The stone that was dug out looks like a dragon claw with scales on it. Is it really a dragon claw fossil?

Produced by: Science Popularization China

Author: Earth's Gravity

Producer: China Science Expo

Not long ago, during a road construction project, some rocks resembling dragon claws with "scales" on them were dug up on the roadside in Wugang, Hunan. The video recording the dragon claw rocks became a hit on a short video platform. Many people speculated whether these were fossils of dragon claws.

Pictures of "Dragon Claw Fossil" circulated on the Internet

(Photo source: Xiaoxiang Morning News)

Let me first give you the answer: they are not dragon claw fossils, but just some ordinary limestone. So why can limestone look like dragon claw fossils? We can break this question into two sub-questions: 1. Why does it look like a dragon claw? 2. How are "dragon scales" formed?

1. Why does it look so much like a dragon’s claw?

This is the result of water-dissolved limestone and is a typical karst landform.

The chemical composition of limestone is calcium carbonate, which is relatively soluble in water compared to other types of rocks. Because carbon dioxide in the air tends to dissolve in water, when they come into contact with limestone, a chemical reaction will occur to form soluble calcium bicarbonate. As the water flows, the calcium bicarbonate will be carried away, and the limestone will be dissolved.

In order to help you review junior high school chemistry, we have thoughtfully put the chemical reaction equations mentioned above below:

This process is the reaction of rock dissolution, which domestic scholars vividly call karstification . In areas covered by soluble rocks such as limestone, the interaction between water and soluble rocks (karstification) is very common, forming a series of magnificent landscapes, among which the most famous ones may be caves and tiankengs. These special landscapes are called karst landforms (internationally known as karst landforms).

Caves and tiankengs are common karst landforms. Guilin has also formed famous landscapes due to karst action.

(Image source: Wikipedia)

The global distribution of karst landforms shows that most of southern my country (especially Yunnan, Guizhou, Hunan, Chongqing, and Guangxi) is covered.

(Image source: Wikipedia)

However, caves and tiankengs are formed inside the limestone strata, while the "dragon claw fossils" we see today occur on the surface of the limestone.

The formation process is as follows: when rainwater or other water flows into limestone, since limestone is impermeable, the water will initially flow on the surface of the limestone - due to the slope of the terrain, the uneven surface of the limestone, and possible cracks on the surface of the limestone, the water will naturally flow along the small grooves on the surface of the limestone.

Cracks on the surface of limestone are very common (of course this picture is just an extreme example). At the same time, the internal composition of limestone is not uniform. Some places are difficult to dissolve, while others are more soluble. This is the reason why grooves appear on the surface of limestone.

(Image source: Wikipedia)

As we know from the previous article, limestone is soluble in water, so these grooves will become wider and wider, thus forming a karst trough. The rest of the karst trough will bulge into ridges or columns (because the cracks on the surface of the limestone are crisscrossed, the karst troughs are also crisscrossed, so the rest of the karst troughs are separated into blocks, and as the blocks continue to be dissolved, they will become columns). These ridge-like or columnar protrusions can be called stone buds .

Stone buds and karst troughs

(Image source: Wikipedia)

The ridge or column shape mentioned above is the situation where the limestone layer is directly exposed on the surface. In the "dragon claw shape" situation we are discussing today, the limestone is buried under the soil layer, but there is no difference in essence - because as we all know, soil is also permeable.

The dragon claw-like shapes we see are actually karst troughs and stone buds half-buried in the soil. If we continue to dig down, we will find that there is a complete limestone layer underneath them, and these "dragon claws" are just small protrusions on the limestone layer. But it is this half-hidden and half-exposed situation that makes them look like dragon claws.

For a larger stone bud, you can imagine that its bottom is buried in the soil, with only the top layer exposed above the ground. Doesn’t its shape look somewhat similar to a "dragon claw"?

(Image source: PxHere)

2. How are scales formed?

The scaly structure of the "dragon claw fossil" is formed by the weathering of the special mesh texture in the limestone. The limestone in Wugang is most likely pagoda limestone, which was discovered and named by Mr. Li Siguang, a famous Chinese geologist, in the 1920s. It is called pagoda because it is rich in Ordovician paleontological fossils. The most conspicuous fossil is called Sinoceras chinense. The hornstone is long and conical. After being preserved in the rock layer, it looks like a small pagoda, so it is called pagoda limestone.

This kind of limestone has another obvious feature - a reticular pattern. Some people named it horseshoe pattern, while others named it turtle crack because the crack structure after weathering is hexagonal and the whole rock layer looks like a turtle's back.

A pagoda-shaped right-angled stone fossil on the pagoda limestone, and the reticular pattern of the pagoda limestone

(Image source: Reference 1)

As for how this network of cracks was formed, geologists still have no unified answer. Initially, some people believed that this was formed when the rock layer was directly exposed to sunlight during its formation and cracked.

This phenomenon of cracking is very common in our daily life (especially for people living in rural areas). Take modern cracking as an example. They are often formed in mud after heavy rain. Once exposed to the sun, the surface of the mud will quickly lose water and shrink, forming hexagonal cracks, which are called mud cracks.

Modern mud cracks

(Image credit: Jonathan Wilkins)

Mud cracks preserved in mudstone

(Image credit: David Tanner, Leibniz Institute for Applied Geophysics (LIAG), Hannover, Germany)

Since limestone is often formed in shallow sea or coastal environments, some scientists believe that these pagoda limestones may have been formed in a very shallow environment. The limestone was intermittently exposed to sunlight, and under the sunlight, the silty limestone dried and cracked, forming these cracks. However, this theory has rarely been mentioned recently.

Others believe that the network of cracks may be caused by dehydration and shrinkage of limestone when it was formed underwater. As mentioned above, limestone is formed in shallow sea environments, either because of the large amount of evaporation of seawater, which increases the concentration of calcium ions in it, thus forming mud precipitation, which finally solidifies to form limestone; or because of the death of calcium-rich organisms in the ocean, such as shells, corals, etc., their calcium exoskeletons precipitate to form biomass-rich mud, which further forms limestone.

The formation of limestone is mainly related to the precipitation of calcium ions in seawater and the precipitation of biological exoskeletons.

(Image source: Reference 2)

Regardless of the cause, after it settles on the seabed, it will continue to be covered and compacted by subsequent sediments. During this compaction process, the originally water-rich sediment (which may be a colloid at this time) will gradually dehydrate due to increasing pressure. During the dehydration process, the volume naturally decreases and shrinks, thus forming cracks. The cracks are then filled with muddy materials, and finally this regular grid-like structure is formed.

This is an example of the formation of hexagonal basalt columns (the famous Giant's Causeway is a typical example of this landform). The principle of the formation of cracks after the plaster is dehydrated in this article is similar to this. After the plaster is dehydrated, local tension is generated, which naturally produces regular shapes and regular cracks.

(Image source: Wikipedia)

Some people also believe that the reason for the formation of network cracks is as follows: when this kind of limestone is formed, there is a thin layer of clay between the mortar layers. When they are compacted as a whole, the mortar is first dehydrated, and a large part of this water is absorbed by the clay layer. Then, as the pressure continues to increase, the clay layer also begins to dehydrate. Due to the presence of bacteria and other organic matter in the clay layer, the water in the clay layer becomes acidic. This acidic water will squeeze into the limestone layer (the original mortar layer, which has been solidified into limestone due to dehydration) during dehydration, and partially dissolve the limestone layer (that is, pressure solution), causing this kind of cracks to appear.

Under shallow burial conditions, the mortar is first dehydrated and the water is concentrated in the clay; under deep burial conditions, the water in the clay gushes out and begins to dissolve the limestone, causing cracks in the limestone.

(Image source: Reference 1)

Others believe that this is related to tectonic action. Simply put, when the rock in this area was formed, it was subjected to complex pressure or tension, which caused the unconsolidated sediments to be pulled or fractured, forming a network of fractures. Subsequent sedimentation has transformed these fractures into what we see now.

(a) During the sedimentation stage, the mud was still loose sediment; (b) Under the influence of tectonic forces, cracks appeared in the sediment; (c) After deep burial, it was subjected to pressure solution; (d) After being exposed on the surface, it was further transformed by weathering into the form we see today.

(Image source: Reference 1)

In short, scientists are not sure why the limestone looks like dragon scales. But what we can be sure of is that what we found is not dragon claws, but ordinary limestone. As for the layers of scales on it, they are just a structure of the limestone itself, not the so-called dragon scales.

References:

[1] Liao Jijia, Ma Sihao, Liao Mingguang, et al. Research progress and new discoveries on the origin of the Ordovician Pagoda limestone reticular structure[J]. Acta Sedimentologica Sinica, 2017, 35(2): 241-252.

[2] Wheeley JR, Cherns L, Wright V P. Provenance of microcrystalline carbonate cement in limestone–marl alternations (LMA): aragonite mud or molluscs?[J]. Journal of the Geological Society, 2008, 165(1): 395-403.

[3] Fang Shaoxian, Hou Fanghao, Lan Gui, et al. Genesis and hydrocarbon content of the “horse-shoe pattern” structure in the limestone of the Middle Ordovician Baota Formation in Sichuan and Guizhou [J]. Marine Petroleum Geology, 1994 (1): 36-40.

[4] Zhou Shuxin, Wang Jianguo. Origin of the Pagoda Limestone in Shiqian, Guizhou Province[J]. Petroleum Experimental Geology, 1992, 14(3): 291-295.

[5] Huang Leqing, Liu Wei, Bai Daoyuan, et al. Turtle crack structural characteristics, genesis and resource significance of the Ordovician Pagoda Formation limestone in northwestern Hunan [J]. Earth Science, 2019, 44(2): 399-414.

Editor: Guo Yaxin

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