Telling time of year when dinosaurs died out

About 65 million years ago, an asteroid slammed into Earth in a cataclysmic impact that, among other things, wiped out the dinosaurs. But incredibly, scientists now know at what time of year this happened. PhD student Melanie During, from Uppsala University in Sweden, has found tiny balls of congealed glass made by the impact lodged in the gills of fish that died that day and were fossilised. And because fish grow at different rates at different times of the year, she was able to work out when they must have died, as she explains to Harry Lewis…

Melanie - When the meteorite hits, it's like throwing a bowling ball in a sandbox: molten rock immediately gets expelled into space, and in space they crystallise. But there's no gravity, it's a vacuum. So, the lightest elements, they stay in the centre which means you've got these balls that are often hollow in the centre, or glossy, and then they rain back down on earth. Those are tektites, and they have a fall back time of 15 to 30 minutes. The second thing is that, when the impact was made, you get this shock wave going through the earth. Of course, it's going to generate tsunamis in the ocean, but also, on the continental plate, the overlying bodies of water (lakes, rivers) are going to slosh back and forth like a pool during an earthquake. And so, this deposit with these fishes is caused by such a wave which went back and forth, meaning they alternate in the way that they are stacked up.

Harry - So, the fish are following this motion of the water? They're either pointed East or West - they're in polar opposites?

Melanie - Yep. It's East and West. Some of them are pointing in different directions when they've hit a tree or, if they're split open, it's a very violent deposit. But the majority of them are following the direction we expect.

Harry - We expect that these small spherical bits of meteorite that they...

Melanie - They're actually earth rock that's been ejected into space.

Harry - And they are penetrating actually into the bodies of the fish?

Melanie - No, they're not. They are actually taken up in their gills. They were still swimming and looking for food when the first impact hit, and they just sucked them up like they do with plankton and they choked on them.

Harry - Right. And so, through that, you are able to estimate what time of year these fish died?

Melanie - Yes. So these fish, they must have died on that particular day. The incoming wave and the impact both should have come within an hour. We know that they died on a particular day because these fishes have bones that grow like tree rings, registering a new year every year, but growth is not homogenous. It's not like growth, non-growth, growth, non-growth. No, we can actually tell that in spring growth gets started. You can tell that food uptake begins - but it's not as high as in summer, for instance. And then autumn is much lower, and in winter there's absolutely no growth. That's what we were looking for, and we've got multiple seasons in multiple fish. That what we were documenting and all of them clearly died at the same time and all of them clearly died when they were increasing their food intake again, after winter, but not yet at the summer maximum.

Harry - When this meteorite struck, we know roughly where it was as well, right? So, whereabouts was it?

Melanie - Yeah, it's a little town now called Chicxulub that is closest to the crater and that's in the Mexican peninsula. So if you have the Gulf of Mexico, it's on that arch on the bottom, which was in the Northern hemisphere when it struck. So, I would say it's roughly three and a half thousand kilometers south of the location that I studied.

Harry - What were the effects for the Northern hemisphere and the Southern hemisphere? Did they differ?

Melanie - In the North, it was spring and in the South, it was therefore autumn. So, when you look at the behavioral life cycles of the organisms - in the spring, plants were producing seeds, their first leaves, flowers, animals were looking for food, tending to offspring while, in the Southern hemisphere, they were preparing for winter; plants were shedding their leaves, making them a lot more robust, and many animals were seeking shelter and trying to prepare for winter. That latter category especially, those underground may have just been in the right place at the right time when the meteorite struck.

Harry - So, we know that there's been transition from what's around us today from the dinosaurs to a mainly mammal populated ecosystem and this could have been due to the time of year in which the meteorite struck?

Melanie - Yeah. Which is insane.

Harry - To be able to extrapolate that out as well, as a jigsaw puzzle not just for when it happened but for why our world looks the way it does, that's got to be pretty exciting for you to be able to connect these dots.

Melanie - It really is. And then you start looking up, "Okay. So, what was the recovery time?" And then you see that in the Southern hemisphere, it looks like they recovered twice as fast. Where did turtles survive? Turtles survived in the Southern hemisphere while any of the modern birds survived in the Southern hemisphere. I don't think that's a coincidence.