Dinosaurs were already in decline before the asteroid.

Britain’s University of Bristol has looked at a few million years of evidence and found that it points to a grim fact about the dinosaurs. Their population was already not doing so well for the 10 million years leading up to the big extinction event:

Lead author, Fabien Condamine, a CNRS researcher from the Institut des Sciences de l’Evolution de Montpellier (France), said: “We looked at the six most abundant dinosaur families through the whole of the Cretaceous, spanning from 150 to 66 million years ago, and found that they were all evolving and expanding and clearly being successful.

“Then, 76 million years ago, they show a sudden downturn. Their rates of extinction rose and in some cases the rate of origin of new species dropped off.”

The team used Bayesian modelling techniques to account for several kinds of uncertainties such as incomplete fossil records, uncertainties over age-dating the fossils, and uncertainties about the evolutionary models. The models were each run millions of times to consider all these possible sources of error and to find whether the analyses would converge on an agreed most probable result.

Guillaume Guinot, also of the Institut des Sciences de l’Evolution de Montpellier, who helped run the calculations, added: “In all cases, we found evidence for the decline prior to the bolide impact.

“We also looked at how these dinosaur ecosystems functioned, and it became clear that the plant-eating species tended to disappear first, and this made the latest dinosaur ecosystems unstable and liable to collapse if environmental conditions became damaging.”

You can read the research here, in Nature Communications.