Dinosaurs ‘were not in decline when an asteroid hit 66 million years ago’

It is well known that the reign of the dinosaurs came to an end when a giant asteroid smashed into Earth 66 million years ago.

But where there is less scientific consensus is whether the creatures were already in decline before the Chicxulub space rock wiped out more than 75 per cent of the planet’s species.

One recent study suggested that climate change may have been to blame for their struggles — but a new piece of research disputes this and provides the strongest evidence yet that dinosaurs were in fact thriving shortly before they were killed off. 

An international team of experts from the UK and Spain analysed 1,600 fossil records from North America to come to their conclusion. 

They wanted to establish why non-bird dinosaurs, including Tyrannosaurus rex and Triceratops, became extinct — whereas mammals and other species such as turtles and crocodiles survived.

To do this, the experts modelled the food chains and ecological habitats of land-living and freshwater animals during the last several million years of the Cretaceous, as well as the first few million years of the Paleogene period after the asteroid hit. 

They discovered that prior to the impact mammals were diversifying their diets, adapting to their environments and becoming a more important part of ecosystems as the Cretaceous unfolded. 

Dinosaurs, on the other hand, already ruled the world and were in their prime so had no reason to adapt in the same way. They had already succeeded in doing so to get to where they were.

This discovery suggests that mammals didn’t just take advantage of the dinosaurs dying, but were already creating their own advantages by diversifying prior to the asteroid impact.

Such evolution meant they had more varied diets and were better equipped to small shifts in climate, so were better prepared than the dinosaurs to cope with the abrupt destruction caused to Earth.

Study author Professor Steve Brusatte, of the University of Edinburgh, said the researchers found that dinosaurs were going strong, with stable ecosystems, right until the asteroid suddenly killed them off. 

‘Meanwhile, mammals were diversifying their diets, ecologies and behaviours while dinosaurs were still alive,’ he said.

‘So it wasn’t simply that mammals took advantage of the dinosaurs dying, but they were making their own advantages, which ecologically pre-adapted them to survive the extinction and move into niches left vacant by the dead dinosaurs.’

While previous studies have shown that a wide range of dinosaurs were on Earth just before the asteroid struck, until now it has been unclear whether they were in their prime, or already in decline. 

Another previous study, published in 2016, claimed that dinosaurs were already dwindling 50 million years before Chicxulub.

The University of Reading researchers suggested that the creatures were in long-term decline because they could not cope with the ways Earth was changing.

Their analysis showed that long-necked giant sauropod dinosaurs were declining the fastest, whereas theropods, which included T.Rex, were in a more gradual decline.

But the new study disputes this and insists that the dinosaurs were thriving until the asteroid ‘changed the ecological rules of the time’. 

Lead author, Alfio Alessandro Chiarenza, of the University of Vigo, said: ‘It seems that the stable ecology of the last dinosaurs actually hindered their survival in the wake of the asteroid impact, which abruptly changed the ecological rules of the time. 

Researchers wanted to establish why non-bird dinosaurs, including Tyrannosaurus rex and Triceratops, became extinct — whereas mammals and other species such as turtles and crocodiles survived

Researchers from the Chinese Academy of Sciences studied more than 1,000 fossilised dinosaur eggs and eggshells, and claimed in a paper published in September that the animals were already in decline when the asteroid struck. This is at odds with the latest research

‘Conversely, some birds, mammals, crocodilians, and turtles had previously been better adapted to unstable and rapid shifts in their environments, which might have made them better able to survive when things suddenly went bad when the asteroid hit.’

Dinosaurs first emerged 230 million years ago when the warm conditions from the pole to equator provided the perfect environment for them to evolve and dominate over mammals for more than 100 million years. 

Some scientists think that when the climate cooled and sea levels shifted, this was the beginning of the mammals taking over, but the authors of the latest study say the asteroid impact still played the most pivotal role because at the time dinosaurs were going strong.

Co-lead author Jorge García-Girón, of the University of León in Spain, added: ‘Our study provides a compelling picture of the ecological structure, food webs, and niches of the last dinosaur-dominated ecosystems of the Cretaceous period and the first mammal-dominated ecosystems after the asteroid hit. 

‘This helps us to understand one of the age-old mysteries of palaeontology: why all the non-bird dinosaurs died, but birds and mammals endured.’

The research is published in the journal Science Advances.