Dinosaurs were thriving before the deadly asteroid strike that wiped them out 66 million years ago, according to a new study.

For decades scientists have believed the animals were already in decline because of changes in the climate.

But researchers have now refuted that theory after new analysis suggested they were flourishing before the Cretaceous period came to a sudden end.

"Dinosaurs were likely not doomed to extinction until the end of the Cretaceous, when the asteroid hit, declaring the end of their reign and leaving the planet to animals like mammals, lizards and a minor group of surviving dinosaurs, birds," said Alessandro Chiarenza, lead researcher and PhD student at Imperial College London.

"The results of our study suggest that dinosaurs as a whole were adaptable animals, capable of coping with the environmental changes and climatic fluctuations that happened during the last few million years of the Late Cretaceous. Climate change over prolonged time scales did not cause a long-term decline of dinosaurs through the last stages of this period."

Scientists unearth 'supercroc' that dined on dinosaurs Show all 2 1 /2 Scientists unearth 'supercroc' that dined on dinosaurs Scientists unearth 'supercroc' that dined on dinosaurs 264538.bin Professor Paul Sereno with 'supercroc' Mike Hettwer/National Geographic Scientists unearth 'supercroc' that dined on dinosaurs 264539.bin A drawing of 'boarcroc' Todd Marshall/National Geographic

Experts have previously argued the number and diversity of dinosaurs was already in decline before the asteroid strike.

But the new study, published in the journal Nature Communications, suggests the change in conditions for fossilisation led previous studies to underestimate how many dinosaurs roamed the earth in the lead up to the asteroid strike.

Scientists from Imperial, University College London and Bristol University focused on North America where the majority of dinosaur fossils are found.

During the time when these huge creatures walked the Earth, the continent was split in two by a large inland sea.

Orthodox belief has generally had it that there was a dwindling population of dinosaurs because more later fossils were found in the west of America, suggesting they were already starting to die out in the east when the asteroid struck.

But the new study argues this discrepancy is actually because the ground near the Rocky Mountains in the west is in perfect condition to preserve fossils when compared to the rest of the continent.

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The team also examined the environmental conditions the dinosaurs would have endured, such as temperature and rainfall.

Their final picture suggested dinosaurs could have survived right across America – rather than simply where their fossils were found millions of years later.