Birds Are The Last Dinosaurs. Why Did They Survive?

IRA FLATOW: This is Science Friday. I’m Ira Flatow. I spent a lot of time watching my bird feeder this pandemic. It was kind of a nice thing to do with all that time, and we’ve talked about birds a lot in the past on this program, how smart they are, how their vision works, even some of the physics of how their feathers can be so brightly colored. And with me today is another SciFri staff member who likes birds a lot herself, Producer Christie Taylor. And I have a feeling this story is– I’m sorry to say it– bird brained but in a good way. Hey, Christie, right?

CHRISTIE TAYLOR: Hey, Ira. Yeah, I think that’s a fair assessment.

IRA FLATOW: All right, tell us about your birdy news. What’s on your mind today?

CHRISTIE TAYLOR: Well, I don’t know if it counts as news, but remember the end Cretaceous mass extinction event?

IRA FLATOW: Sure. It was that 65-million-year-old meteor strike, and I remember just like it was yesterday.

CHRISTIE TAYLOR: It was a really long time ago, but it was also a really big deal, right? The climate changed really drastically thanks to all the dust in the air, and we can think that event for the loss of 85% of species on Earth at the time.

IRA FLATOW: Wow, 85%.

CHRISTIE TAYLOR: Yeah, that’s more than four out of every five species, and that includes most of the dinosaurs that were living on the Earth at that time.

IRA FLATOW: Yeah, but we do have some dinosaurs around now, don’t we? Aren’t birds dinosaurs?

CHRISTIE TAYLOR: You’re right, yeah. Chickens, herons, all of them– ancestors of modern birds survived, and they exploded into the thousands of species that we see today. So did mammals, which is why we’re here. And in a lot of ways, we can think about the existence of modern birds as a murder mystery, kind of like a game of Clue, only the mystery isn’t who killed them. It’s, why didn’t they die? What traits did birds’ ancestors have that let them stay alive and keep evolving?

IRA FLATOW: Like Clue, I’m going to take a wild guess, though, and suspect the answer does not involve a lead pipe.

CHRISTIE TAYLOR: You’re correct, Ira, though I think that may be the most sure we can be about anything at this point. But I brought in two researchers who have been exploring this question, gathering the evidence, asking lots of questions. And with the help of a brand new fossil skull finding, they have some ideas that the size and shape of the brain is involved in this survival mystery.

So Dr. Julia Clarke is a professor of vertebrate paleontology at the Jackson School of Geosciences at the University of Texas, and Doctor Chris Torres is a postdoctoral researcher in bird paleontology at Ohio University. And I should note that we talked to them in front of a live studio audience on Zoom. I started by asking Julia to help us set up the murder scene a little bit better. Why is it so mysterious that the ancestors of modern birds survived while a lot of other species did not?

JULIA CLARKE: I think it’s really interesting to think– when we think of the question of why we have one group of dinosaurs that’s still around with us today and we lack all of these other species that are so charismatic and are all the dinosaurs that we probably grew up thinking about as a kid. But I think it’s also important to think about all the other survivors. So the other major group, the cousins of the dinosaurs, these are the crocodilians, they also survived. Turtles survived. Lizards survived.

And then we have mysteries like, pterosaurs are gone. They’re gone at this– by 66 million years ago. And they share a lot of problems with our murder mystery, our survivor mystery in birds because they tend to not fossilize, not to be present in a lot of different fossil settings, fossil settings in which fossilization occurs, so they tend to– what we’re doing is we’re trying to solve this survivor mystery but with very limited data.

And so we have to approach this question two ways, which you’ll see in the work that Chris has led on this new fossil, which is thinking, OK, what can we study? If we look at all our living birds today, these are the survivors, and what they have in common, what is not present in the ones that go extinct, their close cousins that flew, that are about the same body size, that also went extinct? And why do things that are very different stick around?

The fossils we still have to decode this mystery are very limited, and so every time we get a new insight, it shifts that picture a little bit. But what’s so fun about this mystery is we still have a heck of a lot of hypotheses to rule out, and we have– if we were at the murder mystery party, we would need– we still have Colonel Mustard. And so every new fossil gives us new important insight, but are we there yet? Or are the murders are still occurring inside the house?

I don’t know. Maybe I took that metaphor too far. But that’s my brief murder mystery summary.

CHRISTIE TAYLOR: Chris, why investigate the brain as a clue? Why is the brain the part that you wanted to focus on?

CHRIS TORRES: The really cop-out answer is because we could. For the longest time in the bird fossil record, especially of early birds, the brain has been effectively a black box. There’s just been an almost total lack of data from that region for early birds, and this new fossil that we worked on finally preserved the data from the brain. And so we were able to tap into this really rich source of data that we hadn’t been able to access before. But more substantially, it’s because the brain is a really important organ for life as one might expect.

There’s a lot about the way we live our daily lives that’s tied to the brain. And so we can study the brain to gain a lot of really cool insights into how these organisms lived their lives. We’re talking about organisms in the case of Ichthyornis, for example, kind of the star of our new paper, this early bird. It’s 70, 80, 90 million years old. And so we’re dealing with really precious and incomplete remains, and so we’re looking for any kind of data that we can for insight into how the whole organism, when it was out, living its life, was living its life. And the brain, at least the external shape of the brain, which is what we’re really talking about here, is a really important source of information, and it’s a really useful set of clues for reconstructing what these organisms were like when they were still alive

CHRISTIE TAYLOR: How do you look at the brain of an animal whose brain has not been fossilized? Because we don’t have fossil brains here. We have skulls, right?

CHRIS TORRES: Right, yeah, exactly. So we’re dealing with proxies for the brain, and so this is something that makes birds so convenient to study because birds have really complex gigantic brains. And part of what makes them so complex is that their skulls– just like in mammals, their skulls totally wrap around the external surface, the outside surface of the brain, which means that any given bird’s skull is effectively a mold for its brain or at least the external surface. It’s important to know what we’re talking about the outside, not the inside. So it gives us– it’s basically a mold of the brain. And so if we have a particularly complete fossil bird skull, we can use that to reconstruct the outside of the brain and compare it to other extinct birds, near relatives of birds, non-avian dinosaurs, and living birds that we can go out and observe today.

CHRISTIE TAYLOR: OK, so we found a skull. We are looking at skulls to interpret what the brain was shaped like. I think this is the part where we talk about what you found, so you looked at– and you mentioned this bird, Ichthyornis, and you compared that to modern birds. What did you see?

JULIA CLARKE: So this was actually one of the earliest fossil birds that was described or known in detail shortly after Archaeopteryx, so within 20 or so years of Archaeopteryx. And Charles Darwin actually wrote to ornithologist Charles Marsh, the original describer of the Ichthyornis– that’s the genus of the species that we refer the skull to Ichthyornis dispar, and he said, wow, what great evidence for evolution. It looks so close to living birds, but it has teeth. And that clearly means it’s not part of a living species. So Darwin was so excited about the species back when it was described.

And so you asked what– you asked Chris about the brain, and I’ll let him get into the brain here in of Ichthyornis in a second. But what was important about Ichthyornis was that it was this very close cousin of the– we say the radiation of all the birds we have today, so that’s like the common ancestor of all the birds we have today and all of its descendants. So Ichthyornis was a close cousin to that. In fact, it’s the best represented in the fossil record of things that are close to that radiation, and that’s why it was going to be so cool to finally get some insight into its brain.

CHRIS TORRES: Yeah, so, when it comes to their brain– so exactly why Ichthyornis is so interesting is why we were so fortunate to have this particular specimen because it has a mostly complete skull, and that includes three dimensionality, which is also a crucial aspect [INAUDIBLE]. It’s not just that we have the parts, that we have them in position relative to where they would have been in life, which led us to reconstruct most of the brain and compare it to a bunch of living birds and then a relatively small sample– but it’s the best we’ve got– of early birds and near bird relatives.

Early birds, in this case, really just refers to Archaeopteryx, which is the earliest known bird. And so we had an idea of what– the brain of Archaeopteryx and some more distantly related dinosaurs looked like. And so we had an idea of why living bird brains, at least the external surface, why it was shaped differently from these other dinosaurs. A lot of the differences revolved around the forebrain, the cerebrum.

Living birds have enormous cerebral hemispheres, and those are a really important part of the brain and how it works. The cerebral hemispheres are where a lot of higher cognitive functions occur like that’s where memories are stored, so where learning happens, language is processed, senses are processed. So we knew there was a major difference between non-bird dinosaurs and living birds.

Living birds have these enormous cerebral hemispheres. More distant relatives don’t. A major question has been, where did that come from? Where along the line leading towards birds did that evolve? When did that first appear?

When we looked at Ichthyornis, we found that it’s cerebral hemispheres were much more like Archaeopteryx and other non-bird dinosaurs and very unlike living birds. It had relatively small cerebral hemispheres. And because Ichthyornis is so closely related to living birds, we were able to hypothesize that trait of having really large cerebral hemispheres probably was unique to living birds.

And so it fit the pattern that Julia referenced earlier, which is we’re looking for traits that are present in the survivors and absence in the casualties to try to help explain why the survivors survived. And so when we observed this state in Ichthyornis, this condition of having relatively small cerebral hemispheres, it indicated that having these large cerebral hemispheres fit that pattern and likely contributed, in some way, to the exceptional survivorship of living birds through that end-Cretaceous mass extinction event.

CHRISTIE TAYLOR: Tell us about what you see in these brains because it’s not just that the cerebrum is bigger.

CHRIS TORRES: The real estate is at a premium in the skull, and so, if you’re going to expand something, something else is going to have to give way. Something else is going to have to shift out of the way so that can happen. And so we see this general reshaping of the brain.

So if you look at the brains of early birds, their brain is vaguely linear. You get one structure, the cerebrum, out front, the optic lobes, or the midbrain, in the middle as might be expected by its name, and then the cerebellum is at the back. But in living birds, that is totally shifted so that what was formerly the midbrain is now totally underneath what is formerly the forebrain. And so the changing shape is so fundamental that even those words are not very descriptive anymore. So we see this general reshaping that happens as a consequence of this relative expansion of the cerebral hemispheres.

CHRISTIE TAYLOR: One way I would describe in looking at the different images of brains is that the Ichthyornis brain is almost like a train. I see a little train cars from the cerebellum at the back to the cerebrum at the front. But the modern bird brain is much more smushed. I don’t know.

CHRIS TORRES: Sure.

CHRISTIE TAYLOR: It’s folded and smushed. It’s not very technical.

CHRIS TORRES: It’s hard to describe, and that’s an obstacle we faced in the paper itself like, what’s the most reasonable way and conservative way of describing the shape here without using words that means something else? Yeah, it’s a very abstract shape.

CHRISTIE TAYLOR: Just a quick reminder, this is Science Friday from WNYC Studios. I’m talking to paleontologist Julia Clarke and Chris Torres about fossil birds and what we can learn about their brains. I want to get to what this shape might actually mean. We’re talking about Ichthyornis, which didn’t make it, and we’re talking about modern birds, which did. So what about a brain shape like this might actually promote survival?

CHRIS TORRES: So I can talk about what the expansion of the cerebrum might mean. The short answer is, [MUMBLING] I don’t know. It’s not super clear. So like I mentioned earlier, the brain is incredibly complex. There’s so many functions associated with the brain. Just saying it’s incredibly complex is an understatement.

And again, I want to emphasize that we’re looking at the external surface of these brains. The brains themselves are three-dimensional structures. And so the neurons, the brain cells, are all below the surface, and it has to do with their organization, and their connectivity, and their density that is where all of the cognition of the brain function actually happens.

And so what we’re looking for is some fossilizable feature, some feature in the fossils that give some hint as to what’s going on below the surface. And shape, relative size and shape of these structures, hints at that. And so what this is telling us is that there’s, of the myriad functions associated with the cerebrum, some or many or few of those would have been linked to survival.

Again, it’s not clear what those functions actually were. It may never be clear what those functions are because the brain is so incredibly complex, and surface, shape is still relatively simplistic, it may be that we’re never able to directly say, OK, it’s bigger, which means this. But it does help narrow down some functions that may be associated with that expansion of the forebrain.

And if we can turn to modern neuroscience and look at bird brain researchers, looking at the actual functions of birdbrains and comparing those to the brains in other reptiles, we start to get an idea of what those functions may have been. And we can start to hypothesize or at least speculate as to what the links would have been.

JULIA CLARKE: Well, I’m going to pick up here, yeah, so I think the key thing to point out in kind of a context is that we actually have, as you get these fossils, these incompletely known fossils from the Mesozoic of things that did not survive. So there are very few features we know are unique to living birds, and what Chris has described is something that we newly point out may be unique to those survivors. What it means in terms of ecological flexibility, is it enhanced visual system?

But this is speculation at this point because it’s sort of like if you found a computer chip. And that has a ton– it runs your whole phone, everything that your phone does. And you want to say, what’s the reason my phone survived the break that’s on this chip? It’s hard because it’s such a– it controls so much. It’s involved in so many different pathways like the forebrain alone in birds. So that’s not a easily– that’s why Chris went, huh?

IRA FLATOW: We’re going to have to take a short break, but, when we come back, what the end-Cretaceous mass extinction has to do with us now and more bird paleontology with guests Julia Clarke and Chris Torres.

This is Science Friday. I’m Ira Flatow. We’re listening to Producer Christie Taylor’s interview with bird paleontologist Julia Clarke and Chris Torres about their research suggesting modern birds escaped extinction 65 million years ago with some help from their brains. This interview was recorded in a live taping on Zoom, and, to find out how you can join us in a future Zoom call-in, visit sciencefriday.com/livestream. All right, now back to the interview.

CHRISTIE TAYLOR: We’re actually going to go to a listener question now, and I’m going to start with Candace from Arizona. Candace, if you’re able to unmute yourself, go ahead and ask that question.

CANDACE: Yes, thank you. This is a great program. I’m interested in your research and whether it shows if the ancestors of today’s birds had the ability for the bird to change rapidly when there was a changing environment such as what we have with climate change right now.

CHRISTIE TAYLOR: That is a great question. Julia or Chris, does either one of you feel more eager to answer it?

CHRIS TORRES: Yeah, Yeah, yeah, I think in terms of rapid climate change– global rapid catastrophic climate change is thematically similar to what we would have seen then and what we’re seeing now, causes obviously vastly different. So the environment is changing incredibly rapidly, and so, like Julia indicated, there’s all these functions that would be associated with the cerebral hemispheres, these higher cognitive functions, so to speak.

And so all of these functions or at least some of these functions may have been associated with behavioral plasticity, the ability for the organisms that had those organs, the ancestors of living BIRDS to have a wide repertoire of possible behaviors, wider, and more plastic, more changeable, more adaptable than their coevolved species, other species that would have been around at the time, which would have conceivably, potentially, have prepared them for rapidly changing environments. And so we might have expected to have seen this plasticity in a wide range of behaviors, whether it is moving across the world, where they’re feeding, when they’re feeding, what they’re feeding on, learning new behaviors, so basically modifying their own behavior as the world is rapidly changing. I think that [INAUDIBLE] question. Julia, do you have anything to add to that?

JULIA CLARKE: Well, I think it’s an interesting question. I was just reading a question in the chat that was sort of around the same thing, and what we can’t parse is whether the survivors are, as Chris alluded to, perhaps have ecological differences that then creates natural selection for larger brains or if larger brains have evolved and that’s behavioral flexibility, if we are very vague about it, precedes the actual extinction event. So oftentimes, what we have is that, for example, you can’t have natural selection for a more, let’s say, efficient wing until you have the natural selective force of moving in air.

And so this can be a difficult thing to parse in terms of cause. The brain, which is responding? Is there something that leads to overall larger size? And this comes up a lot in human evolution debates for example, but it’s a really good question on the part of several of our listeners.

CHRISTIE TAYLOR: Well, and I think, actually, this is a great time to sit back and ask, who cares? Why would it matter why Ichthyornis didn’t make it and all of these other birds and their ancestor did? Does it change anything about the world that we inhabit today?

CHRIS TORRES: Yeah, because it’s happening again. We’re in a mass extinction. It’s happening right now. And so a lot of research, rightfully so, is focused on, how do we stop this? How do we fix this?

But I think a lot of research needs to focus on, what’s the world going to look like afterwards? Assuming we can stop things, assuming that we can do something to rein in what we’re doing, how do we predict what the world is going to be like? And a lot of that revolves around, who’s going to be around when this is all done?

And by understanding that, by understanding what traits influence survivorship, we can make really informed scientific decisions about, for example, conservation. It helps us to figure out, OK, we’re spending money. Where do we spend our money? And what science is going to influence our decision making when it comes to that?

And so by understanding these traits in birds, by understanding comparable traits in all of the other lineages that survived, we get an idea of what traits influence survivorship. We can start to make predictions about which organisms may be successful and, if they are successful, how are they going to change? What’s going to change about them, going forward? And so we only get those insights by looking at how it’s happened before, and so insights like these are those insights.

CHRISTIE TAYLOR: Is it oversimple, looking at these comparisons of brains, to say that smarter animals are probably more likely to make it?

JULIA CLARKE: Gosh, that is such a great question because I always think about crows and my class at UT, talking not about– we do human evolution, and then I talk about crows because I think that we too often emphasize the peculiarities of our own intelligence and not that of our co-travelers on this planet who look very different from us. And crows and ravens, where I grew up in California, have been expanding their range so enormously. And there are so many amazing videos you can watch of crows and ravens really– and read scientific studies of their adaptability in human-modified environments, and they are the biggest brained of our survivors.

So parrots and corvids, the crows and ravens, have the absolutely largest brain per body size of our extant birds. And it doesn’t mean survival of the smartest necessarily, but we do see a lot of adaptability in these large-brain species to human-modified environments, which is most of the Earth at this point. And I also want to plug that what we’re seeing now and trying to understand in real time, we’re looking back 66 million years through a dirty lens at incomplete data. Our world would look very different without this mass extinction event that just, boom, shifted everything. So in some ways, it’s a good reminder short-term events have completely reshaped the Earth system, and what we’re doing is kind of trying to make sense of how exactly that worked because identifying causal features or causal factors in survivorship and extinction is really difficult to do.

CHRIS TORRES: I actually want to piggyback off something that Julia just said, which is this human-centric notion of intelligence and smartness, and I think it’s important to remember that any trait that influences survivorship is entirely contextual. And so what we call smart starts to lose meaning when we’re talking about things that are increasingly further removed from us. So fish, what we consider intelligence start to unravel when we’re looking at fish, let alone insects, let alone plants. It’s not even a term that’s really applicable to plants, and yet they’re wildly successful and all over the world.

CHRISTIE TAYLOR: We have one anonymous question that I’m just going to read, and their question is, looking at hummingbirds and hawks, one wonders about the cerebellum and its role in balance during flight. Is there evidence of changes that led to backward flight in hummingbirds or rapid descent as we see in raptors?

CHRIS TORRES: Yeah, so that question is one that we would love to be able to answer [INAUDIBLE] associating function with external shape. It’s really non-trivial, and it’s really difficult. Like I mentioned earlier, and I keep stressing, the brain is really complex, and so a change in shape, a change in relative size, relative volume or mass, can have cascading effects. They can affect– probably do affect every function associated with that shape.

But because brains are so incredibly complex, they could have really major neuronal change, major changes at the cellular level which affect the functions of the brain. You could conceivably have really major changes below the surface with no change at the surface, which means that we would have– we may not have any direct evidence for that shift in function and any associated shifts in behavior. And so trying to constrain that, getting an idea of how much neuronal change can there be, how much neurological change, how much behavioral change can there be before you start to see meaningful changes, observable changes in the shape of the brain is a direction of research that lots of labs around the world revolve around, trying to answer that question.

Number one, how much change can there be before you start to see it at the surface? Number two, how much surficial change do you see? And number three, can you go backwards? Once you see that change in structure, can you make a meaningful connection– ideally you can– to the changes in behavior, changes in function, changes in neurology that preceded that shift?

JULIA CLARKE: I just wanted to add to it. The way we are studying this is we want to look at closely related species to see how fast can the brain shape evolve. And that’s Chris is directly involved in, studying now. It’s like, so if a totally new feeding behavior occurs within the last million years or less on what are geologically tiny time scales, how much shift in the shape of the brain occurs? And that is, I think, the necessary next step that he’s pursuing and we’re pursuing to look at that to start answering these questions that we can bring to the fossils in [INAUDIBLE].

CHRISTIE TAYLOR: Is there anything fossils themselves can’t tell us about why animals survive these mass extinction events and how birds have been evolving in this time?

JULIA CLARKE: There is a lot that we do not have in our fossils. We don’t have the whole insides. We don’t have, what were their lungs like? What we’re looking for is fossilizable correlates, things that can enter the rock record, and those are generally, generally, mineralized tissues like bones, or teeth, or the tight wrapping of the brain. So those are things that we get fairly regularly when we have a fossil. So everything else, we don’t have.

I think the big question with the extinction event is also where our fossils come from. We need a global record. We can’t just know what happened in North America, and almost everything that we know about extinction comes from– at this event comes from North America. And so we need a global perspective, and that’s the reason that we’ve been trying to get new data from the farthest conceivable regions from the impact site, which was in Mexico, present-day Mexico, to see what’s happening there.

And I think the other challenging thing, the thing that we may– we’re getting a lot of new proxies for like, OK, what was the first day like? My colleagues at UT here drilled the crater itself. How fast does life come back? We are learning new things about really core questions related to extinction, but there are so many to ask. So if you know any budding paleontologists out there, there is a lot still to learn about this particular event and extinction in general.

CHRISTIE TAYLOR: Just a quick reminder that this is Science Friday from WNYC Studios, talking to paleontologists Julia Clarke and Chris Torres about what fossil birds can teach us about who survives and who doesn’t, especially in times of mass extinction. Julia, you refer to this need for a global fossil record, but what kind of fossil find, as you sort of scour the globe, both of you, would kind of change the game? What is your dream in terms of answering these questions we’ve been discussing?

CHRIS TORRES: Yeah, so my– I want more brains. I just want more brains.

CHRISTIE TAYLOR: So you’re the zombie in the room, OK.

CHRIS TORRES: I’m the zombie in the room, yeah. No, having really nice skulls, which would be great for 1,000 different reasons– so having this one specimen, specimen Ichthyornis, threw so much light on the extinction of most dinosaurs and helped us understand so much how and why living birds were the only dinosaurs that know of that survived. But the grand total of early bird brains that we now have data on has gone from one to two, which is still incredibly tiny.

If we’re going to have a second brain, if we’re going to have two brains, the earliest known bird and then, arguably, the closest relative of living birds are the two best ones to have. But there are so many other species out there that we don’t have data for, and each of those species could represent a totally unheard of unpredicted brain shape that could give us all this insight into how those organisms were living their lives and what the environment was like, what the ecosystems were like. Having more brains would help us fill in the context within which the ancestors of living birds were living when this extinction happened, and so I just want more really nice complete skulls so I can have their brains.

CHRISTIE TAYLOR: And just so I’m hearing you, you’re saying like, we could even find a skull that completely trashes, in scientific terms, everything that you’re proposing in this paper.

CHRIS TORRES: Oh yeah, absolutely. Yeah, that’s true of every paper, and I think it’s really important for authors, especially to acknowledge, that it’s conceivable that there’s data we could collect totally trash our hypotheses. And so what we did in this paper was we had the newest and the broadest source of data yet published because we were able to build off of everybody who came before us and provide something new, and that gave us some new perspectives on what may have happened and helped us refine some hypotheses, reject others. And so more data could totally change the game, and so I want those data, whether they change the game or not. I want those data.

CHRISTIE TAYLOR: I am picturing a Secret Santa gift exchange in which Chris just gets tons of skulls, so thank you for that.

CHRIS TORRES: I am also expecting that.

CHRISTIE TAYLOR: Well, thank you both so much for joining me. We are out of time. We have gone longer than I could have ever dreamed in such a wonderful way, so thank you both for joining us today.

JULIA CLARKE: Yes, thank you.

CHRIS TORRES: Thanks for having us. This is great.

CHRISTIE TAYLOR: Julia Clarke is a professor of vertebrate paleontology at the Jackson School of Geosciences. That’s at the University of Texas in Austin. And Chris Torres is a post-doctoral researcher studying bird paleontology and their brains at the University of Ohio in Athens. Thank you everyone so much for joining us today. I’m Christie Taylor.

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