The Super Time and Space Detective invites you to solve the case and solve the dinosaur "mystery" together

Produced by: Science Popularization China

Author: Sun Jiafeng

Producer: China Science Expo

Dinosaurs, which once dominated the earth, are always one of the focuses of people's curiosity. However, they have been extinct for 66 million years, so it is not easy to learn about them. However, geologists are like detectives, unraveling the mystery from each dinosaur fossil.

The morphology, diet, reproduction methods, etc. of dinosaurs can be studied directly with the help of dinosaur bone fossils. However, information such as where dinosaurs lived, what their living environment was like, whether they lived in groups or alone, etc. is difficult to obtain directly from dinosaur bone fossils.

At this time, the key evidence is not the dinosaur skeletal fossils, but some things that most people think are insignificant - such as the rock strata where dinosaur fossils are found, dinosaur egg fossils, feces fossils and footprint fossils.

Dinosaur bone fossils are the focus of all museums. They can help us directly understand the body structure of dinosaurs, but they are not everything. More materials are needed to restore the living environment of dinosaurs.

Image source: wikipedia

Where are the dinosaur footprints?

To explore the above content, we have to go to a place - Zhucheng, Shandong. Zhucheng, Shandong is a famous dinosaur fossil production area in China. In recent years, scientists have discovered a large number of dinosaur fossils here, including duck-billed dinosaurs, large horned dinosaurs - Zhucheng Sinoceratops (the first discovery outside North America), ankylosaurs, coelurosaurs, and one of the largest tyrannosaurus in Asia - the giant Zhucheng Tyrannosaurus. In addition to these dinosaur fossils, Zhucheng also discovered large-scale dinosaur footprint fossils.

Zhucheng is an important dinosaur fossil producing area in China. The picture shows the dinosaur fossils in the Dinosaur Fossil Gallery in Dinosaur Stream.

Image source: provided by the author

These footprint fossils are mainly preserved in three areas: Zhangzhuhewan, Tangdi Gezhuang area and Huanglonggou area in the south. The footprint fossils preserve the wonderful moments of dinosaurs in their daily lives and provide information about the type, size, number, walking direction and speed of dinosaurs and their living environment, while the dinosaur bone fossils reflect the burial environment after their death.

Among them, the dinosaur footprint fossils in Huanglonggou are the largest and most famous in the world. It is located in Dashan Community, Huanghua Town, in the south of Zhucheng. The explored area here is about 5,000 square meters, and more than 11,000 different types of theropod and sauropod dinosaur footprints have been discovered. They lived in the middle of the Early Cretaceous more than 100 million years ago, and are recognized by experts as a rare geological wonder in the world.

The densely populated area of ​​dinosaur footprints here is located on a 50° steep slope. The entire slope is composed of hard rock strata. Within an area of ​​nearly 5,000 square meters, more than 11,000 dinosaur footprints of various shapes, sizes, depths are densely arranged on the same rock layer, existing in yellow-green fine sandstone, accompanied by mud cracks and ripples.

Mud cracks refer to the traces of wet mud on the shore that dry out and crack after being exposed to the sun in the dry season. They will be preserved in the rocks. The same is true for ripple marks. The mud and sand on the river bottom, lake bottom, seabed and shore will leave ripple marks due to the fluctuations of water, and they will also be preserved in the rocks.

The whole picture of the dinosaur footprint fossils in Huanglonggou

Image source: provided by the author

These footprints are extremely well preserved, with the toes, soles and heels of each dinosaur fossil clearly visible, as natural and lifelike as the footprints left by people stepping on soft soil.

The dinosaur footprints come from many types of dinosaurs, including large theropods, sub-large theropods, small theropods (similar to Scaphodon), sauropods, etc., among which small theropods are the main ones. Among these dinosaur fossil footprints, traces of turtles and tortoises swimming can occasionally be seen.

Huanglonggou footprint fossils (the large round pit is a sauropod footprint, and the lined pit is a small theropod footprint)

Image source: provided by the author

The Tangdi Gezhuang and Zhangzhuhewan dinosaur footprint fossil sites are both located in the north of Zhucheng City, dating back about 100 million years, and the two places are 1 kilometer apart.

The Tangdi Gozhuang dinosaur footprint site is relatively small in scale, and those with important scientific significance are all on the same rock layer. Here, scientific researchers discovered a dinosaur turning footprint consisting of 28 sauropod dinosaur footprints. The average length of the hind feet is about 30 cm, and the entire track is distributed in a semicircular shape, which just preserves the entire process of the dinosaur's turning. This is the first 180° turn of a dinosaur discovered in the world.

These footprints are concentrated on the same layer with an inclination of about 45°, and are found in purple-red fine sandstone, accompanied by mud cracks, cross-bedding, and rain marks.

Tangdi Gezhuang Dinosaur Footprint Fossils

Image source: provided by the author

The Zhangzhuhewan dinosaur footprint fossil site is located on the east bank of the Donghe River in Zhangzhuhewan Village, Shiqiaozi Town. It consists of 4 sauropods and 1 ancient bird footprint. Among them, the ancient bird footprint is particularly precious, and the footprints are concentrated in the same angle and close to the horizontal stratigraphic level.

Dinosaur footprint fossils in Zhangzhuhe Bay

Image source: provided by the author

How to infer the living environment of dinosaurs?

The above descriptions may seem a bit boring, but just as detectives can solve a case through the crime scene, geologists can infer the environment in which dinosaurs lived at the time - arid to semi-arid lakeside - from the information in these rock strata and fossils.

This judgment is not made casually. Let's analyze it bit by bit according to the geologist's thinking:

1. Fine sandstone is produced around water bodies such as rivers, lakes and seas.

Fine sandstone, as the name implies, is a rock formed by the consolidation and dehydration of fine sand a long time ago. Just as most of the beaches we see today appear near the sea, lakes, rivers, etc., fine sand in the geological period also appeared in these places.

In addition, the ripple marks on the fine sandstone layers also confirm that this is a beach by the water. Combined with the analysis of local geological maps and other data, it can be further concluded that this is a lakeside rather than a river or seaside.

Sandstone is made of sand and was formed in the same environment as modern beaches.

Image source: wikipedia

2. Dinosaurs once lived by the lake.

The footprints at the three fossil sites in Zhucheng are all found in fine sandstone, accompanied by horizontal stratification, rain marks and mud cracks, indicating that the dinosaurs at the three sites left their footprints when they were walking on the lakeshore.

The presence of a large number of dinosaur and bird footprints also proves that there were water bodies at these three locations in the Early Cretaceous period. Dinosaurs and birds regarded the lakes here as water sources and came to drink water regularly.

Moreover, this should be their long-term and fixed activity place. After all, the conditions for preserving fossils are very harsh. Not all footprints can be left behind and become fossils. Only if they are frequent enough and numerous enough can the probability of becoming fossils be increased.

3. The lake is deep in the south and shallow in the north.

Mud cracks in the Huanglonggou strata (in the dry season, shallow water areas often dry up first, exposing bottom sediments to form mud cracks) are mainly concentrated in the north, and ripple marks are mainly developed in the south/southwest, indicating that the deep water area of ​​the lake basin should be located in the south/southwest, while the north/northeast is the lakeshore.

Wave marks on the footprint layer of Huanglonggou

Image source: provided by the author

4. This was once an arid-semiarid environment.

It is very rare to preserve so many dinosaur footprints. First of all, the beach on the lakeshore must be fine enough, and the humidity, viscosity and granularity must be just right to leave intact and beautiful footprints. Moreover, even if the beach conditions are met, specific environmental conditions must be met.

After the dinosaur footprints were formed, if it rained heavily or the area was flooded, the footprints that had not yet dried would soon melt in the water. Therefore, footprints need a certain amount of time to dry after they are formed on wet ground. After the surface where the footprints are preserved is dry and hardened, and then buried by later sediments, it is easy to form fossils.

Therefore, footprints are more likely to be preserved in sedimentary environments such as lakeside, seaside, and riverside in arid areas.

5. Mud cracks themselves reflect an arid-semiarid environment.

Mud cracks in sandstone are also a sign of climate and environment. Mud cracks, also known as dry cracks or cracks, are common in the shores of seas, lakes, rivers, or low-lying areas where water accumulates temporarily. People with rural life experience can often see this phenomenon.

This is caused by the fine-grained sediments (clay, silt or mud) deposited underwater or the fine-grained sediments on the surface being soaked in water and then exposed above the water surface. In addition, the dry climate causes the water to evaporate quickly, causing the sediments to dehydrate and shrink.

Mud cracks in modern environment

Image credit: Jonathan Wilkins

Although mud cracks are common in the modern environment, they are easily affected by weathering and erosion after they are formed in geological periods and are difficult to preserve on the surface. It is generally believed that the appearance of mud cracks in geological periods indicates drought or alternating dry and wet environments.

Because the cracks in the geological period are formed in low-lying areas with short-term water accumulation in arid and semi-arid areas, short-term precipitation causes water accumulation in low-lying areas. Due to the large evaporation, the water evaporates quickly, which easily forms mud cracks. The mud cracks formed in this environment are above the water surface for a long time, the later flooding time is short, and the preservation time is long.

Based on the presence of a large number of dinosaur footprint fossils on mud crack structures in the Cretaceous strata of Zhucheng, it can be inferred that dinosaurs lived in a lakeside environment and that intermittent droughts had occurred.

6. Sauropod footprints often appear in semi-arid environments.

From a global perspective, sauropod footprints are often found in semi-arid inland basin environments or coastal deposits in low-latitude arid areas, and are rarely found in humid climate environments. Sauropod footprints were found in all three fossil sites in Zhucheng, indicating that Zhucheng was already dry at the time. Sediments exposed to the air shrink due to evaporation and dehydration, resulting in mud cracks.

7. The drought here has intensified over time.

The Huanglonggou dinosaur footprint fossils are found in the sandstone of the Longwangzhuang Formation in the Laiyang Group during the middle Early Cretaceous. Analysis of large-scale measured profile sedimentary phases shows that over time, this area has gradually evolved from a shallow lake environment to a lakeside environment, with the water becoming gradually shallower and the climate becoming drier.

Mud cracks on the footprint level are mostly thin layers sandwiched between middle-layer sandstone, indicating that floods caused water to accumulate on the mudflats. The floods receded in a short period of time, leaving the mudflats exposed for a long time. Due to the large evaporation rate, the water evaporated quickly, forming mud cracks. The climate environment should be a seasonal semi-arid to arid climate.

Compared with the mud cracks in the footprint layer, the size and range of mud cracks in the later layers have obviously expanded, indicating that the degree of climate drought is intensifying.

H- Mud cracks on the later-formed layer of the footprint layer in Huanglonggou; I- Mud cracks on the footprint layer

Image source: Journal of Palaeogeography

The above reasoning process is just a small example of geological research. Over the past hundreds of years, geologists have been doing this kind of work similar to time and space detectives, and these studies have continuously enriched our understanding of the history of the earth.

Nowadays, any popular science book can tell us in detail about the magnificent evolutionary history of the Earth over the past 4.6 billion years. But in fact, behind every story, there is a seemingly small but actually interesting reasoning process.

References:

[1] Xing Lida, Jared D. Harris, Wang Kebo, et al. 2010c. Non-avian dinosaur and bird footprint fossil assemblages from the Lower Cretaceous Laiyang Group of Zhucheng Basin, Shandong Province [J]. Geological Bulletin, 29(8): 1105-1112

[2] Liu Yongqing, Kuang Hongwei, Peng Nan, et al. 2011. Burial sedimentary facies and paleogeographic environment of Cretaceous dinosaur footprints and skeletal fossils in Jiaolai Basin, Shandong Province [J]. Earth Science Frontiers, 18 (4): 9-24

[3] Wang Baohong, Liu Yongqing, Kuang Hongwei, Wang Kebai, Chen Shuqing, Zhang Yanxia, ​​Peng Nan, Xu Huan, Chen Jun, Liu Hai, Xu Jialin, Wang Mingwei. New discovery of dinosaur footprint fossils from the late Early Cretaceous at Tangdigezhuang, Zhucheng, Shandong and its significance [J]. Journal of Palaeogeography, 2013, 15(04): 454-466.

[4] Xu Huan, Liu Yongqing, Kuang Hongwei, Wang Kebai, Chen Shuqing, Zhang Yanxia, ​​Peng Nan, Chen Jun, Wang Mingwei, Wang Baohong. Super-large-scale dinosaur footprints from the middle Early Cretaceous in Zhucheng, Shandong Province and their paleogeography and paleoecology[J]. Journal of Palaeogeography, 2013, 15(04): 467-488.