Why are there differences between the north and south of China's duck-billed dinosaur fossils?

Produced by: Science Popularization China

Author: Sun Jiafeng and Chen Shuqing (Zhucheng Dinosaur Culture Research Center)

Producer: China Science Expo

Hadrosaurs are well-known for their iconic duck-billed snouts and can be considered the "star player" in the dinosaur world. The hadrosaurs are a large family with many members, and the corresponding fossils are also widely distributed. Needless to say, my country is vast, and there are also a large number of hadrosaurs fossils distributed in this vast land of China.

However, when researchers were counting and sorting out my country's hadrosaur fossils, they discovered something strange: China's hadrosaur skeletal fossils were mainly found in the north, while in the south, hadrosaur footprints and dinosaur egg fossils were mainly found.

What is the reason for this north-south distribution difference?

A huge family of duck-billed dinosaurs!

Hadrosauroidea, also known as duck-billed dinosaurs, are large terrestrial herbivorous ornithischian dinosaurs that lived in the Late Cretaceous. The most obvious feature of hadrosauroids is their long and flat heads with duck-like snouts, as well as their complex dentition and powerful chewing system. Hadrosauroidea includes the more primitive basal groups and the more advanced hadrosauridae.

In the middle of the Late Cretaceous, hadrosaurids separated from basal hadrosaurids and gradually occupied the terrestrial ecosystem at that time; basal hadrosaurids were gradually replaced by hadrosaurids and became completely extinct in the Early Maastrichtian. This may be because hadrosaurids acquired more complex tooth structures, the ability to walk on two legs, and a larger body size.

Some hadrosaurids had different headdress structures. Due to the different headdress morphology, hadrosaurids can be divided into flat-headed hadrosaurinae and hadrosaurinae with solid headdresses, and lambeosaurinae with hollow headdresses. The osteological differences between hadrosaurinae and lambeosaurinae are mainly reflected in the morphological characteristics of the facial bones and girdle bones.

Restoration of the duck-billed dinosaur

Image source: veer gallery

Restoration of Lambeosaur

Image source: veer gallery

Giant Shandong Dragon (Hadosaurinae)

Image source: provided by the author

Skeletal reconstruction of typical representatives of basal hadrosaurines, hadrosaurinae and lambeosaurinae

Image source: References

Some hadrosaurs were very large. For example, the Edmontosaurus, Giant Paulosaurus and Charonosaurus of North America were up to 12-13 meters long. The giant Shandongosaurus of Zhucheng, Shandong, China was 15 meters long, the giant Zhuchengosaurus was 16.6 meters long, and the giant Huaxiasaurus was 18.3 meters long! In the later stages of the evolution of hadrosaurs, there were almost no species shorter than 7 meters. Each one weighed several tons, or even 10-20 tons. The seemingly majestic ceratopsians and ankylosaurs became small next to them.

Distribution of Hadrosaurs in China

Hadrosaurs are numerous and numerous, and their fossils are also very rich, from newly hatched individuals, juvenile individuals to adult individuals, and even their egg fossils, skin imprints, footprints, and fecal fossils have been found. Fossil materials have been found in non-marine strata from the Early Cretaceous to the Late Cretaceous in Europe, Asia, North America, South America, and Antarctica, and are most abundant in North America and Asia.

China has a very rich number of hadrosaur fossils, which are distributed over a wide area, all over China. In addition to Shandong, many hadrosaur fossils have been found in Inner Mongolia, Ningxia, Heilongjiang, Xinjiang, Sichuan and other places.

Phylogenetic tree of hadrosauroidea and its distribution in China

Image source: Journal of Palaeogeography

Shandong Province is famous for the production of hadrosaurs in China. In 1923, Chinese geologist Tan Xichou discovered the earliest hadrosaurs named in China, Tansaurus sinensis, in Jiangjunding Village, Laiyang, Shandong. Later, he discovered the first dinosaur fossil skeleton after the founding of the People's Republic of China, the famous hadrosaurs with headdresses, Qingdaosaurus spinosaurus, and Tansaurus jingangkou and Laiyangsaurus yang in Xigou, Jingangkou Village, Laiyang.

Zhucheng is a rich source of Late Cretaceous hadrosaur fossils. Since 1964, three large-scale dinosaur fossil excavations have been carried out, discovering the world's largest dinosaur fossil cluster. The hadrosaurs discovered include the giant Shandongsaurus, the giant Zhuchengsaurus, and the giant Cathaysaurus. Among them, the giant Cathaysaurus is the tallest hadrosaur in the world. China's largest horned dinosaur and China's largest carnivorous dinosaur have also been discovered.

The giant Shandongsaurus in Zhucheng area and the North American Edmontosaurus are sister groups. The duck-billed dinosaurs originated in Asia about 86 to 83 million years ago. One branch evolved into the native giant Shandongsaurus, and the other branch migrated from East Asia to North America via the Bering Road Bridge, evolved into Edmontosaurus, and underwent further differentiation and radiation.

Beipiao City in Liaoning Province is an important source of Cretaceous dinosaur fossils. The famous Gilmore Dimitron was discovered here and lived in the Early Cretaceous. Yixian County in Liaoning Province is the most important fossil treasure house of the Early Cretaceous. A large number of important fossils of dinosaurs, birds, mammals, and flowering plants have been discovered here. It is the "Jehol Biota" that shocked the world and involves some major issues in biological evolution. The Yixian Bosaurus discovered here is one of the most primitive duck-billed dinosaurs discovered in Asia.

The Wulaga area in Heilongjiang Province is one of the important burial sites of hadrosaurs. The Wulagaosaurus found in the area, the Brachylophosaurus and the Maiasaura found in North America together constitute a basal branch of the hadrosaurine subfamily. The Longgushan area in Jiayin, Heilongjiang Province is an important source of dinosaur fossils from the Early Cretaceous to the Late Cretaceous.

Heilongjiang Manchuriasaurus was discovered in Jiayin County, on the banks of the Heilongjiang River in my country. This is the first dinosaur fossil site in China and also the first hadrosaur fossil site in China. The Late Cretaceous Yuliangzi Formation in Longgushan, Jiayin, Heilongjiang, preserves abundant hadrosaur fossils, from juveniles to adults, as well as hadrosaur egg fossils and hadrosaur embryo fossils, which are very rare. Adult and juvenile Charonosaurus were discovered west of Jiayin County, Heilongjiang Province. Charonosaurus is a large Lambeosaur subfamily dinosaur that survived until the end of the Cretaceous.

The Fuyun Yaksasaurus, which lived in the Late Cretaceous, was discovered in Fuyun County in northern Xinjiang.

The Bakosaurus jiangyi found in Erlianhot, Inner Mongolia, is a duck-billed dinosaur from the Early Cretaceous. It also retains many characteristics of an ornithopod and is a species that has been studied the most. The Alxa Proto-Bakosaurus and the Gobi Proto-Bakosaurus were found in Alxa, Inner Mongolia, and lived in the Early Cretaceous.

Next, let's turn our attention to southern my country. A considerable number of hadrosaurs-related fossils have also been discovered in the Guangdong and Guangxi regions. For example, the large stone Nanningsaurus discovered in Guangxi lived in the Early Cretaceous and is the first hadrosaurid discovered in southern China. The Nanxiong Small Hadrosaurus discovered in Guangdong is a smaller hadrosaurid from the Late Cretaceous.

The Nanxiong Basin in Guangdong Province is an important source of hadrosaur footprints. A new species of Nanxiong hadrosaur was named based on the footprints from the Yangmeikeng footprint site. This is the first description of the genus of hadrosaur footprints in China, expanding the distribution of the footprints from North America to China, and providing new fossil evidence for the existence of large hadrosaur in the Nanxiong Basin.

More than 10,000 dinosaur egg fossils have been unearthed in Heyuan City, Guangdong Province, which has also won the Guinness World Record for the largest collection of dinosaur egg fossils. It is an area rich in well-preserved Late Cretaceous duck-billed dinosaur egg fossils.

It can be seen that China's hadrosaur skeletal fossils are mainly found in northern China, while dinosaur footprints and dinosaur egg fossils are mainly found in the south. Why does this phenomenon occur?

There are only footprints and egg fossils in the south? The reason may be fear of dry heat!

Hadrosaurs mainly lived and reproduced along the coast and river basins, and their bones, eggs, nests and footprints are widely found in Late Cretaceous river channels and alluvial plain deposits around the world. The Upper Cretaceous in China is widely distributed, characterized by river channel, alluvial plain and foothill flood deposits with a small amount of lake phase and volcanic debris deposits, and produces a large number of hadrosaurs bones, footprints and dinosaur egg fossils.

A sketch of basin distribution in China during the Late Cretaceous

Image source: Journal of Palaeogeography

As can be seen from the figure above, hadrosaur skeletal fossils were mainly found in the northern basins of China during the Late Cretaceous, while the southern basins mainly produced hadrosaur dinosaur eggs and footprint fossils, with only a small number of skeletal fossils reported in the Guangxi and Nanxiong basins. It is speculated that there are two reasons for the north-south difference in the distribution of hadrosaur skeletal fossils:

One possibility is due to differences in research levels, with the north having a higher level of research and the south having a relatively lower level of research. Another possibility is that the climate in the south was hot and dry in the Late Cretaceous, which was not suitable for hadrosaurs to live there all year round. They only migrated or reproduced through the southern river basins in suitable seasons, so the skeletal fossils are scarcely preserved.

Although hadrosaur skeletal fossils are scarce in the southern region, a large number of hadrosaur dinosaur egg fossils belonging to hadrosaur have been reported, which are widely distributed in Guangdong, Hubei, Zhejiang and other places. Hadrosaur footprint fossils have also been found in Nanxiong, Guangdong. Therefore, the lack of Late Cretaceous hadrosaur skeletal fossils in the south is not due to differences in research levels.

Since northern and central China belonged to the subtropical to temperate zone in the Late Cretaceous, the climate was warm and humid, the flora was represented by northern subtropical to temperate climate warm and humid vegetation, and coal-bearing rock systems were widely developed. The Songliao Basin, Jiaolai Basin, Northern Jiangsu Basin, Bohai Bay Basin and South North China Basin were relatively independent large depression basins. Rivers and lakes were widespread in these large depression basins, and river and lake deposits were widely developed. Angiosperms were relatively abundant, which was very suitable for hadrosaurs to live in basins such as Jiayin, Erlian, Nanyang and Shandong Jiaolai.

In most parts of southern China, the climate was hot and arid in the Late Cretaceous, represented by tropical and subtropical dry-hot vegetation. Angiosperms were very rare in the southern region, and the sedimentary strata (such as the Maodian Formation in Jiangxi, the Guankou Formation in Sichuan, the Mankuanhe Formation in Yunnan, the Nanxiong Group in Guangdong, etc.) were characterized by paleosols and red bed deposits rich in gypsum and calcareous nodules.

The hot and dry climate in the dry season may not be suitable for hadrosaurs to live here all year round. They may only reproduce or migrate along the water systems in the southern region during the rainy season. There are a certain number of Lauraceae angiosperm fossils in Nanning, China, reflecting a tropical semi-arid environment, which may be suitable for hadrosaurs to live.

The distribution range of hadrosaur species in China is extremely limited. Each species of hadrosaur has only been found in one location or a few locations very close to each other. This shows that after these hadrosaur species spread over a large area in the Late Cretaceous, they were isolated in relatively isolated small environments. At the same time, these small environments provided an isolated and differentiated environment for hadrosaur species, leading to their rapid spread and differentiation from the Coniac to the Maastrichtian stages of the Late Cretaceous.

Conclusion

The hadrosaurs have long been submerged in the long river of history, but thanks to the efforts of scientific researchers, we have restored the original story of tens of millions of years ago. Even the hadrosaurs did not expect that their "moving" experience would be talked about by people in later generations!

References:

【1】Xing Hai. 2015. New fossil materials of hadrosauroids from China and their morphology, phylogeny and ontogeny. PhD thesis, China University of Geosciences (Beijing).

【2】Wang Rui, Jiang Baoyu. 2021. Distribution of paleo-river systems in China during the Late Cretaceous and its impact on the distribution of hadrosaurs. Acta Palaeogeographica Sinica, 23(3): 581-599.