It is widely accepted that a wide variety of dinosaurs lived around the world at the end of the Cretaceous period, 66 million years ago. But scientists question whether the dinosaurs were at the height of their dominance when the asteroid hit, or whether their best days had already passed. New research published Monday in the journal Proceedings of the National Academy of Sciences appears to suggest that dinosaurs were already in decline. A large amount of dinosaur research focuses on fossils found in North America, some of which suggest that dinosaurs were thriving right before the asteroid hit, and some of which suggest the opposite. By examining the fossil record in China and comparing it to this existing data, researchers at the Institute of Paleoanthropology and Vertebrate Paleoanthropy in China hoped to paint a more complete picture. They examined more than 1,000 fossilized dinosaur eggs and egg fragments excavated from the Shenyang Basin in Central China. The distinct layers of earth in which different eggshells were found allowed them to create a timeline spanning nearly two million years at the end of the Cretaceous period to examine the patterns of dinosaurs at that time. What they found suggests that dinosaur diversity in the basin had declined at the end of the Cretaceous period. Although they found evidence of some species of tyrannosaurs and sauropods, the majority of the eggs belonged to just three species. And of those three, two were from the same group of dinosaurs, a toothless species called oviraptors, ironically thought to have been egg thieves. This low diversity was maintained throughout the two-million-year time period, and when the researchers compared it with data from North America, they found that collectively it suggested that dinosaurs had already declined when the mass extinction came for them. The researchers say further study is needed to determine the causes of this decline, but they believe it could be due to global climate fluctuations and massive volcanic eruptions in what we now call India and other regions, destabilizing global ecosystems.
