Dinosaurs had already declined in China two million years before the extinction event

(Donald E. Davis / Wikimedia Commons https://bit.ly/3S1anCC)

Scientists of the China University of Geosciences examined dinosaur eggshell fossils from the Shanyang Basin in Central China.

The team of geologists and paleontologists, led by Fei Han concluded that the dinosaurs in the region experienced a two-million-year decline before becoming extinct.

This new study supports the hypothesis that the asteroid impact only hastened the extinction of the dinosaurs.

The Cretaceous-Paleogene extinction event, which occurred 66 million years ago, affected many groups of animals, and caused the extinction of non-avian dinosaurs.

At present, the impact hypothesis of extinction prevails among scientists: according to it, an asteroid with a diameter of about ten kilometers caused the biospheric crisis.

However, there is an alternative point of view, in which the extinction was caused by volcanism on the Indian plate.

In case, paleontologists disagree on the details of the extinction of dinosaurs: some experts believe that their extinction was sudden, others believe that this group was already in decline and the impact event simply accelerated their extinction.

The dispute is complicated because a detailed and continuous paleontological record of terrestrial vertebrates in the late Cretaceous is known only for North America.

This may not reflect the state of affairs in the rest of the world.

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THE STUDY
Han's multidisciplinary team studied in detail the state of the Late Cretaceous dinosaur fauna in Central China.

Using cyclostratigraphy, magnetostratigraphy, and biostratigraphy, the researchers have dated rocks from the Shanyang Basin in East Qinling to within 100,000 years.

It turned out that the basin contains continuous deposits ranging in age from 71.7 to 64.6 million years, this means it covers the end of the Cretaceous and the beginning of the Paleogene.

At the same time, fossil eggshells and whole dinosaur eggs found here come from deposits ranging in age from 68.2 to 66.4 million years.

The team also found bones of hadrosaurids, sauropods and theropods, including an unknown tyrannosaurid in these deposits.

The team analyzed more than 1,000 fossil dinosaur shells and eggs and determined that they belong to just three ootaxa:

• Macroolithus yaotunensis
• Elongatoolithus elongatus
• Stromatoolithus pinglingensis

The first two ootaxa belong to the Elongatoolithidae and most likely belong to small theropod oviraptorosaurs, while S. pinglingensis is attributed to hadrosaurids.

According to previous work, the diversity of ootaxa and dinosaur taxa is higher in earlier deposits of other East Qinling basins.

The results of scientists suggest a low level of diversity of dinosaurs in this region for almost two million years before the extinction.

The scientists tentatively attribute this decline to climatic changes at the end of the Cretaceous.

However, it is necessary to increase the sample of taxa, its spatiotemporal coverage, and also use high-precision dating of fossils to finally resolve the dispute about the extinction of dinosaurs.

Source:

• Proceedings of the National Academy of Sciences: https://www.pnas.org/doi/full/10.1073/pnas.2211234119
• SCMP: https://www.scmp.com/news/china/science/article/3193044/ancient-eggshells-show-dinosaurs-already-their-way-out-asteroid

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