New discovery! New time limit of Yanliao Biota: a legend of life spanning tens of millions of years

Produced by: Science Popularization China

Author: Zou Zuyang (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences)

Producer: China Science Expo

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The Anthropocene is only a brief moment in the long history of life on Earth. Long before humans appeared, there were many brilliant and declining epics of life, among which dinosaurs are the most well-known. Thanks to well-preserved fossil materials, scientists can restore some of the features of history.

Since the 1990s, the release of a series of movies such as "Jurassic Park" and "Jurassic World" have firmly linked the Jurassic and dinosaurs together.

(Photo source: veer photo gallery)

my country is a country of dinosaurs. The "Yanliao Biota" with the southeastern Inner Mongolia Autonomous Region, northern Hebei Province and western Liaoning Province as its core distribution area was discovered in the Jurassic strata. It has produced a large number of beautifully preserved fossils of various categories, which is an important window for us to understand the Mesozoic terrestrial ecosystem. It is of great significance to understand the early evolution of mammals, the earliest feathered dinosaurs, the origin of early insects and angiosperms and other scientific issues.

The Yanliao biota is distributed in the lake-facies strata of multiple volcanic sedimentary basins. The lake-facies strata, which are mainly composed of conglomerate and shale, are interspersed with multiple layers of volcanic ash (tuff). Volcanic ash is an ideal dating object for geological history. It is the debris formed by volcanic explosive eruptions. It erupts from the crater into the atmosphere, is transported by the atmosphere, and then settles in various environments to be preserved and consolidated into rocks. The whole process is very short and can be regarded as a moment in the history of the earth. Its age can be regarded as an absolute time to limit geological events.

Representative basins of the Yanliao biota

I am a doctoral student in the team of Academician Xu Yigang of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. My main research object is the Jurassic volcanic activity on the northern margin of the North China Craton. Considering the close temporal and spatial connection between the Jurassic volcanic activity and the evolution of the biota in this region, we focused on the volcanic rock samples related to the Yanliao biota, hoping to establish an accurate age scale that can be compared regionally. Through research on previous studies of the Yanliao biota, the author found that their discussion of its age focused on the two most representative basins, namely the Ningcheng Basin in Inner Mongolia and the Jianchang Basin in Liaoning.

Unlike the Jianchang Basin, which has a definite stratigraphic and chronological understanding, the age and stratigraphic affiliation of the volcanic-sedimentary strata hosting the biota in the Ningcheng Basin have remained unresolved. There are many opinions, including being classified as the Jiulongshan Formation or Haifanggou Formation of the Middle Jurassic, the Tiaojishan Formation of the Middle and Upper Jurassic, and the Yixian Formation of the Lower Cretaceous.

We believe that the main reasons for the above differences of opinion are the unclear relationship between the fossil layers in the Ningcheng Basin and their overlying volcanic lava, as well as the lack of direct age data constraints on the fossil layers.

In recent years, Ningcheng has developed a number of bentonite mines to establish China's largest bentonite cat litter production base, exposing many fresh geological sections, which provide us with a good reference for correctly understanding the contact relationship of the strata.

Section of the three-province boundary marker on the west side of Ningcheng Basin

Image source: Ma Qiang

Reconstruction of stratigraphic sequence of the Yanliao biota

In response to this scientific question, we conducted detailed field investigations in the Ningcheng Basin in Inner Mongolia with experts from the Institute of Geology and Geophysics, Chinese Academy of Sciences, the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, and the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and examined many sections with clear contact relationships. Under the guidance of Researcher Xu Yigang and Professor Ma Qiang of Wuhan University of Geosciences, we remapped the area and found that the Jurassic strata in the Ningcheng Basin can be divided into four rock series from bottom to top, and identified two early and late fossil-bearing sedimentary units, namely, the early fossil layer is a lacustrine tuffaceous sedimentary layer under the volcanic rock, and the late fossil layer is a sedimentary interlayer in the volcanic rock.

Section of Guanzaishan Mountain on the east side of Ningcheng Basin

Image source: Ma Qiang

In addition, we realize that the Jurassic strata in the basin are unconformably located on the ancient basement of the Precambrian system, and the strata are unevenly distributed in space. The strata on the east side are relatively intact, while only the early fossil layers are preserved on the west side. This may mean that the sedimentation center gradually migrated to the northeast in history. These geological changes are the culprits for the difficulty in comparing strata today.

A hand-drawn diagram of the unconformity contact by Professor Meng Qingren in the field

Source: Photographed by the author

Determination of geological age of Yanliao Biota

On this basis, we further obtained tuff and adjacent volcanic lava samples in the fossil layer of the Ningcheng Basin. Zircon, an accessory mineral contained in tuff and volcanic lava, is one of the most important and widely used minerals in isotope geochronology because of its stable physical and chemical properties and high closure temperature, which can successfully preserve the closed U-Pb decay system.

Some zircon images of dating samples (ovals represent ion probe points)

Source: Photographed by the author

The U-Pb decay system refers to the fact that the radioactive isotope of uranium is unstable and will eventually become stable lead after a series of decays. Although the decay occurs randomly at the microscopic level, the time required for the radioactive parent to decay to half of its original amount is fixed (half-life) at the macroscopic level. It is not affected by environmental conditions and depends only on the element itself. With this habit of radioactive isotopes, we can reverse the time it has taken for it to decay to date through the current isotope ratio of the mineral (it must be a closed system). The half-life of uranium is very long and is suitable for dating ancient samples. We finally obtained the age of the geological samples by means of high-precision secondary ion mass spectrometry (SIMS). The results show that the four rock series units in the Ningcheng Basin are gradually getting younger from bottom to top, which is consistent with stratigraphic observations.

By comparing with the nearby Jianchang, Luanping and Beipiao basins, we found that the Yanliao biota in northern China spanned more than 11 million years, and its development stage can be divided into two periods, the first period was earlier than 167-163Ma (million years), and the second period was 161-156Ma. The former is represented by the early fossil layers of the Ningcheng Basin, and the latter is represented by the late fossil layers of the Ningcheng Basin and the Jianchang Basin fossil layers.

This study on the Yanliao Biota not only reestablished a clear stratigraphic sequence, but also redefined the time span of this paleontological community, laying a solid foundation for a deeper understanding of the evolution of Jurassic organisms.

Perhaps in the near future, in the interdisciplinary research fields of paleontology, paleoenvironment and geochemistry, we will see how the Yanliao Biota can further help scientists reveal the evolutionary process and development laws of life on Earth, and how deep geological processes affect the response of shallow ecological environments, thereby providing inspiration for current environmental and ecological protection.