A study of fossilized eggs in China indicated that dinosaurs were not very diverse, and generally declined even before their extinction nearly 66 million years ago.
The long-term decline in global dinosaur diversity and a continuing decline in the number of dinosaur species over the past few million may have resulted from climate fluctuations and massive volcanic eruptions from the Deccan Traps in India, the researchers said.
These factors, they said, may have led to ecosystem-wide instability, making non-avian dinosaurs vulnerable to mass extinctions in conjunction with the asteroid impact.
A large asteroid that struck Earth at the end of the Cretan period – 145 to 66 million years ago – is believed to have contributed to the global extinction of the dinosaurs, leaving birds as their only living descendants.
It has been widely debated whether dinosaurs were at their peak or were already in decline before their demise.
Most of the scientific data on the last days of dinosaurs comes from North America.
Although some published studies indicate that dinosaur groups were thriving well before the extinction, other, more detailed research has indicated that dinosaurs were in decline, paving the way for their eventual mass extinction.
Researchers from the Chinese Academy of Sciences studied more than 1,000 fossilized dinosaur eggs and eggshells from the Shanyang Basin in central China.
These fossils came from rock chains with a total thickness of about 150 meters.
The research, recently published in the journal PNAS, obtained detailed age estimates for rock layers by analyzing and applying computer modeling to more than 5,500 geological samples.
This allowed scientists to put together a timeline of nearly two million years at the end of the Cretaceous period – to an accuracy of 100,000 years – representing the period immediately leading up to the extinction.
This timeline allows direct comparisons to be made with data from around the world. The researchers identified a decrease in dinosaur diversity based on data from the Shanyang Basin.
For example, the 1,000 dinosaur egg fossils collected from the aquarium 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 belonged to a group of toothless dinosaurs known as aviraptors, while the other was from the group of plant-eating hadrosaurids, also known as duck-billed dinosaurs.
They said a few additional dinosaur bones from the area show that tyrannosaurus and sauropod also lived in the area between about 66.4 and 68.2 million years ago.
This low diversity of dinosaur species persisted in central China for the past 2 million years before the mass extinction, according to the researchers.
They added that these findings, along with data from North America, suggest that dinosaurs may have been in decline globally before they became extinct.