Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to get started learning STEM for free, and if you’re interested in an annual premium subscription you’ll get 20% off using our link. [♪ INTRO] Around 66 million years ago, a massive asteroid slammed into the Earth, making a huge crater in what’s now Mexico. But it also left its mark all over the globe.

The impact filled the atmosphere with dust, causing long-term disruptions to the climate, and that was only after the more immediate side-effects like wildfires and tsunamis. This whole catastrophe led to the extinction of about 75% of Earth’s species, bringing the Cretaceous Period and the Age of Dinosaurs to an end. All in all, it was a terrible day.

And now paleontologists have pinpointed during what time of year that millions of years old event happened, all thanks to new fossil evidence. The clues come from a fossil site in North Dakota called Tanis, where paleontologists uncovered the remains of a mass death event, a direct result of the asteroid impact itself. When the asteroid hit, seismic tremors would have taken only tens of minutes to travel the 3,000 kilometers to North Dakota, where they shook up an ancient inland sea called the Western Interior Seaway.

Water sloshed around and crashed onto the shoreline, burying plants and animals from freshwater and marine habitats in one big jumble. And this happened while debris from the impact was still raining from the sky, tiny chunks of vaporized and re-cooled rock, found today among the Tanis sediments, and even inside the fossilized gills of the poor fish buried there. The rain of debris is thought to have only lasted a couple of hours after the asteroid impact, so finding those tiny rocks inside the fossils means that they died right after the asteroid struck and they can tell a story of when it all happened.

In a 2022 study published in the journal Nature, scientists examined the fossils of paddlefish and sturgeon from Tanis. See, some of the bones in these fish grow in yearly layers, kind of like tree rings. Each layer has two parts: a fast-growth section in spring and summer when food is abundant, and a slower-growth section in the tougher fall and winter.

There’s also a chemical cycle. As these fish dine on zooplankton, they pick up more of a carbon isotope called Carbon-13. So the Carbon-13 levels in the bone grown during the spring and summer are higher because the fish eat more of these little critters.

Researchers identified these same cycles in the fossil fish bones and they noticed that, in all of the fossil fish, their very last cycle was interrupted at the beginning of the spring-summer phase. And this suggests that the event that killed them happened in the spring. This is also supported by another 2022 study that came out in the journal Scientific Reports.

This team of scientists found a similar pattern in the yearly cycles of fish bones from the Tanis site, and they also found several other signs of springtime. They measured the bones of the youngest fish in the deposit, estimating that they were only a few months old at most when they died. Since these fish are thought to have spawned throughout the spring like their living cousins, this suggests the demise occurred in late spring or summer.

On top of that, the researchers also identified fossils of adult mayflies. Mayflies are a type of insect that spend most of their lives as aquatic larvae. Then when spring and summer roll by they emerge as adults to mate and only live for a few hours or days before dying.

Finding adult mayfly fossils suggests that they were buried during the time of these spawning events. And the researchers also noted that the leaves fossilized at the site show high amounts of insect feeding traces, and insects do more leaf-eating during spring and summer. Just one piece of evidence would be exciting enough, but with several lines of evidence all pointing toward the same answer, we have good reason to think the asteroid landed during spring in the Northern Hemisphere.

Now, it’s amazing, wild even, that we can narrow the timing down to the season an event happened 66 million years ago! But wait, there’s more! This information also gives us better insights into the extinction that followed the asteroid impact.

Spring is a crucial time for reproduction and early development of plants and animals. So these disasters hitting the ecosystems in the Northern Hemisphere during this vulnerable time would have devastated a fresh generation of life and weakened the foundation of future generations. And it’s even possible that the extinction might have been worse in the north than in the south.

Because when it’s spring in the north, it’s fall in the Southern Hemisphere, a time when many plants and animals are beginning to hunker down for the harsher months. Dormant plants and hibernating animals might have had a better shot at surviving the calamitous effects of the asteroid. And there’s some evidence to support this hypothesis, but more research will be needed to be sure.

Now, we can’t say for sure that the extinction would have been better or worse if the timing were different. These scientists certainly aren’t claiming that the dinosaurs would still be around if only the asteroid had waited a few months. Because, you know, it’s still a massive chunk of rock that impacted the Earth.

But the more specific information we have, the better we can understand the details of this mass extinction. A pretty useful thing to know, not only for interpreting disasters of the past, but also for anticipating problems of the future. And if you want to learn more about anticipating things in the future, you should check out today’s sponsor, Brilliant.

They are a fully interactive online platform with courses on science, engineering, computer science, and math. Brilliant helps you gain a deeper understanding of these concepts by taking you through the subjects piece by piece in visually stimulating and hands-on ways. And they have a whole course on “Probability Fundamentals,” where you can learn to analyze the probability and up your chances to win things like probabilistic games.

To sign up, visit the link in the description or visit Brilliant.org/SciShow to get 20% off the annual Premium subscription, and checking them out also helps us, so thank you! ♪ OUTRO]