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The time of year the dinosaur-killing asteroid hit explains why some species survived

SACHA PFEIFFER, HOST:

Our next story is also about climate change and a new clue that helps scientists understand a past disaster. It took place 66 million years ago. Dinosaurs roamed the Earth, and then an asteroid hit.

MELANIE DURING: You can imagine this to be worse than any earthquake that you know of times a thousand.

PFEIFFER: Paleontologist Melanie During of Sweden's Uppsala University says it was a terrible time to be alive.

DURING: You have immediate tsunamis near the impact itself, but you also have these shock waves going through the continental crust.

ARI SHAPIRO, HOST:

Those shock waves jolted bodies of water thousands of miles from the asteroid's impact site near Mexico's Yucatan Peninsula, and all that sloshing buried even faraway animals in crashing walls of water and debris.

PFEIFFER: The asteroid's impact also triggered a hailstorm of glass.

DURING: So when the meteorite hit, it almost vaporized immediately, but you can still compare it to throwing a bowling ball in a sandbox. A lot of sand jumped up, and in this case, a lot of molten earth rock that fired into space where it crystallized.

PFEIFFER: What went up eventually came down and pelted the planet with glassy rock.

SHAPIRO: In the days and months that followed, intense radiation, global wildfires and acid rain punished life on Earth. Three-quarters of the world's species went extinct. But why some and not others?

DURING: The dominant group, the non-avian dinosaurs, they went extinct. How is this selection possible? Why them? Why not also sharks or turtles or crocodiles or mammals?

PFEIFFER: Now Melanie During and her team have uncovered a clue that might help answer that question. They looked at fossilized fish dug up in North Dakota. The fish were buried alive the day the asteroid hit. They even have some of those shards of glass lodged in their gills.

SHAPIRO: During says fish bones grow sort of like tree rings, adding a new layer every year. So by looking at their bones, they could tell what time of year they died. Writing in the journal Nature, During's team says it was springtime in the North when the asteroid hit.

DANIEL FIELD: This paper really made me happy, right? I mean, it's always going to be difficult to figure out exactly what happened on the day of the asteroid impact from our vantage in the present day.

PFEIFFER: Daniel Field of the University of Cambridge was not involved in that work, but he says the paper provides new ideas about how extinction might have been more severe in some parts of the world than others.

FIELD: It's plausible, at least, that the seasonality may have had implications for determining the winners and losers of this mass extinction event.

SHAPIRO: Remember; spring in the North is autumn in the South. So while animals in the North were tending to newly hatched young, creatures in the South might have been hunkered down for the winter, helping them endure apocalyptic conditions.

PFEIFFER: Melanie During says studying more Southern Hemisphere species might help advance that idea, but she doesn't expect they'll be able to narrow down the time of impact even further.

DURING: You cannot pinpoint it to, like, a holiday or something specific. That would be great. Like, it was on my birthday or - but I don't think we can do that.

SHAPIRO: Even so, she proposes picking a day in the spring.

DURING: April 1, I think we should all just stop for a minute and just...

SHAPIRO: Hold a moment of silence for the dinosaurs.

(SOUNDBITE OF FLORATONE'S "SWAMPED") Transcript provided by NPR, Copyright NPR.