Thanks to Linode for supporting this episode of SciShow.

You can go to linode.com/scishow to learn more and get a $100 60-day credit on a new Linode account. [♪ INTRO] It turns out getting hit by a mountain-sized asteroid is pretty bad for the planet. So when it comes to the one that killed off the non-avian dinosaurs 66 million years ago, it might make sense to assume that the specifics of exactly when it hit and at what angle and all that probably don’t matter much.

Big space-rock bad. But, remarkably, recent research has suggested that it may not have been just the size of the rock that led to the extinction of the dinosaurs, but the timing of it, too. Granted there probably isn’t a great time for your planet to be hit by a 10-kilometer-wide asteroid.

Reconstructions of the event point to a really, let's say “intense” day, with the atmosphere boiling due to ejected material returning to Earth as a kind of burning rain. But there are definitely worse times. Because the exact timing affected the impact site.

And for reasons that we’ll explain, that rock hit our planet in one of the worst possible places at one of the worst possible angles. This all leads to an interesting question: What if things had been just a little different? Would a few minutes and a few degrees really have made a difference for the dinosaurs?

And, if it did, would we be here? Well first off, it turns out the asteroid may have hit during an especially vulnerable few million years for the dinosaurs. Or at least a couple really important groups of them.

A paper from 2014 did a big review of different dinosaurs, trying to see how they were doing in terms of diversity ahead of the impact. And the researchers made an interesting suggestion: that large-bodied herbivores were having trouble. And they might have been really important.

In this paper, the scientists reviewed the types of fossils we have, particularly from sites like Hell Creek, Montana, which is a famous fossil bed that contains specimens from right up to the impact. The scientists also looked at other things that were happening in the world around then, such as sea level changes and a spate of massive volcanic eruptions that were happening roughly around this time in what would be modern-day India. Based on that, the researchers suggested that two groups of dinosaurs seemed to be going through a loss of diversity around this time.

One was the ceratopsids, which includes triceratops and its kin, and the other was the duck-billed dinosaurs called hadrosaurids. And this loss of diversity may have made them particularly vulnerable to big shocks in the environment. A more diverse group has more potential tools to solve problems.

And this might have been detrimental for dinosaurs as a whole, since ceratopsids and hadrosaurids made up a significant portion of the food-web. So if the duck-bills and three-horns started to disappear, the meat-eaters may have followed. Now this doesn’t explain everything; there were plenty of other herbivores like sauropods, the longnecks, that also went extinct.

And this is only one interpretation of how these groups were doing. The question of whether dinosaurs were already declining before the end-Cretaceous event has been a point of discussion for a while. Other scientists have argued they might have been doing fine.

But if they were in trouble, and the impact had taken place before or after these potentially troubled times, what would the food web have looked like then? Would some have made it through? At this point, we’re still talking about spans of millions of years.

But scientists have been able to zoom in on this case of cosmically bad timing a lot further. For instance, we think the asteroid hit during the Northern Hemisphere springtime. And that might have been pretty bad.

This is according to a paper published in 2022. Now, you might ask, how is it possible to tell what season it was? That seems really specific.

The answer? Fish bones. Specifically, they were studying the bones of fossilized, sturgeon-like fish that the scientists think actually died on the very day of impact.

The fish came from a site in North Dakota, and we think this particular site looks like it contains evidence of the capital-V Very Bad Day, since it’s in the right layer of rocks, has the kind of fossil animals and pollen we’d expect for the time, and, perhaps most importantly, a layer of iridium-rich clay. This iridium is a kind of signature of the asteroid impact, since iridium is an element that’s relatively rare in the Earth’s crust, but fairly common in asteroids. So if you see it suddenly show up in a layer of dust, there’s a good chance that dust came from an asteroid impact or explosion.

So we know the fish lived in the, like, years or centuries close to the impact, but can we really say they died on the day of? Well, looking carefully at the fish’s gills, the scientists found something pretty special: small spheres of glassy material that they identified as molten impact material; bits of the ground melted and tossed into the upper atmosphere by the asteroid. This seemed to confirm that these fish died on the day of the impact.

And, luckily, sturgeon-like fish grow in a specific way over the course of the year as the seasons change. By slicing the fossil fish bones into thin sections and looking at growth rings in the bone, kind of like tree rings, they concluded that these fish died during the Northern Hemisphere spring. The scientists say that this may have been another case of bad timing, since a lot of animal populations are kind of vulnerable to disruption during springtime.

That’s when many creatures mate or go through growth spurts as youngsters, for instance. A natural disaster here may have been more impactful than at another time of the year. It may help explain why plants and insects in the Southern Hemisphere, which was going through its fall, seem to have recovered twice as fast.

Okay, so that’s pretty cool, BUT! We can zoom in even further. We can get more precise, on the range of minutes … It turns out, if the asteroid had been just slightly earlier or later, the impact may not have been as deadly.

Because the rocks underneath the precise spot it hit in modern Mexico may have been kind of exactly the kind of rocks you don’t want hit with an asteroid. You see, rocks can be made of different elements, like silicon or carbon or sulfur. And in 2017, scientists were interested in knowing if the composition of the rocks around the impact site would have made a difference.

They used computers and a model of the Earth’s climate and plugged in the composition of different types of rocks into the model. The computer then spat out the amount of soot and sulfates that would be ejected into the atmosphere as well as what that would mean for Earth, since both soot and sulfates can block the sun’s light. This is important because blocking off the sun would send the Earth into the years-long global winter that many scientists believe ultimately killed off the dinosaurs.

In the end, the researchers found that the rocks under the impact site were especially bad for the planet, producing a lot of sulfate. And because the Earth is constantly spinning and moving in space, this means that if the asteroid had just arrived, say, 10 minutes earlier or later, it would have hit in a place without such problematic rocks. Or better yet, out in the ocean, where a lot of its impact would have been lessened, at least in terms of the amount of rock that got ejected into the atmosphere.

Now three years later in 2020, scientists took this line of reasoning further, noting that even if it had hit the same type of rock, that particular timing may have been especially bad due to the impact angle. Because the Earth turns, different timing would have resulted in the asteroid hitting at a steeper or shallower angle. Which affects how much stuff gets ejected into the atmosphere.

The 2020 scientists looked at what are known as gravity anomalies in the impact site. “Gravity anomalies” is a pretty cool term, and kudos to whoever coined it. But they aren’t all that strange. They’re mostly a way to measure the density of the rocks below our feet by detecting extremely minute changes in the force of gravity above them.

Gravity is based on mass, after all, so more dense rocks exert a slightly stronger pull than lighter ones. It’s not really something you can feel by yourself, but with the right instruments, scientists can detect it. So, the scientists used these anomalies to map out the ripples of light and dense rock beneath the impact site and compared it to a computer model that simulated what different angles of impact would look like.

And they concluded that the asteroid hit from a pretty steep angle, about 45-60 degrees, and came in from the northeast. This vaporized more rock than a shallow strike and released more climate-changing gases than other angles, with 2-3 times as much carbon dioxide released as a vertical impact and 10 times as much as a shallow impact. Again, if the asteroid had just slightly better timing, the Earth would have moved beneath it, possibly changing the angle of the strike and making things less catastrophic.

Still bad, but better. And all of this leads us to the question: What would have happened if the timing had been better? Could dinosaurs have survived?

And, if so, what would that mean for us? We think mammals were mostly small-bodied generalists at the time, about the size of a badger or smaller, with many being nocturnal. The other niches were filled by other animals.

After the dinosaurs died off, mammals and birds were able to go through a dramatic expansion. They got big, numerous, and weird, quickly. But if those niches had remained filled, that might not have happened.

Mammals may have stayed small, night-going creatures. There may have been no room for stuff to evolve into cows and elephants and lions if triceratops and sauropods and T-rex were still around, after all. It’s impossible to confidently say what it would have meant for us if things had been different.

The changes to the Earth were widespread and complex, and we can never go back and determine how things would have changed if the impact was more or less bad. Even if dinosaurs had been at their full strength, and the asteroid had hit during another season, and 10 minutes later, and not at that precise place at that precise angle … dinosaurs may have still died out. And even if they did survive, some have suggested that mammals still may have flourished due to things like changing vegetation.

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Thanks to Linode for supporting this SciShow video! [♪ OUTRO]