The death of dinosaurs is widely accepted to have been caused by the impact of an asteroid on the Earth about 66 million years ago. However, palaeontologists have long debated whether they were in decline before the mass extinction. In a study published today in the journal Nature Communications, an international team of scientists suggests that they were in decline for as much as ten million years before the death blow, reports Phys.org. Lead author Fabien Condamine 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.” Professor Mike Benton from the University of Bristol added: “In the analyses, we explored different kinds of possible causes of the dinosaur decline. It became clear that there were two main factors, first that overall climates were becoming cooler, and this made life harder for the dinosaurs which likely relied on warm temperatures. Then, the loss of herbivores made the ecosystems unstable and prone to extinction cascade. We also found that the longer-lived dinosaur species were more liable to extinction, perhaps reflecting that they could not adapt to the new conditions on Earth.” Condamine added: “This was a key moment in the evolution of life. The world had been dominated by dinosaurs for over 160 million years, and as they declined other groups began their rise to dominance, including the mammals. The dinosaurs were mostly so huge they probably hardly knew that the furry little mammals were there in the undergrowth. But the mammals began to increase in numbers of species before the dinosaurs had gone, and then after the impact they had their chance to build new kinds of ecosystems which we see today.”
