(CNN) — It’s 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 came?
Most of the information about the dinosaurs of the late Cretaceous comes from what is now the United States, particularly from the hell creek formationwhich offers a picture of the relatively rich diversity of dinosaurs during the last million years of that period.
However, fossil information from that time in other regions is much scarcer, and it is not known whether the pattern observed in North America is representative of the global diversity of dinosaurs at that time.
To fill this gap in the fossil record, researchers in China have 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 at the end of the Cretaceous.
The eggs and eggshell fragments represent the last 2 million years of the dinosaur age, with fossils representing each 100,000-year interval. The study consisted of 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 that period, 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 plant-eating hadrosaurid, or a member of the group of duck-billed dinosaurs.
The researchers said their find of the fossilized eggs matched fossilized dinosaur bones found in and around the same region, although they did uncover some additional dinosaur bones from the region that show tyrannosaurs and sauropods also lived in the area. between 66.4 and 68.2 million years ago.
In the image, a fossilized egg belonging to Macroolithus yaotunensis, which was examined as part of the investigation. Credit: Qiang Wang/Institute of Vertebrate Paleontology and Paleoanthropology
“Our results support a long-term decline in global dinosaur biodiversity before 66 million years ago,” the study says, “which likely produced the conditions for the late Cretaceous mass extinction of non-avian dinosaurs.”
Most of the dinosaurs died out, but some smaller, bird-like dinosaurs survived and evolved into the birds we know today.
Those who oppose the theory of sudden death due to the asteroid point to a period of global cooling that may have made life difficult for many species of dinosaurs. Its disappearance has also been linked to a series of huge volcanic eruptions in the Deccan Traps in present-day India.
Paleontologist Alfio Alessandro Chiarenza, a postdoctoral researcher at the University of Vigo, Spain, who has worked on paleoclimatic records from that 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.
In addition, eggs of many dinosaur species have not been found, even the best-known ones such as Tyrannosaurus rex, Chiarenza said.
“These results also contradict what is apparent from the egg remains and from the diversity of bones, teeth, and other remains found in places like Spain, (and) what we know from the North American record,” he said by email. . “So I think these authors are misreading these signals.”
Chiarenza remains convinced that the asteroid crash was the real driver of the extinction of the dinosaurs.
“Dinosaurs were probably fine and diverse and if it hadn’t been for the late Cretaceous asteroid (they might) be dominating today, as far as we know.”