The long-term decline in global dinosaur diversity and the low number of their lineages over the past million years may be due to climate fluctuations and massive volcanic eruptions from the Deccan Traps in India, researchers said. These factors may have led to ecosystem-wide instability, thus making non-avian dinosaurs vulnerable to the mass extinction that coincided with the asteroid impact, they said. A large asteroid that struck Earth at the end of the Cretaceous period — 145 to 66 million years ago — is believed to have contributed to the global extinction of the dinosaurs, leaving birds as their only living descendants. It has been widely debated whether the dinosaurs were at their peak or already in decline before their demise. Most of the scientific data on the last days of the dinosaurs comes from North America. Although some published studies suggest that the dinosaur populations there were quite thriving before the extinction, other more detailed research has suggested that the dinosaurs were in decline, which set the stage for their eventual mass extinction. Researchers from the Chinese Academy of Sciences studied over 1,000 fossilized dinosaur eggs and shells from the Shanyang Basin in central China. These fossils came from rock sequences with a total thickness of about 150 meters. The research, recently published in the journal PNAS, obtained detailed age estimates of the rock layers by analyzing and applying computer models to more than 5,500 geological samples. This allowed scientists to create a timeline of nearly 2 million years at the end of the Cretaceous — with a resolution of 100,000 years — representing the period just before the extinction. This timeline allows direct comparisons with data from around the world. Researchers found a decline in dinosaur diversity based on data from the Shanyang Basin. For example, the 1,000 fossil dinosaur eggs collected from the basin represent only three different species: Macroolithus yaotunensis, Elongatoolithus elongatus and Stromatoolithus pinglingensis, they said. The researchers also discovered that two of the three dinosaur eggs came from a group of toothless dinosaurs known as oviraptors, while the other came from the group of herbivorous hadrosaurs, also known as duck-billed dinosaurs. Some additional dinosaur bones from the area show that tyrannosaurs and sauropods also lived in the area between about 66.4 and 68.2 million years ago, they said. This low diversity of dinosaur species persisted in central China for the past 2 million years before the mass extinction, according to the researchers. These results, combined with data from North America, suggest that dinosaurs were probably declining globally before their extinction, they added.
