Why are all marine reptile fossils in China found in the south?
China is extremely rich in marine reptile fossils, but there is a striking feature of these fossils: they are all found in the Triassic strata of South China. The Triassic was the first period of the Mesozoic Era and also the time when marine reptiles first appeared on Earth. During the Triassic, southern China was an ocean, which extended westward to what is now Europe, called the Paleotethys Ocean, the predecessor of the modern Mediterranean Sea. This body of water was home to a large number of marine reptiles, while northern China at the same time was a vast land area, making it impossible for marine reptiles to exist there, and thus no such fossils have been preserved. With the geological changes of the ages, by the Jurassic and Cretaceous periods, the ancient oceans in South China had also become land, and marine reptiles became extinct in this area.
Chinese Triassic marine reptile fossils have been found in Hubei, Anhui, Guizhou, Yunnan, Guangxi, and Tibet, with the most abundant ones in Guizhou and Yunnan. The Guanling area in southwestern Guizhou is rich in large ichthyosaurs and sea lizards, as well as a small number of placodonts and rare primitive turtles—the semi-armored turtle. In Xingyi and its surrounding areas at the border of Guizhou and Yunnan, such fossils are even more abundant. In addition to various ichthyosaurs, sea lizards, and placodonts, there are also abundant finback dinosaurs and original dinosaurs. In addition, Pan County in Guizhou and Luoping in Yunnan produce a variety of dragon turtles and mixed-form Qian alligators. The marine reptile fossils found in the above-mentioned areas are mostly well-preserved skeletons, indicating that the marine environment here during the Triassic period was calm, and after the animals died, they were quickly buried by underwater sediments, less affected and damaged by other organisms and water currents. In contrast, the marine reptile fossils found in Dingri and Nyalam in Tibet are fragmented bones and scattered tooth fossils, indicating that it was a turbulent and dangerous marine environment, which was completely different from that in Guizhou and Yunnan.