Dinosaurs may have already been declining in biodiversity in large numbers before they disappeared some 66 million years ago, adding new insight to the theory about their extinction, a new study by Chinese researchers suggests.

Analyzing dinosaur fossil records derived from eggs, eggshells, and bones discovered in central China’s Shanyang Basin, researchers from the Chinese Academy of Sciences found that the prehistoric reptiles that roamed the earth for millions of years had decreased in numbers and variety during the final 2 million years of the late Cretaceous Period in China. They said this may have weakened the dinosaurs’ ability to adapt to the changing environment.

The new findings were published in the academic journal Proceedings of the National Academy of Sciences on Monday.

“We are still trying to understand what caused this,” Wang Qiang, a researcher of paleontology at the Institute of Vertebrate Paleontology and Paleoanthropology at the CAS and a lead author of the study, told Sixth Tone. “Our preliminary hypothesis is that it is linked with the change of global temperatures in the late Cretaceous, which affected the hatchability of the dinosaur eggs.”

The extinction of dinosaurs is hotly debated among scientists. While some believe that it happened abruptly and coincided with the impact of an asteroid that hit parts of what is today’s Mexico, others argue that it was a gradual process lasting over millions of years with the asteroid worsening their chances of survival.

Researchers analyzed samples of dinosaur eggshells and rock sediment to identify hadrosaurs, or duck-billed dinosaurs, and the birdlike carnivorous oviraptorosaurs. They’re believed to be the two primary dinosaur groups that lived in the Shanyang Basin, located in the Qinling Mountains which split China’s northern and southern regions.

Wang said the new findings present new evidence regarding the theory that the demise of dinosaurs was a long-term process and that the global decline of their biodiversity had already begun before their mass extinction. However, he added that further research was needed to determine whether the findings apply to areas beyond East Asia.

The study’s lead author said the decrease in biodiversity damaged the dinosaurs’ ability to “repair and restore themselves” when facing disasters, such as the impact from an asteroid in Mexico and the volcanic activity in India’s Deccan Traps which preceded large-scale environmental destruction.

“It is similar to the demise of many creatures that we now witness every year,” said Wang, referring to how a loss in biodiversity contributed to their extinction. “Due to the overall decrease in overall variety, they were less able to adapt to the changing environment.”

Editor: Bibek Bhandari.

(Header image: Details from a depiction of dinosaurs that lived in central China. Courtesy of Chuang Zhao)