It's the most dramatic mass extinction in the history of Earth. About 66 million years ago, a giant meteorite smashed into the Gulf of Mexico, sending toxic gases into the atmosphere and causing extreme climate change that wiped out most dinosaurs. Except that's not the whole story. Mass extinction, like modern love, is complicated. A new study from a group of UK researchers reveals that most dinosaur clades were already in decline long before the Chicxulub impact that changed Earth's ecosystems forever.

After an exhaustive statistical analysis of dinosaur fossil frequency over time, the researchers published their findings in Proceedings of the National Academy of Sciences. What they found was that dinosaur populations were declining about 24 million years before the bolide from space smashed into our planet. The researchers show that dinosaurs from three major sub-clades—Ornithischia, Sauropodomorpha, and Theropoda—reached a deadly tipping point about 90 million years ago. That's when dinosaur species began going extinct at a higher rate than they were speciating, or spawning new species. Put simply, new dinosaur species weren't evolving to replace the ones that went extinct.

Extinction is a normal part of the lifecycle of any species, but in a healthy clade you expect to see new species evolving at the same or higher rate than they are going extinct. This wasn't so among most dinosaurs for millions of years before the Chicxulub event. That said, a few lucky dinosaur subclades, Hadrosauriformes and Ceratopsidae, actually saw speciation rates rising during the tens of millions of years before the meteorite impact. Indeed, there is evidence that Hadrosaurs lived for hundreds of thousands of years after the meteorite impact. And of course, early mammals were running around and happily speciating in the millions of years before and after the meteorite impact.

This "gradual decline" scenario suggests that the Chicxulub catastrophe wasn't so much the cause of dinosaur extinction as it was the final death blow to an already-weakened group of animals.

There are a number of reasons why the dinosaurs began dying off 90 million years ago. The planet was undergoing a major transformation, as the Earth's two supercontinents broke apart, and sea levels fluctuated wildly. At the same time, megavolcanoes were erupting in India's Deccan Traps, which geoscientist Gerta Keller has long argued spurred extinctions among dinosaurs in the seas and on land. All of these events, taken together, would have had the effect of shrinking the dinosaurs' once-massive tropical territories that stretched across the supercontinents Laurasia and Gondwana. As habitats shrank, so too did dinosaur populations. That left them extremely vulnerable to extinction when disaster struck and killed off huge numbers of dinosaurs. The smaller the population, the harder it is to bounce back when many individuals die.

Though it's tempting to blame the dinosaurs' demise on a single, apocalyptic disaster, the truth is that mass extinctions are always a long, messy process. The standard definition for mass extinction, when 75 percent or more of Earth's species die out, comes with the caveat that the process usually takes more than a million years. That's because it's actually phenomenally hard to kill off so many species. It takes more than one disaster. Now we know that the Earth itself, with its deadly tectonic shifts, was destroying the dinosaurs long before that space rock came in to finish the job.

Proceedings of the National Academy of Sciences, 2016. DOI: 10.1073/pnas.1521478113