Dinosaurs weren’t wiped out by that meteorite after all

For some reason, the scientists neglected to mention "fighting with a giant ape" as one possible cause of the dinosaur population decline. (credit: King Kong)

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.

Here you can see the slow decline of several subclades of dinosaurs, long before the Chicxulub impact. Note that two subclades were actually speciating at a higher rate, but they were the exceptions that proved the rule. (credit: Proceedings of the National Academy of Sciences)

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.

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