(CNN) — This is a hotly contested paleontological debate. Did the massive asteroid that hit Earth 66 million years ago wipe out a thriving and diverse population of dinosaurs, or were they already struggling to survive when that cataclysmic day arrived?
Most information about Late Cretaceous dinosaurs comes from what is now the United States, especially the hell creek formationwhich offers a picture of the relatively rich diversity of dinosaurs over the last million years of this period.
However, information about fossils from this era in other regions is much scarcer, and it is unclear whether the pattern observed in North America is representative of the global diversity of dinosaurs at this time.
To fill this gap in the fossil record, Chinese researchers studied more than 1,000 fossilized dinosaur eggs from the Shanyang Basin in central China. The studypublished on September 19 in the scientific journal Proceedings of the National Academy of Sciencessuggests that dinosaur diversity was already in decline by the end of the Cretaceous.
Eggs and eggshell fragments represent the last 2 million years of dinosaur age, with fossils representing every 100,000 year interval. The study involved obtaining detailed estimates of the age of rock layers by analyzing and applying computer models to more than 5,500 geological samples.
The analysis found whole eggs and eggshell fragments from just three dinosaur species, suggesting low dinosaur biodiversity during this time, the researchers said.
The Macroolithus yaotunensis and the Elongatoolithus elongatus belonged to a group of toothless dinosaurs known as oviraptors, while the third, the Stromatoolithus pinglingensiswas a herbivorous hadrosaurid, or a member of the duck-billed dinosaur group.
The researchers said their discovery of the fossilized eggs matched fossilized dinosaur bones found in and around the same area, although they found a few additional dinosaur bones from the area that show tyrannosaurs and sauropods also lived. in the region between 66.4 and 68.2 million years ago. .
“Our results support a long-term decline in global dinosaur biodiversity before 66 million years ago,” the study states, “which likely produced the conditions for the mass extinction of non-avian dinosaurs in the Cretaceous. late”.
Most dinosaurs went extinct, but some smaller, bird-like dinosaurs survived and evolved into the birds we know today.
Those who oppose the asteroid sudden death theory point to a period of global cooling that may have made life difficult for many dinosaur species. Its demise has also been linked to a series of huge volcanic eruptions in the Deccan Traps of present-day India.
Paleontologist Alfio Alessandro Chiarenza, a postdoctoral researcher at the University of Vigo, Spain, who worked on paleoclimatic records from this period, said fossilized eggs are not a reliable record of dinosaur diversity. Chiarenza, who was not involved in the study, noted that according to recent research, many dinosaurs likely had soft-shelled eggs that would be difficult to fossilize.
Additionally, the eggs of many dinosaur species have not been found, even the best-known ones like Tyrannosaurus rex, Chiarenza said.
“These findings also contradict what is apparent from egg remains and the diversity of bones, teeth and other remains found in places like Spain, (and) what we know from North American records,” he said via email. . “So I think these authors are misinterpreting these signals.”
Chiarenza remains convinced that the asteroid crash was the real driving force behind the extinction of the dinosaurs.
“Dinosaurs were probably beautiful and diverse and if it hadn’t been for the Upper Cretaceous asteroid (they might) dominate today, as far as we know.”