Dinosaurs were in decline long before the asteroid struck that spelt their doom, new research suggests.

Dinosaurs were on the up and up from the late Triassic about 220m years ago, with new species arising faster than others went extinct. But the study reveals their fortunes had begun to change long before the catastrophic six-mile-wide asteroid hit what is now the Gulf of Mexico, 66m years ago.

The research, the authors believe, could resolve a longstanding controversy among palaeontologists.

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“One of the things that has been long debated about dinosaur evolution is whether they were reigning strong right up until the time of the meteorite impact, or whether there was a slow, gradual decrease in [the emergence of new species] or an increase in extinction before that time,” said Chris Venditti, an evolutionary biologist at the University of Reading and an author of the paper.

Writing in the Proceedings of the National Academy of Sciences, researchers from the University of Bristol and the University of Reading describe how they carried out a new kind of statistical analysis based on large “family trees” of dinosaurs, allowing them to explore the rise and fall of species as time marched on. The study looked at the three major groups of dinosaur: the ornithischians (beaked herbivores such as Stegosaurus), theropods (flesh-eating beasts such as Tyrannosaurus rex) and sauropods (long-necked plant-eaters that included Diplodocus).

The results showed that while dinosaurs flourished from about 220m years ago, with species arising faster than they went extinct, about 140m years ago their success started to stall. Eventually, about 90m years ago, 24m years before the asteroid hit, the dinosaurs entered a long-term decline, with species going extinct faster than new ones emerged.

That, the authors reveal, could have left the dinosaurs more vulnerable to extinction when the six-mile-wide disaster struck. “Because diversity was lacking, because species were going extinct and not being replaced, it might have made them more susceptible,” said Venditti.

But it was not all doom and gloom. While dinosaurs overall experienced a downturn, some were on the up: duck-billed dinosaurs and horn-faced dinosaurs appear to have been booming just before the impact occurred. While the study does not reveal the reasons for their success, the authors suggest the rise of flowering plants could have helped. “They might have been able to exploit new plant matter that the others couldn’t,” said Venditti.

Further insights were gleaned when the researchers teased apart the trends for each of the three major dinosaur groups. Theropods, they discovered, went into decline around 120m years ago, with sauropods following around 6m years later. At that timethe ornisthischians – apart from the duck-billed and horn-faced dinosaurs – also experienced a downturn.

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The research also suggests that rising sea levels could have helped fuel the emergence of new species by fragmenting landscapes and isolating creatures. But Venditti believes a number of phenomena could have played a part in the loss of dinosaur species, from changes in global climate to mammals competing for resources and even nibbling at dinosaur eggs. “What is certainly true is that the decline of the dinosaurs would have left room for other groups of animals like mammals to fare better when that impact hit,” added Venditti.

While the authors stress that the asteroid impact 66m years ago ultimately marked the end of the dinosaurs’ reign, Venditti believes their days may already have been numbered. “If they continued on that trajectory, even if that meteor didn’t hit, they may well have been very species-poor in some millions of years or even have gone extinct all together,” he said.

Not that everyone agrees. While palaeontologist Stephen Brusatte from the University of Edinburgh describes the decline as “very, very plausible,” he believes that doesn’t mean dinosaurs were “wasting away” towards an inevitable end. Rather, he says, the fact that the duck-billed and horn-faced dinosaurs were doing well suggests that dinosaurs could have bounced back.

“It may be that the effects of the asteroid were a bit worse because you had dinosaurs that maybe weren’t as strong in an evolutionary sense as they once had been,” he said. “But I think if there was no asteroid you would still have dinosaurs around today.”