What Was It Like To Witness The End Of The Dinosaurs?
66 million years ago, a massive asteroid hit what we know today as the Yucatán Peninsula of Mexico. Many people have a general idea of what happened next: The age of the dinosaurs was brought to a close, making room for mammals like us to thrive.
But fewer people know what happened in the days, weeks, and years after impact. Increased research on fossils and geological remains from this time period have helped scientists paint a picture of this era. For large, non-avian dinosaurs like Triceratops and Tyrannosaurus rex, extinction was swift following the asteroid impact. But for creatures that were able to stay underwater and underground, their post-impact stories are more complicated.
Joining Ira to discuss her book The Last Days of the Dinosaurs is Riley Black, science writer based in Salt Lake City, Utah. You can read an excerpt of the book here.
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Science writer Riley Black writes the “Laelaps” blog at Scientific American. She is also the author of the books Skeleton Keys (Penguin Random House, 2019) and My Beloved Brontosaurus: On the Road with Old Bones, New Science, and Our Favorite Dinosaurs.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. For the rest of the hour, we’re going back in time– way back– to explore what happened on Earth after that massive asteroid wiped out the dinosaurs. A new book traces what happened, from the immediate aftermath to thousands of years later.
The world changed literally overnight, if not faster. As for humans, says the author, “we wouldn’t exist without the obliterating smack of cosmic rock that plowed itself into the ancient Yucatán.” Wouldn’t you like to know more? I would.
Joining me is Riley Black, author of The Last Days of the Dinosaur, based in Salt Lake City, Utah. Welcome back to the show.
RILEY BLACK: Oh, thank you so much for having me back on.
IRA FLATOW: You’re quite welcome. You know, I know you have written several books, many about dinosaurs. Why did you want to focus on the last days of the dinosaur in this book?
RILEY BLACK: Yeah, I realized that I hadn’t really done justice to the story. To borrow that Seinfeld line, I kind of yadda yadda’d this extinction, right? Because a big rock strikes the planet, we assume that it’s going to cause a mass extinction somehow.
But there have been other impacts at other times that had nothing to do with any major extinction event. So this seemed different. And I realized I didn’t know as much about it as I probably should. And the more that I started to research on this– and I mean, paleontology is my beat, I write stories about some of these new discoveries– I realized that I had the story kind of, not entirely wrong, but I didn’t understand how much we had learned about it.
IRA FLATOW: Well, let’s begin where you just left off there. You say in your book that there have been other impacts of similar or greater scale that did not trigger biological disasters. So what was it about this impact that did?
RILEY BLACK: Yeah, that’s the really strange thing, because it’s not as if this asteroid or whatever this body of rock was– we’re pretty sure it was some kind of asteroid. A carbonaceous chondrite, I think, is the best working hypothesis right now. It wasn’t just hanging out near Earth and decided to stop in. It had been traveling towards our planet for a very, very long time.
And this was kind of like a galactic skill shot, in a sense. Like, it could have just as easily missed or come close or hit somewhere else on the planet. But the fact that it hit at an incredible amount of speed, was so very big, and it hit all this limestone– so basically, these ancient fossil deposits, the ancient remnants of reefs that had existed millions of years before the impact itself, that contained all these chemical compounds that contributed to the impact winter.
So when you put all these things together– the size and the speed, the angle at which it hit, the sheer force of it– all these things came together in basically the worst case scenario, that nothing quite like this has ever happened before in Earth’s history, and certainly not at such a vulnerable moment for life on Earth. And all these things came together, not just in the first 24 hours where you had this incredible heat pulse and all this debris, but in the years following. So it really was every single way that this could have gone wrong for life on Earth, just about. That’s how this played out. It’s really spectacular how quick and violent this was.
IRA FLATOW: Yeah, so it was the perfect asteroid storm, you’re saying, is what it is.
RILEY BLACK: Oh, absolutely.
IRA FLATOW: Yeah. Let’s talk about the sequence of events. And that’s what you do in the book. You go through the first days, months, years, eons, thousands, million years. Let’s look at the timeline on this. You write that “this calamity was as immediate and horrific as a bullet wound.” Explain that.
RILEY BLACK: Yeah, so when we think about this mass extinction, or at least a lot of the visuals that I get or got growing up about this mass extinction, you’d see these emaciated dinosaurs wandering through this, like, nuclear winter kind of scenario, that they made it through the first day, but it was really the debris clouds and the cessation of photosynthesis and all these big environmental changes. But we now know that they probably didn’t even make it that far, that basically all our favorite non-avian dinosaurs– T. rex and Triceratops and Edmontosaurus and all those– were probably gone within about the first 24 hours, because what happened in the minutes to hours following this impact, you had all this pulverized rock. So millions of cubic miles of rock that’s been thrown up into the atmosphere that start to spread.
Basically, they start to spread all over the planet. And as they’re coming down, each one, each little piece, is creating a significant amount of friction. By itself, any one of those isn’t very much. But you do enough of that, it’s just so much debris and so much, basically, damage created by this impact, that all that friction creates an infrared pulse. So it raises the air temperature all around the planet to about 500 degrees Fahrenheit.
So if you ever broiled a chicken, that’s about what you broil a chicken at. And T. rex was more or less a broiled chicken within about 24 hours of this impact. If you couldn’t get underground, if you couldn’t get underwater in somewhere, had some other way to block yourself from this pulse–
IRA FLATOW: Wow
RILEY BLACK: –you’re basically out in the open. It was so hot that some forests were spontaneously catching fire based upon some of these models that geologists and paleontologists have put together. So it really was incredibly extreme.
IRA FLATOW: And then we had a cold period.
RILEY BLACK: That’s right, about three years of impact winter. So you had this terrible heat pulse that did most of the major initial damage. But in the years that followed, you not only had the soot from forest fires all over the planet, you not only had all the dust and debris thrown up by the impact itself, but all these sulfur-based compounds that we know from observations during our own history are really good at reflecting back sunlight. So it’s estimated that the sunlight reaching the Earth was reduced by at least about 20%. And that was enough to curtail, if not stop, photosynthesis over much of the planet.
And if you take out plants, that’s the basis of our ecosystem. It’s the basis of our oceans. It’s the basis of how we get our oxygen. It’s something over those three years that not only temperatures dropped, but ecosystems almost entirely collapsed. And those survivors, during those three years, had to get by on scraps.
IRA FLATOW: There was one factor that made this extinction not as bad as it could be, you write, and that’s a very unlikely source. I’m talking about volcanoes. Can you explain this?
RILEY BLACK: Yeah, so in the past, we’ve had at least five mass extinctions so far. We may be entering a sixth. But most of those five, or at least a significant number of them prior to this asteroid impact at the end of the Cretaceous, were caused by volcanic activity. And in particular, prior to the asteroid impact and after the asteroid impact 66 million years ago, you had incredible outpourings in what’s called the Deccan Traps in what’s now India, just thousands of miles just covered by molten rock and all the greenhouse gases that are being spewed into the atmosphere as part of it.
And those greenhouse gases, in fact, counteracted some of the effects of impact winter. This is not what we think of when we think of volcanic eruptions like this. We often think about them in terms of causing extinctions. But in this case, it kept the impact winter from being as bad as it otherwise could have been. It raised temperatures just enough to allow some forms of life to be able to survive when otherwise they would have gone extinct in the chill of that impact winter. So even though those volcanic eruptions were previously considered to be a contender for this extinction, it turns out that they kind of mitigated the effects of the asteroid impact and kind of came to the rescue for at least some forms of life.
IRA FLATOW: Yeah, that is really something new that we haven’t heard before, because we’ve heard of research that says the dinosaurs were already weakened by natural forces and possibly volcanoes. They were on their way out, and the asteroid just provided that final push. But are you saying that’s not true?
RILEY BLACK: That’s right. It seems to be the opposite case, that volcanic eruptions actually assisted some of these surviving animals. Most of the non-avian dinosaurs, if not all of them, were already gone by the time this counteracting force would have come into play. But that’s the other part of this, is that so much of what we understand about this extinction comes from Western North America. It comes from the Hell Creek Formation and the overlying rock layers in Montana and the Dakotas.
There’s so much that we don’t know. So the decline that paleontologists previously thought they saw is because there are fewer rocks from the relevant time period. So just at an absolute level, we have less dinosaur diversity because there aren’t as many rocks from the very end of the Cretaceous that actually preserved them as compared to 10 million years before. So we’re really learning, in a sense, how much we didn’t previously know about this mass extinction and how it played out.
IRA FLATOW: And how long did it take for plant life to come back?
RILEY BLACK: Yeah, there’s a seed bank, or there was a seed bank, in the soil. So a lot of plants, they spread their seeds, they spread nuts, they spread their fruits as far as they possibly can. And some of those already exist in the soil and would have been shielded by some of the heat effects. It really only takes about a couple of inches of soil to really shield what’s in the soil from the effects of things like forest fires. We know from modern-day forest fires that get about as hot as that infrared pulse that it doesn’t take all that much.
So that seed layer was there. It actually allowed beaked birds to survive. That’s why we have dinosaurs around us now, is because beaked birds were able to subsist on the seeds and nuts that still existed.
But it took at least about 100,000 years before you started to see vegetation make a real recovery. You have what’s called a fern spike, where we see fossil ferns and their spores everywhere in the fossil record around this time. And that’s because ferns are what we call disaster taxa. They’re really good at coming into spaces that have been disturbed, that have been disrupted. And they’re kind of the first initial signs that life is beginning to recover.
And then by about a million years after impact, that’s when you start to have these thick, dense forests starting to grow up, that you have the rise of flowering plants and angiosperms rather than conifers. So it took about a million years before anything recognizable as a forest started to re-establish itself.
IRA FLATOW: Very interesting. And one of the interesting topics you talk about in the book is the evolution of the bird life following the impact. In fact, I found it fascinating to learn that it was the evolution of a beak and not the teeth of the dinosaur birds–
RILEY BLACK: That’s right.
IRA FLATOW: –that allowed these birds to survive. What’s there about a beak that nature likes?
RILEY BLACK: Yeah, we’ve had beaks evolve multiple times, over and over again. And in the case of dinosaurs, why beaked birds were able to survive, if you think about what birds and bird-like dinosaurs were doing prior to the impact, you had basically things like velociraptor covered in feathers, very sharp teeth. We had toothed birds that were able to eat insects and little lizards and things like that.
And then you had beaked birds that primarily ate seeds, nuts, plant material. They were already adapted to this kind of diet. They probably already had things like a gizzard or ways to grind up that plant food.
So they, in a sense, were pre-adapted to life after impact, whereas all those carnivorous species, there’s nothing for them to eat because there are no more plants, therefore no more insects. There are very few small little critters for them to eat. So basically, if you were a carnivore, trying to survive through this impact winter is much, much more difficult, whereas beaked birds, they are already adapted to eating things that had survived. And that’s why they’re able to hang on.
IRA FLATOW: Interesting. So what else was different about plant and animal life post-impact?
RILEY BLACK: You see forests grow a lot denser. If you think about forests and habitats in the age of the dinosaurs, basically in that end Cretaceous heyday, it would have looked somewhat similar to areas in Eastern Africa today, so more conifers than flowering plants. There were certainly no grasses, but that kind of open habitat and open woodland, because many dinosaurs were big. Where they walked, where they pushed over trees, where they fed, this all influenced the ecosystem. It shaped it around them. So you’re going to have dinosaur-sized holes, basically, through any ecology that you’re looking at.
But once they were gone, once you don’t have things like Triceratops mowing down vegetation or pushing over trees anymore, forests could grow a lot denser. They could grow a lot closer together, and they could grow tall. And that provided a multi-tiered ecosystem for their survivors, whether you were a bird or a mammal or an insect. Life could be different at the canopy than on the trunk of the tree than at the surface of the soil or down below that soil. So you had all these different new opportunities for evolution and pioneering new niches open up.
IRA FLATOW: Right. This is Science Friday from WNYC Studios, talking to Riley Black, author of The Last Days of the Dinosaurs. Really interesting. As I said in the opening, I read a portion of your book where you said “We”– meaning humans– “We wouldn’t exist without the obliterating smack of cosmic rock.” Why is that?
RILEY BLACK: There’s no reason to think that the age of dinosaurs would have stopped without this. I mean, in a sense, we still are because birds are still here. But the kind of dinosaurs that we think about and see in the movies all the time, they would still be here. If you think about 66 million years, that is a very long time.
But if you were to start from the day before impact– so you know, T. rex and Triceratops are still doing fine– project that backward 66 million years further into the Cretaceous, dinosaurs are still around. They’re doing fine. Basically, there’s more time between Stegosaurus and T. rex than there’s been since T. rex went extinct. So dinosaurs were the dominant vertebrates on land. They were the most prominent vertebrates on land for so very long, and they’d survive so many different changes between the continents moving around, volcanic eruptions, climate changes, sea level changes, they would have made it through. It took something really unexpected and unprecedented to really change up life and what it was.
And our ancestors could have very well gone extinct in this very same extinction. That’s one of the things that blows my mind, honestly, is that there were primates around during the last day of the Cretaceous. This little animal called Purgatorius is the earliest known primate, and it was able to survive where the big and terrible dinosaurs weren’t. So it’s not just that we evolved as a result of this extinction, but our ancestors, our primate ancestors, actually eked right through it.
IRA FLATOW: That is cool. Are there other things we can see now that are direct remains of this extinction?
RILEY BLACK: You can look almost anywhere, basically, whether it’s seeing all the flowering plants and their pollinators– that’s something that those interactions and those kinds of plants were around before impact, but they’re much more prominent now. Or things like beans– I love a good taco. I like to put refried beans on it sometimes. Beans only came about because legumes evolved about a million years after impact, that plant life got this reinvigorated kind of evolutionary pulse after the impact. And basically, plants like legumes that are rich in protein are part of that as well. So whether it’s just our own existence or what we eat or the sort of vegetation we see around us, there are so many little hallmarks that we can draw back to this mass extinction.
IRA FLATOW: Who knew how important beans were? As you say, Riley, you’ve written a lot about dinosaurs and written many books. What surprised you the most about writing this book, in your research?
RILEY BLACK: I felt like so much of it was a surprise, because I had so many assumptions going into it. I think what really struck me was the way the world recovered after impact, how relatively quick that was. I mean, a million years is a long time. But to think that prior to the mass extinction, the largest mammals that we know about were about the size of a house cat, and then a million years later, the largest mammals that we know about were about the size of a German shepherd, that’s quite a bit bigger.
And we’re starting to understand so much of how and why they evolved and the pattern of their evolution. There was a paper that just came out– I wish I could have included it in the book. But it’s still fascinating to me about how mammals were getting big so quickly that their brains pretty much remained at the same size as they had been post-impact, so that you have much bigger-bodied mammals, but their brains are about the same size. And it wasn’t until about another 10 million years or so after that, after impact, that you start to see a lot more changes to the prefrontal cortex and changes in behaviors and interactions and things like that.
So life really raced to fill in the voids that were left by this mass extinction in such a way that it was a really formative and interesting time for evolution in general. Like, it wasn’t a sense of progress or mammals picking up the torch where dinosaurs had left it, but it was something entirely new happening. And seeing and understanding some of those interactions that we never got to view before, so much of this research has come out in the past five or 10 years. Even in the last year, we’ve made a lot of new discoveries. So this is rapidly changing and giving us this view, this timeline, that we never really had before.
IRA FLATOW: Time for a new book, Riley. We’ll have you back on that next one. Thank you for taking time to be with us today.
RILEY BLACK: Oh, it’s always a pleasure. Thank you so much.
IRA FLATOW: Riley Black, author of The Last Days of the Dinosaurs. And coincidentally, this month the SciFri Book Club is reading The Last Days of the Dinosaurs. You can find out how you can become a member and read along with us at sciencefriday.com/bookclub. That’s sciencefriday.com/bookclub.