Dinosaurs were not very diverse and had already declined in general even before they became extinct nearly 66 million years ago.
The impact of asteroids on Earth was not the only reason for the extinction of dinosaurs. Dinosaurs weren’t very diverse and, in general, had already declined before going extinct nearly 66 million years ago, a study of fossilized eggs in China suggests.
The prolonged decline in global dinosaur diversity and the continued low number of their descendants over the past few million years may be due to climate fluctuations and massive volcanic eruptions from India’s Deccan Traps, the researchers said.
These factors may have led to instability throughout the ecosystem, leaving non-avian dinosaurs vulnerable to mass extinctions coinciding with the asteroid impact, they said.
A large asteroid that hit Earth at the end of the Cretaceous Period — 145 to 66 million years ago — is believed to have contributed to the global extinction of dinosaurs, leaving birds as their only living descendants.
There has been much debate about whether dinosaurs were at their peak or already in decline before they died.
Most of the scientific data on the last days of the dinosaurs comes from North America.
While some published research suggests that dinosaur populations there thrived quite well before going extinct, other, more detailed research has suggested that dinosaurs were in decline, setting the stage for their eventual mass extinction.
Researchers at the Chinese Academy of Sciences studied more than 1,000 fossilized dinosaur eggs and eggshells from the Shanyang Basin in central China.
These fossils come from rock sequences with a total thickness of about 150 meters.
The research, recently published in the journal PNAS, obtained detailed age estimates of the rock layers by analyzing and applying computer modeling to more than 5,500 geological samples.
This allowed the scientists to create a timeline spanning nearly 2 million years to the end of the Cretaceous Period — with a 100,000-year resolution — representing the period just before extinction.
This timeline allows for direct comparisons with data from around the world. The researchers 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 just three different species: Macroolithus yaotunensis, Elongatoolithus elongatus and Stromatoolithus pinglingensis, they said.
The researchers also found that two of the three dinosaur eggs came from a group of toothless dinosaurs known as oviraptors, while the other is from the herbivorous hadrosaurid group, also known as duckbill dinosaurs.
A few additional dinosaur bones from the region show that tyrannosaurus and sauropod also lived in the area between 66.4 and 68.2 million years ago, they said.
This low diversity of dinosaur species was maintained in central China for the last 2 million years before the mass extinction, according to the researchers.
These results, combined with data from North America, suggest that dinosaurs were likely in decline worldwide before they became extinct, she added.