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A reconstruction showing an asteroid hitting the Earth. The blue planet features in the foreground, as a greyasteroid hits, with a circle of debris being kicked up into the blackness of space.

It had been suggested that when the asteroid hit, (non-avian) dinosaurs were already on the path to extinction. This was unlikely the case © NASA/Donald E Davis

Dinosaurs were not in decline before the asteroid wiped them out

New research suggests that the dinosaurs were doing well, and in some cases even flourishing, before they were wiped out by an asteroid.

It goes against earlier suggestions that the non-avian dinosaurs were already on the decline and heading towards extinction when an asteroid hit Earth 66 million years ago.

When the asteroid hit the Earth at the end of the Cretaceous Period some 66 million years ago, it put an end to the reign of the dinosaurs and allowed the mammals to diversify.

Recently there have been suggestions that the downfall of the dinosaurs was all but inevitable. It has been argued that the diversity of these ancient animals was declining long before the arrival of the infamous asteroid, and if the tape of life was left to play out on its own the dinosaurs would have gone extinct anyway, asteroid or not.

A new paper has come to a different conclusion, instead showing that many groups of dinosaurs were actually doing rather well right up until the end.

Joe Bonsor is a PhD student at the Museum, who added more up to date and diverse data to conduct the computational analysis that came up with the differing view, now published in the journal Royal Society Open Science.

A reconstruction of a hadrosaur dinosaur known as Edmontosaurus. It has a large beaked head, big, elephant-like body with a long tail and four legs,. It is orange-red in colour.

Big hadrosaur dinosaurs, such as this Edmontosaurus, were doing incredibly well when the asteroid hit the Earth © DataBase Center for Life Science/Wikimedia Commons

'What we found is that the dinosaurs were still dominant, they were still widespread and still doing really well, ' explains Joe. 'If the asteroid impact had never happened then they might not have died out and they would have continued after the Cretaceous.'

Instead, Joe and his colleagues caution against the pessimistic claims that dinosaurs were already going extinct, as much of our information on these events is based on patchy and therefore biased fossil records. 

Dinosaur diversity

The history of dinosaurs spans an incredible amount of time. Having first appeared sometime during the Triassic around 230 million years ago, they evolved into a huge diversity of shapes and forms.

From tiny animals just 50 centimetres long to huge creatures pushing on 35 metres, some would go on to evolve elaborate head crests and frills, huge teeth and crushing jaws. Some were covered entirely in scales and armour, while others evolved tufts of feathers or even a complete downy covering.

It is this extraordinary range that, in part, would help them to dominate the land for over 150 million years.

Previous studies have suggested that the dominance of the dinosaurs was coming to a natural end and that this incredibly diversity was beginning to decline. They came to this conclusion by looking at the fossil record to see which species were around, and then modelling these dinosaur family trees over time.  

But only a small fraction of what was alive at any one time gets turned into a fossil. 

A reconstruction of a dinosaur nest. A number of broken eggs ie in the middle of a dirt nest, as three small bipedal dinosaurs crouch among the eggs from which they have just hatched.

Dinosaurs were an incredibly diverse group of animals, ranging from small bipedal animals to huge hulking sauropods

By looking at a greater number of dinosaur groups to include more up to date and detailed family trees, this latest piece of research questions the conclusions drawn by the earlier studies.

'The main point of what we are saying is that we don't really have enough data to know either way what would have happened to the dinosaurs,' explains Joe. 'Generally in the fossil record there is a bias towards a lack of data, and to interpret those gaps in the fossil record as an artificial decline in diversification rates isn't what we should be doing.

'Instead we've shown that there is no strong evidence for them dying out, and that the only way to know for sure is to fill in the gaps in the fossil record.'

Thriving ecosystems

The newly updated trees instead show a picture in which dinosaurs on every continent were doing as well as they ever were.

Right up to the end of the Cretaceous, huge plant-eating animals such as hadrosaurs, ceratopsians and ankylosaurs were the dominant herbivores in North America, while gigantic titanosaurs could still be found globally. These animals would have been shaping the environment in a hugely significant way. 

The head and front limbs of an animatronic T. rex. It is looking to the left of the frame, with its mouth wide open showing all its long teeth.

Some of the largest carnivores ever to exist were still stomping around right up until the end of the Cretaceous

These massive creatures would have been the main prey for the largest ever predators as tyrannosaurs stalked the herds of herbivores. But they weren't alone, as other big predators were still surviving right up until the end, with abelisaurs continuing to dominate in South America.

This all builds to show a picture in which these ancient animas were not seemingly in decline and were still in fact the main component of most land-based ecosystems.

'We might never know the true rates of evolution of dinosaurs,' says Joe. 'But the only way to know for sure is to fill in the gaps in the fossil record to get a better answer.

'We simply don't think we're there just yet. In the meantime, the data doesn't currently show they were in decline. In fact some groups were thriving and there's no evidence to suggest they would have died out around that time had the extinction event not happened.'