The strange "Swallow in the Stone" is actually an ancient shellfish

In the Paleozoic Era, there was a thriving class of "shellfish" with peculiar shapes. They were similar in appearance to the shellfish we see today, but there were obvious differences when we looked closely. The most obvious difference was that the shells were bilaterally symmetrical, with a dorsal ridge and a ventral groove, while the shells of the shellfish we commonly see today are symmetrical from top to bottom. These special shellfish are called brachiopods. Today, we will introduce to you the "bird in the stone" among the brachiopods, which is particularly valuable for viewing and collecting - the stone swallow.

The appearance of the stone swallow is like a swallow with its wings spread out, and the central ridge, central groove and shell decoration (radial folds and radial lines) are very beautiful, making the fossil beautiful and lovely. Stone swallow fossils are produced in Hunan, Guangxi, Sichuan, Guizhou, Yunnan and other places. In the geographical work "Xiangzhou Ji" of the Jin Dynasty, there was a bizarre record: "There are stone swallows in Lingling Mountain. They fly when there is wind and rain, and turn back into stone when the rain stops." It is said that there are stone swallows in Lingling Mountain in Yongzhou, Hunan. They fly when there is wind and rain, and turn back into stone when the rain stops.

This strange phenomenon was revealed through personal investigation by Northern Song Dynasty mineralogist and geoglyph Du Wan, who recorded it in the "Yunlin Stone Catalogue": "Stone swallows are found in Lingling, Yongzhou. It is said that they fly when it rains. A few years ago, I was crossing a high cliff and saw many swallow-shaped stones. I marked them with pen. The stones were exposed to the scorching sun and a sudden rain came. All the ones I marked fell to the ground one by one. This is because the cold and heat combined to make them fall off and they could not fly." It turned out that the stone swallows did not really fly, but the rocks became loose after being exposed to the scorching sun and then expanded and contracted due to the sudden rain. The fossils in the rocks fell off and fell to the ground.

In the Upper Devonian mudstone and limestone in central and southeastern Hunan, there are abundant stone swallow fossils buried. People can often pick up complete fossil individuals that have been weathered and fallen off, and also find fossil groups covering the rock layers. If you are lucky, you can even find precious fossil specimens that coexist with Yunnan clam and quasi-Yunnan clam.

Back to the stone martin itself, the reason why they look like swallows is firstly because of their long wings. On their back shells, they have developed a pair of spiral exoskeletons supporting the soft body - brachiopods, which extend spirally from the middle of the shell to the left and right sides of the wings. This makes their wings longer than those of other brachiopods, and the overall look is like the open wings of a swallow.

Secondly, from the side view, the shell of the stone swallow is biconvex, with a small dorsal shell and a large ventral shell. The earliest formed part of their shell is shaped like a bird's beak, called the shell beak. The particularly prominent ventral beak on the ventral shell looks more like a swallow's short beak.

The stone swallow fossil is deeply loved by people because of its unique shape. Due to its large production, it has not become exclusive to wealthy collectors. It can be said that it is one of the most suitable ancient fossils for collection. The reason for collecting it is not how high its economic value is, but what is important is that it inspires our interest in exploring the mysteries of the earth and science.