Fossil eggs from China suggest that dinosaur numbers and diversity were declining even before the asteroid impact which is widely believed to have caused a mass extinction 66 million years ago.
The asteroid which hit Earth at the end of the Cretaceous period saw the end of the age of the dinosaurs and is said to have killed off three-quarters of all species on Earth.
Striking waters off what is now Mexico, the 10-kilometre asteroid unleashed energy equivalent to 10 billion atomic bombs. According to the impact-driven extinction theory, all the non-avian dinosaurs were wiped out either by the explosion itself, or the acid rain and “impact winter” believed to have lasted decades after it.
The removal of the non-avian dinosaurs from the scene set the stage for the emergence of mammals as Earth’s dominant animal group in the subsequent Palaeogene geological period which spanned the next 43 million years.
But there is ongoing debate among palaeontologists as to whether dinosaurs were at their peak, or already on the decline in the late Cretaceous.
New research published in the Proceedings of the National Academy of Sciences (PNAS) journal seem to add weight to the suggestion that dinosaurs were already on the way out even before those fateful events.
Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences have found evidence that dinosaur diversity in central China was waning in the couple of million years before their extinction.
The researchers hope to determine whether this trend extends throughout Asia.
While the jury is still out in the palaeontological community over this hypothesis, the study in China does back up other detailed research, predominantly from North America, which suggests that dinosaur populations were already in a state of decline.
The researchers looked at over 1000 fossilised dinosaur egg fragments from the Shanyang Basin in central China.
Fossils were uncovered from rock sequences totalling around 150 metres in thickness. Over 5,500 Rock samples were analysed using computer modelling to accurately date the fossils. Overall, the fossils spanned a nearly 2-million-year period right up to the end of the Cretaceous and allowed the palaeontologists to zoom in to 100,000-year intervals to study the relative abundance of dinosaur species in the sediments.
What they found was a decline in dinosaur diversity in the region. The 1,000 dinosaur egg fossils came from only three species: Macroolithus yaotunensis, Elongatoolithus elongatus, and Stromatoolithus pinglingensis. Two of these are from a group of toothless dinosaurs known as oviraptors, while the other is from the herbivorous, duck-billed hadrosaurid group.
Additional fossil bones from the area show that a tyrannosaur and sauropod (large-bodied, long-necked dinosaur) also lived there in the same time period between about 66.4 and 68.2 million years ago.
A picture emerges of a worldwide decline in dinosaur diversity in the last days of the Cretaceous.
Such a sustained low number of dinosaur species for their last few million years may have resulted from global climate fluctuations and massive volcanic eruptions. There are such events known in the geological record like the Deccan Traps in India.
“Our results demonstrate low dinosaur biodiversity during the last 2 million years of the Cretaceous, and those data indicate a decline in dinosaur biodiversity millions of years before the Cretaceous/Paleogene boundary,” the authors write. “The end-Cretaceous catastrophic events, such as the Deccan Traps and bolide impact, probably acted on an already vulnerable ecosystem and led to nonavian dinosaur extinction.”