Dinosaurs were already in decline up to 10 million years before their extinction was caused by the impact of a huge asteroid on Earth, according to a study.
An international team of scientists, including from the University of Bristol, examined the six most popular dinosaur families through the Cretaceous period, spanning from around 150 to 66 million years ago.
They found the dinosaurs were all evolving and expanding over that period, but showed a sudden downturn around 76 million years ago.
The study, published in the journal Nature Communications, concludes that this means the reptiles were already in decline before their death was caused by the impact of an asteroid 66 million years ago.
Lead author Fabien Condamine, of the Institut des Sciences de l’Evolution de Montpellier in France, said: “Their rates of extinction rose and in some cases the rate of origin of new species dropped off.”
He 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.”
More than 1,600 records of dinosaurs through the Cretaceous period were used for the study.
The team used modelling techniques to account for uncertainties such as age-dating the fossils, the evolutionary models and incomplete fossil records.
Models were each run millions of times to consider all the possible sources of error and determine whether the analyses would meet on an agreed most probable result.
Guillaume Guinot, also of the Institut des Sciences de l’Evolution de Montpellier, said results showed evidence for the decline prior to the asteroid “in all cases”.
“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,” he said.
Professor Mike Benton, of the University of Bristol’s School of Earth Sciences, said the study examined different kinds of possible causes of the dinosaur decline around 76 million years ago.
“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,” Prof Benton said.
“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.”
The study, Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures, is published in Nature Communications.