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Dinosaurs were heading for extinction millions of years before the meteor impact that is widely believed to have wiped them out, according to Reading Uni scientists.
Until now, most experts have thought dinosaurs were flourishing just before a huge asteroid or comet smashed into the Earth off the coast of Mexico.
New research suggests that in reality, more dinosaur species were disappearing than new ones were emerging up to 50 million years earlier.
Giant long-necked plant-eaters such as Diplodocus were vanishing at the fastest rate, while meat-eating relatives of Tyrannosaurus rex were in more gradual decline.
The scientists, whose findings are based on sophisticated statistical analysis and fossil evidence, believe the meteor impact 66 million years ago finished off a process that was already well under way.
Lead researcher Dr Manabu Sakamoto, from the University of Reading, said: "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.
"Our work is ground-breaking in that, once again, it will change our understanding of the fate of these mighty creatures.
"While a sudden apocalypse may have been the final nail in the coffin, something else had already been preventing dinosaurs from evolving new species as fast as old species were dying out.
"This suggests that for tens of millions of years before their ultimate demise, dinosaurs were beginning to lose their edge as the dominant species on Earth."
The scientists say that for unexplained reasons, possibly linked to the break-up of continental land masses and sustained volcanic activity, dinosaurs stopped producing enough new species to replace those that were lost.
Because of this, they were unable to recover from the environmental crisis caused by the meteor impact, which blacked out the sun with millions of tons of dust, cooled the global climate and caused widespread loss of vegetation.
Huge herbivores with massive appetites would not have been able to feed themselves, and their predators in turn would have been left hungry.
A long gradual dinosaur decline may have set the stage for mammals, according to the researchers, whose findings appear in the journal Proceedings Of The National Academy Of Sciences.
Co-author Dr Chris Venditti, also from the University of Reading, said: "The decline of the dinosaurs would have left plenty of room for mammals, the group of species which humans are a member of, to flourish before the impact, priming them to replace dinosaurs as the dominant animals on Earth."