Paris (AFP)

Long before the fall of the asteroid which ended their reign 65 million years ago, dinosaurs were already in decline, victims of a cooling of the planet's climate, according to a study published on Tuesday.

The responsibility of the extraterrestrial object in their disappearance has been a consensus within the scientific community, since the discovery of the gigantic impact crater, in 1980 in Mexico.

The collision caused such a shock wave that the Earth was enveloped in a cloud of dust and gas, disrupting the climate and permanently wiping off the map three quarters of species, including non-avian dinosaurs.

But paleontologists are arguing over whether this mass extinction came abruptly, or whether the meteorite only dealt the final blow to an already struggling group.

"It's a debate at loggerheads," biologist Fabien Condamine, author of the study published in Nature Communications, told AFP.

Because the data are not robust enough to validate one hypothesis rather than another: "the fossil record (dinosaur bones, editor's note) is incomplete, in poor state of conservation, certain geographical areas such as the tropics are poorly represented, periods of time are better informed than others ... ", develops the CNRS researcher at the Institute of Evolutionary Sciences in Montpellier.

- 1,600 fossils -

To correct these biases, the researcher and his Franco-Anglo-Canadian team used a new statistical modeling method, called Bayesian, which calculates the probabilities of the causes from observation of known elements.

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They selected 1,600 fossils belonging to six families of dinosaurs, including the iconic tyrannosaurs, triceratops and hadrosaurs, popularized by "Jurassic Park".

Three families of herbivores, three of carnivores, rich in 247 species.

Each fossil has been assigned a code making it possible to trace the successive diagnoses made by scientists since their discovery.

Thanks to their model, the researchers were able to estimate an age of appearance and extinction for each species, over a period ranging from 145 million years, at the end of the Cretaceous, to 66 million years, shortly before the catastrophe.

Result: "we see a peak in diversity before 76 million years ago, with a high rate of creation of new species. Then, a slow decline", explains Fabien Condamine.

Thus, over a colossal period of 10 million years - much longer than the reign of the genus Homo - the number of species of dinosaurs decreases, from about fifty to less than twenty 66 million years ago. 'years.

The researchers compared these statistics with environmental data (climatic, marine, geological, etc.) already known and undisputed.

- drop from 7 to 8 degrees -

The results showed a perfect correlation with the climatic curves: "As the rate of disappearance of the species increases, in mirror, the temperatures fall", analyzes the expert in macroevolution.

This big cooling, which caused the Earth to lose 7 to 8 degrees, is starting "right on target" at the same time as that of the decline of the dinosaurs.

“At the time, it was much warmer, there were palm trees and mangrove forests in the Bering Strait,” between Siberia and today's Alaska, explains Fabien Condamine.

However, these giants are not able to produce their own body heat, as humans do.

They depended heavily on their environment for food, movement, reproduction ... Their metabolism would therefore not have been able to adapt to change.

Another key variable: the first disappearances first affected herbivores, around two million years before carnivores.

The depletion of herbivores, prey of carnivores, would have unbalanced ecosystems and led to cascading extinctions among other dinosaur families, the study suggests.

It is in this context that a meteorite 12 km in diameter struck the Earth.

Already weakened, the reign of the giants could not recover from this cataclysm, unlike some small mammals.

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