The three main dinosaur types that lived in the Shanyang Basin at the end of the Cretaceous Period: Oviraptorosaurus, Duck-billed Dragon, and Tyrannosaurus (painted by Zhao Chuang)
Since the advent of dinosaurs in the Late Triassic 235 million years ago, with the change of the earth’s environment, they have also staged the evolution of genera and survived on the earth for up to 170 million years until they disappeared from the earth 66 million years ago, and the mystery of their extinction has become a hot issue of concern to the world.
Scientists have not stopped searching for reasons why dinosaurs disappeared from the earth. In fact, to answer the mystery of the extinction of dinosaurs, the core is to rely on the space-time record of dinosaurs’ survival on the earth, but due to the incompleteness and regional differences in continental stratigraphic records, the solution to this problem has caused difficulties in global comparison.
Recently, the scientific research team composed of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, the Institute of Geology and Geophysics of the Chinese Academy of Sciences and the China University of Geosciences (Wuhan) and other units has carried out systematic paleontology, magnetic stratigraphy, gyropography and other work in the Shanyang Basin of Shaanxi Province, proposing a new understanding for solving the mystery of the extinction of dinosaurs. The results of the study were published as a cover article in the Proceedings of the National Academy of Sciences.
The Shanyang Basin in Shaanxi is one of the few continental basins in China that has studied the mystery of the extinction of the Late Cretaceous-Paleocene dinosaurs.
As the main carrier of dinosaurs breeding and reproducing on the earth, dinosaur eggs can not only reflect the breeding habits of dinosaurs, but also reflect the ancient environment information of dinosaurs during their survival.
The research team collected more than 1,000 in situ buried dinosaur eggs and eggshell specimens in the late Cretaceous Shanyang Formation system of the Shanyang Basin, and the dinosaur egg types were mainly Yaotun giant eggs, long eggs and Pingling stacked eggs, and the parent dinosaurs were oviraptosaurs and duck-billed dragons, which were very consistent with the dinosaur types represented by the dinosaur bone fossils in the Shanyang Basin, and there were also a small number of tyrannosaurus and sauropod bones in the basin. Dinosaur eggs and dinosaur bones indicate that the dinosaur diversity within the basin was at a relatively low level at the time.
In order to determine the geological age of the dinosaur fossil site in the Shanyang Basin, the team carried out detailed magnetic stratigraphy, biostratigraphy and gyratory stratigraphy in the Shanyang Basin. The researchers collected 3538 paleomagneto-oriented samples on 4 representative profiles of the Shanyang Basin, and 9 polar bands were identified after analyzing the results of the samples paleomagnetic experiments.
In the Shanyang Basin, a sign layer consisting of a conglomerate layer and a green round spot layer can be identified, under which a large number of in situ buried Cretaceous dinosaur fossils appear under the sign layer, and above the sign layer there are Paleocene-specific archodontidae, according to which it can be determined that the sign layer is a transitional stratum from Cretaceous to Paleogenesis, including the Cretaceous-Paleogene boundary (KPB). The anomagnetic samples within this marker layer all record negative polarity (R2), so this negative polarity band (R2) can be uniquely corresponded to the 29th negative polarity period (C29r) (previous studies have shown that KPB is located at C29r). The age of the C29r polarity period ranges from 66.38 million years to 65.7 million years, and the stratigraphic age of this polarity (R2) recorded in the Shanyang Basin can be precisely defined.
Regional geological background analysis shows that the Shanyang Basin did not experience major tectonic activities during the Late Cretaceous to Paleogene period, and its sedimentary strata were basically continuous. Therefore, the remaining 8 polar bands can be corresponded to the standard geomagnetic polarity sequence to determine the stratigraphic age of the different layers of the Shanyang Basin.
In order to further improve the age accuracy of the strata, the researchers continuously collected 5466 rock specimens at 5 cm intervals in the Shanyang Basin, and divided the stratigraphic age by using the astronomical orbital tuning method to divide the stratigraphic age by 100,000 years based on the results of magnetic stratigraphy. So far, the researchers have obtained the high-resolution age of 44 continuous dinosaur egg fossil layers in the Shanyang Basin, which provides a reliable chronological basis for analyzing the changes in dinosaur diversity and its driving factors in the late Cretaceous period.
Chronological results show that the time limit for the distribution of dinosaur fossils in the Shanyang Basin is 68.24 million to 66.38 million years ago, indicating that the dinosaur diversity in the Shanyang Basin has been at a relatively low level for about 2 million years before its extinction.
Combining dinosaur fossils found in other Late Cretaceous basins in eastern Qinling, as well as in Laiyang, Shandong, and Nanxiong, Guangdong, the researchers found that there was a significant downward trend in dinosaur diversity in China around 72 million years, which is similar to the distribution characteristics of dinosaur fossils in western North America, so the study believes that the decline of dinosaur diversity in the Late Cretaceous is likely to be a global phenomenon.
The results of this study provide a new basis and perspective for us to understand the extinction process and mechanism of dinosaurs. Dinosaurs are oviparous animals, and the hatching success rate of dinosaur eggs directly determines the prosperity of dinosaur populations. The hatching of dinosaur eggs required the right temperature, humidity and even carbon dioxide concentration.
Therefore, the study argues that during the Late Cretaceous period, with the co-evolution of natural ecosystems and dinosaurs themselves, dinosaur diversity underwent a sustained decline, reducing the environmental adaptability of the dinosaur taxa and leading to its inability to survive and recover from environmental upheavals caused by major catastrophic events such as Deccan eruptions or asteroid impacts, and eventually to extinction.
The research was jointly funded by the National Natural Science Foundation of China and the Pilot Project of the Chinese Academy of Sciences. (Source: China Science Daily Cui Xueqin)
Related Paper Information:https://doi.org/10.1073/pnas.2211234119