 200 million years ago, but later it was separated and drifted to its current position due to the force of the earth's rotation.

Scientists have found a lot of evidence to support the "continental drift theory", one of the most convincing evidence is from the fossil evidence of Lystrosaurus. Lystrosaurus is a mammal-like reptile that lived about 200 million years ago. It mainly lived on land and could not migrate across the sea. However, its fossils are widely distributed in the Late Permian-Early Triassic strata in Xinjiang, southern Africa, India, Antarctica and other places in my country. Its wide distribution pattern across oceans strongly indicates that these fossil sites may have been in the same united continent-Gondwana. Evidence from paleontological fossils played a very critical role in proving the continental drift hypothesis.

In addition to Lystrosaurus, some fish also played a similar role, such as armored fish with limited migration and diffusion capabilities. Armored fish also play an important role in proving the theory of plate drift, and they can even tell us that the Tarim Basin actually "drifted" slowly from the equator.

Armored fish are a type of bottom-dwelling filter-feeding fish that mainly live in shallow seas near the edge of ancient land. Armored fish have flat head armor and lack paired fins, indicating 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. Current fossil records show that the Tarim Basin and South China were home to the same group of armored fish in the early Silurian period (438 million years ago). They included the ancestral types of armored fish and multi-brachial fish, indicating that the two blocks, which are now 4,000 kilometers apart, may have been connected and in the same sea at that time.

The distribution of the Yangtze Sea and the origin of ancient fish fossils (red represents shallow-sea red-bed sedimentary areas) (Shan Xianren drew the paleogeographic map, Zong Ruiwen provided the ecological restoration map of Wuhan, Hubei, and other ecological restoration maps were drawn by Yang Dinghua)

Representative fish fossils from the Tataertag Formation in Xinjiang (Photo by Gai Zhikun, illustration by Yang Dinghua)

In addition to the evidence of ancient fish fossils, the study of invertebrate fossils also shows that the South China and Tarim Plates in my country belonged to the same animal geographical system during the Silurian period. In addition, the sedimentary environment of the two plates in the early Silurian period was basically the same, and the sedimentary rock types were represented by red clastic rock systems formed in arid environments and shallow seas. Furthermore, the Silurian stratigraphic framework of the two plates is basically the same, that is, the age of the Silurian bottom and cover layers and the stratigraphic contact relationship between them are roughly the same.

During this period, the Silurian strata on both plates were covered by the Late Ordovician strata, which were covered by the Middle-Late Devonian strata. According to paleomagnetic research, as early as the Silurian period (440-420 million years ago), the Tarim Plate was located approximately south of 15°N, near the equator, and slightly east. During the Silurian period, the Tarim Plate should have been connected to the South China Plate, known as the Tarim-South China Joint Plate (Figure 9).

So why did the Tarim Plate (equivalent to the current Tarim Basin) later move from the equator to the western border 4,000 kilometers away? Different scholars have given different explanations based on the materials they have in their hands, and there is still no unified view. At present, based on the research of paleontological fossil materials and combined with comprehensive analysis of paleomagnetism, stratigraphy and paleoenvironment, the following more reasonable view is drawn: the Tarim-South China joint plate began to disintegrate in the late Silurian period (420 million years ago). The main reason for the disintegration may be that the Qiangtang and Changdu blocks in the south rushed over with huge energy and knocked the joint plate apart. After the joint plate disintegrated, the Tarim Plate began to drift slowly to the northwest in a clockwise direction, and it reached its current position around the late Permian (250 million years ago), while the South China Plate slid 800 kilometers eastward. In other words, the Tarim Basin was not in its current position 400 million years ago. After a long geological process, it slowly drifted here from the South China region. As the Indian Plate continues to subduct northward, it remains to be seen whether it will drift further north.

Paleogeographic location of the Tarim Basin in the Silurian Period restored from ancient fish fossils

(Together with the South China Plate, it forms a Tarim-South China Joint Plate, provided by Zhao Wenjin)

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