The asteroid was the final nail in the coffin.
But that fatal impact only marked the end of what had been a lengthy struggle for the dinosaurs, according to research just published in Proceedings of the National Academy of Sciences.
Using a combination of statistical analyses of dinosaur family trees and information from the fossil record, researchers at the Universities of Reading, UK and Bristol, UK showed that, for some 50 million years before the asteroid hit, the rate of evolution of new species could not keep up with the pace at which old species were dying out.
The only exceptions to this pattern were some groups of plant-eating dinosaurs that seem to have proliferated right through the end of the Cretaceous Period.
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Dr. Manabu Sakamoto, the palaeontologist who led the research, said: "We were not expecting this result. While the asteroid impact is still the prime candidate for the dinosaurs' final disappearance, it is clear that they were already past their prime in an evolutionary sense."
Why they were declining for so long is unclear. But extreme environmental conditions, the break-up of continental landmasses, and frequent volcanic activity over tens of millions of years might have placed dinosaurs under tremendous stress.
Although extinction was a long time coming for the dinosaurs, we shouldn’t forget that they had an incredibly long reign on Earth — about 150 million years. By comparison, the fossils of the earliest human ancestors are only about 6 million years old.
Professor Mike Benton of the University of Bristol, one of the co-authors of the research, said: "All the evidence shows that the dinosaurs, which had already been around, dominating terrestrial ecosystems for 150 million years, somehow lost the ability to speciate fast enough. This was likely to have contributed to their inability to recover from the environmental crisis caused by the impact."
It was an environmental crisis of epic proportion. The asteroid that crashed into the Yucatan Peninsula 65 million years ago released millions of tonnes of dust into the atmosphere. The earth was thrown into darkness, which quickly killed most plant life and set off a chain reaction of extinction all the way up the food chain.
Dr. Sakamoto points out that the study might help scientists predict future biodiversity loss. "Our study strongly indicates that if a group of animals is experiencing a fast pace of extinction more so than they can replace, then they are prone to annihilation once a major catastrophe occurs,” he said.
“This has huge implications for our current and future biodiversity, given the unprecedented speed at which species are going extinct owing to the ongoing human-caused climate change."
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