Fossil Eggs Show Dinosaur Decline Before End-Cretaceous Extinction

Nearly 66 million years ago, a large asteroid hit Earth and contributed to the global extinction of dinosaurs, allowing the rise of mammals and leaving birds as their only living descendants.

Scientists know that a wide variety of dinosaurs lived around the world at the end of the Cretaceous period just before their extinction. However, scientists have debated whether dinosaurs were at their peak or already in decline prior to their demise. In other words, did dinosaurs go out with a bang or a whimper?

An international team of researchers now has a possible answer. They’ve found evidence to support the hypothesis that dinosaurs were not very diverse before their extinction and had declined overall during the last part of the Cretaceous.

Most of the scientific data on the last days of the dinosaurs come from North America. Although some published studies suggest that dinosaur populations there were thriving quite well before extinction, other more detailed research has suggested that dinosaurs were instead in decline, which set the stage for their eventual mass extinction.

By examining the dinosaur record in China, the researchers hoped to determine whether this declining trend extended to Asia as well.

They studied over 1,000 fossilized dinosaur eggs and eggshells from the Shanyang Basin in central China. These fossils came from rock sequences with a total thickness of approximately 150 meters. The researchers obtained detailed age estimates of the rock layers by analyzing and applying computer modeling to over 5,500 geological samples. This allowed the scientists to create a timeline of nearly 2 million years at the end of the Cretaceous – with a resolution of 100,000 years – representing the period right before extinction. This timeline allows direct comparisons with data from around the world.

The scientists identified a decline in dinosaur diversity based on the Shanyang Basin data. For example, the 1,000 dinosaur egg fossils collected from the basin represent only three different species: Two of species are from a group of toothless dinosaurs known as oviraptors, while the other is from the plant-eating hadrosaurid group (also known as duck-billed dinosaurs).

A few additional dinosaur bones from the region show that tyrannosaur and sauropod species also lived in the area between about 66.4 and 68.2 million years ago.

The small number of species – in concert with data from North America – suggest that dinosaurs were probably declining globally before their extinction.

This worldwide, long-term decline in dinosaur diversity through the end of the Cretaceous Period and sustained low number of dinosaur lineages for the last few million years may have resulted from global climate fluctuations and massive volcanic eruptions poisoning the atmosphere.

These factors may have led to ecosystem-wide instability, thus making non-bird dinosaurs vulnerable to mass extinction coincident with the asteroid impact.

The paper “Low dinosaur biodiversity in central China 2 million years prior to the end-Cretaceous mass extinction” is published in Proceedings of the National Academy of Sciences (2022). Materials provided by the Chinese Academy of Sciences.

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