Why birds were the only dinosaurs that survived mass extinction

Birds are the only lineage of dinosaurs that survived after an asteroid smashed into the Earth 66 million years ago, causing a mass extinction. Now Canadian scientists think they've figured out why.

While many dinosaurs at that time were very bird-like they were covered in feathers, had bird-like proportions,ran around on two legs and some even had wings there was one thing that set birds' ancestors apart: they had beaks instead of teeth.

And that's what made the difference in their survival, the researchers propose in a new paper published in Current Biology.

When the asteroid hit at the end of the Cretaceous period, it produced "basically what would have been a prolonged nuclear winter," said lead author Derek Larson in an interview with Quirks & Quarks.

• Download the MP3 of the interview or listen to the rest of the show

Mass starvation

The energy of the impact would have set off raging forest fires, while the dust and debris that mushroomed into the sky would have blotted out the sun for weeks, killing off plants and the herbivores that ate them.

"While you would think this would be a great time to be if you were a meat-eating dinosaur, because there would be all these dead animals, that's really a short-term resource," said Larson, assistant curator at Alberta's Philip J. Currie Dinosaur Museum and a Ph.D. candidate at the University of Toronto. "The remains of all of these extinct animals would have relatively quickly decayed away."

The only thing left to eat would likely have been seeds.

"Plants produce many, many more seeds than germinate in a given year and many of them can remain dormant in soils for decades," Larson said.

Most dinosaurs would have been unable to crack open these seeds using their sharp, pointed teeth. But the ancestors of modern birds, with their beaks, could happily keep on feasting as other animals starved.

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Even today, Larson said, seeds remain the most plentiful resource in areas devastated by forest fires and seed-eating birds are the first animals to recolonize them.

No decline

Larson and his colleagues came up with their explanation after a painstaking study on how maniraptorans dinosaurs very closely related to birds went extinct.

They examined 3,100 maniraptoran teeth from western North American dating tothe last 18 million years of the Cretaceous period,and theyfound a huge variety of species adapted to different kinds of food, ranging from worms and insectsto the meat of larger animals like mammals.

At that time, other dinosaur groups such as the giant, four-legged, plant-eating sauropods were already in decline and scientists proposed that the asteroid simply dealt the final deadly blow.

But that didn't seem to be the case for maniraptorans, which appeared to be thriving until they suddenly disappeared after the asteroid hit.

"We simply don't have any remains in the fossil record of these animals after the extinction," said Larson.

That's what led Larson and his colleagues to propose their explanation for the vastly different fates of maniraptorans and their very similar, close cousins, the birds.

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Interestingly, while maniraptoran fossils from the end of the Cretaceous period were abundant, the remains of modern birds were not. But because their close relatives were abundant then and their descendants are abundant now, Larson said "we know they were there."

The study was co-authored by Caleb Brown, a postdoctoral fellow at the Royal Tyrell Museum of Paleontology in Drumheller, Alta., and David Evans, Temerty Chair andCurator in Vertebrate Palaeontology at theRoyal Ontario Museum in Toronto and an associate professor at the University of Toronto. It was funded bytheNatural Sciences and Engineering Research Council,the Doris O. and Samuel P. Welles Research Fund and the Dinosaur Research Institute.