#Popular Science in Progress# This "four-legged snake" on the stone slab is more than 200 million years old

In 1957, Mr. Hu Chengzhi, a researcher at the National Geological Museum of China, found fossils of an animal on the stone slabs of a local house in Lvyin Village, Dingxiao Town (now Dingxiao Street), Xingyi, Guizhou. The fossils were initially identified as reptiles. The villagers told him that this was called "four-legged snake slabs" and was very common in the local Langwu Mountain.

The fossils of this creature aroused his interest. Under the guidance of the locals, he came to the mountains, picked up a few fossils and brought them back to Beijing. After research and identification by the famous paleontologist Mr. Yang Zhongjian, he believed that this fossil creature was discovered for the first time in China and even in Asia, and was of great significance. Therefore, it was named Hu's Guizhousaurus after the surname of the discoverer Mr. Hu Chengzhi and the place of discovery, also known as Guizhousaurus.

Guizhousaurus lived in the Middle Triassic period. It was a small marine reptile belonging to the Guizhousaurus family of the Pachyrhinosaurus suborder. Its body length was generally between 15 and 30 cm, with a long neck, and it looked like a mini version of a plesiosaur.

In addition to their appearance, the two also have similar ways of moving, using their relatively flat forelimbs to propel their bodies forward in the water. However, the forelimbs of the plesiosaurs are shaped like paddles, while the forelimbs of the Guizhousaurus retain obvious toes, with webbed toes. This limb shape makes many people believe that the Guizhousaurus can leave the water environment and move on land. Its reproductive method has also caused controversy. Some people believe that they lay eggs on land like turtles, while others believe that they give birth to their young in the water like ichthyosaurs.

As the research progressed, people found that the girdle bones (i.e., waist girdle, including ilium, pubis, and ischium) of Guizhousaurus' hind limbs were movable and not fixed to the sacrum, so it could not support its weight on land with its limbs. It had very obvious aquatic characteristics, and the same characteristics were also present in some early amphibians and modern fish, because their limbs and paired fins did not need to bear weight. Therefore, Guizhousaurus lived in the ocean all its life.

Such a lifestyle ruled out the possibility of Guizhousaurus coming ashore to lay eggs. Later, female fossil specimens discovered also contained larval skeletons preserved in the body cavity, which became evidence that Guizhousaurus gave birth to its young directly in the water. However, their reproductive method was different from the "viviparous" of modern marine mammals such as whales and dolphins. Instead, they laid eggs in the body and gave birth after hatching, which is called "ovoviviparous". The larvae of the former obtain nutrition directly from the mother, while although the larvae of the latter are also given birth directly, the nutrition is provided by the eggs.

Because they breathe with lungs, the larvae leave the mother in the order of tail, body, and then head. If the head is born first, it means death on the spot. The first thing the newly born larvae have to do is to float to the surface to breathe the first breath of air, and then start an independent foraging journey, feeding on fish and shrimps smaller than themselves. Their bodies have 20 cervical vertebrae, 20 dorsal vertebrae, and 37 caudal vertebrae, which ensures flexibility in hunting and escaping from predators.

The narrow trough in Xingyi during the Triassic provided Guizhousaurus with a stable and closed living environment, making them the most prosperous and representative species in the area. However, danger lurked in stability. When the sea level in Xingyi dropped, the seawater circulation was poor, and the low-energy and oxygen-deficient static water environment caused them to quickly go extinct. Only the gray-black bone texture on the fossils and the clear stratified structure of the surrounding rocks tell of their "suffocation" ending.