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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Birds are the only dinosaurs alive today. All the other dinosaur lineages, the tyrannosaurs, the long-necked sauropods, the horned and armored giants, were wiped out 66 million years ago when an asteroid struck the Yucatán. But one small group of beaked, fast-growing dinosaurs made it through, and their descendants now number over 10,000 species. How do we know that birds are dinosaurs? Why did this one branch survive when so many others, including most birds themselves, did not? And what does the fossil record actually tell us?

Steve Brusatte is Professor of Paleontology and Evolution at the University of Edinburgh. His research has traced the evolutionary transition from ground-living theropod dinosaurs to modern birds, drawing on the spectacular feathered fossils unearthed over the past three decades in northeastern China. He has named more than 50 new species, including a feathered, winged raptor he christened Zhenyuanlong. He is the author of The Story of Birds, published earlier this year. Steve Brusatte, welcome to Geology Bites.

Steve Brusatte: Oliver, thank you very much for having me.

Oliver Strimpel: Most listeners will know that birds evolved from dinosaurs. But birds literally are dinosaurs, in the same sense that humans are mammals. What's the difference, and why does it matter?

Steve Brusatte: Birds are dinosaurs in a literal sense. Birds, they're part of the dinosaur family tree. Because of that, they have all the classic features of dinosaurs, the features of the pelvis and the backbone that make dinosaurs dinosaurs. Or maybe a better way to think about it is with an analogy to bats. So bats, what are they? Well, they're mammals. They have hair and feed their babies milk. They do things mammals do because they evolved from other mammals. They are part of the mammal family tree. They're just an unusual mammal that got small and evolved wings and developed the ability to fly. And birds are really the dinosaur equivalent of that. They're a strange type of dinosaur that got small, evolved wings, and developed the ability to fly.

And the big difference is that all the other dinosaurs died with the asteroid. It's morbid to think this way, because it means we wouldn't be here, but imagine there's some extinction that kills off all mammals except for bats. That's really what we're dealing with. So when it comes down to it, we classify species by ancestry, not by appearance. I know a bird doesn't look like a T. rex or a brontosaurus, the same way a bat doesn't look like an elephant or a rhino or a human. Which is, I think, super cool, because it means there is this legacy of the age of dinosaurs, this primeval world of T. rex and Triceratops, that lives on today.

Oliver Strimpel: Many features that we think of as bird features, feathers, hollow bones, large brains, actually evolved in dinosaurs that couldn't fly. Feathers, for example. The fossil from China that settled this in 1996, Sinosauropteryx, wasn't even on the line to birds. So what were feathers for originally, and what does that tell us about how flight evolved?

Steve Brusatte: You're absolutely right. So many of the features we see in birds today, things that are uniquely bird features among living animals, things like feathers, but also the big wings made of those feathers, the wishbones at the front of the chest, the hollowed-out bones that are full of air. Frogs don't have those things. Fish don't have those things. People don't have those things. Those are bird features, but when we look in the fossil record, they are actually dinosaur features. These things evolved one by one over many millions of years in the dinosaur ancestors of birds. And that is the evidence that birds are dinosaurs. It's not like we have the DNA of a velociraptor that proves this story. No, the evidence is in all the shared features of the skeletons.

And this evidence was first recognized back in the 1860s by Thomas Henry Huxley, Darwin's great disciple, in those few years after The Origin of Species was published, when Darwin was first articulating his theory of evolution. It was by looking at these shared features of the skeletons of today's birds and some of the first good dinosaur skeletons that were being discovered and studied. That's what keyed Huxley on to this idea.

Nowadays we have a lot more evidence, and some of the best evidence is that we now know that a lot of dinosaurs had feathers. This was unknown at the time. It was unknown until, as you say, 1996, three years after Jurassic Park came out. That's why the raptors don't have feathers in Jurassic Park: Steven Spielberg didn't know, because the fossils hadn't been found. It's just very hard to preserve soft stuff like feathers and skin and muscles and internal organs as fossils. Most fossils are the hard bits: bones and teeth and shells and such.

But it just so happened in 1996, a farmer in northeastern China, incredible story, a farmer out working his land, started to crack open some of the rocks, and he looked inside and there was a small dinosaur. The bones were all there. It was covered in a halo of feathers, quite simple feathers, downy feathers, and this was the first time somebody had seen a dinosaur fossil with feathers. That really sealed the deal. Huxley was right; birds did evolve from dinosaurs.

What happened was that this part of China back in the Cretaceous period was teeming with dinosaurs, and there were some volcanoes in the background, and occasionally they would erupt and bury these ecosystems, kind of like when Vesuvius erupted and buried Pompeii. What that did was basically capture those dinosaur ecosystems in a freeze frame. It locked in the soft tissues. They didn't have time to decay away. And that's why you see the feathers. That first discovery opened the floodgates, and now there are thousands of feather-covered dinosaur fossils from all across northeastern China. New ones are found all the time, mostly by the farmers.

These fossils tell us definitively that feathers first evolved in ground-living, distant dinosaur ancestors of birds. And they developed first as quite simple structures that looked really like hair, just little strands. They could not have been used for flying any more than we can fly with our hair. It tells us that something as intricate, as beautiful, as advanced as the feather of modern birds evolved in a simpler state for something totally different, probably for insulation, the same reason hair evolved in mammals. But later, those simple feathers were elaborated, and they became the quill pens that became the wings that allowed some dinosaurs to defy gravity and take to the skies.

Oliver Strimpel: So of all the birds that were actually able to fly at the end of the Cretaceous, only one branch made it through. In your book, you've talked about the features that distinguished the survivors from those that perished. They include small body size; beaks instead of teeth; a kinetic skull; a sophisticated digestive system, with things that are only in birds, like a crop, a gizzard and gastroliths; very fast growth from hatchling to adult; full warm-bloodedness; many of them living on the ground or near water rather than in the trees; a diet that included seeds; and, really underlying it, a fairly modern body plan. So let's take a few of these one at a time, the main ones. Let's talk about beaks. Why did they evolve, and what was driving that during the Cretaceous?

Steve Brusatte: Birds have beaks. Birds do not have teeth. Now, in the lab, you can actually manipulate the genes of a little baby bird in the egg, and you can make it start to grow little teeth that actually look like the teeth of raptor dinosaurs. That's some of the evidence that modern scientists have added to support this idea that birds evolved from dinosaurs. But birds today can't grow those teeth. In fact, the beak starts developing as those teeth in the embryo start to waste away.

So birds have beaks. There are lots of things birds do with their beaks today, lots of different shapes and sizes of those beaks. Most of that diversity has come quite recently. Back in the Cretaceous period, there were various groups of birds that were independently losing their teeth and evolving beaks, probably because of diet, because having a sharp beak that could be shaped into many different forms was just more plastic. It was probably more evolvable than having a set of teeth. Different groups were doing this differently, but it was the modern birds that really went all in for the beak. The beak of modern birds is very kinetic, and we can dive into that a little bit more: it can move quite well independently of the rest of the skull. It's really good for eating harder food, for cracking seeds and nuts. And this was happening in these modern-style birds in the latest Cretaceous.

It's not something that only happened in the modern-style birds. Some of the other groups of more primitive birds were independently getting beaks, because beaks were useful. But having that beak probably helped the modern-style birds make it through the asteroid. The beak alone wouldn't have done it, because some other birds had beaks, but having a beak that could eat seeds did. Seeds would have been some of the last surviving food. We know this today from ecological disasters, from forest fires and volcanic eruptions. Seeds can last a long time in the soil, so if you could eat seeds, that would just give you more food for more time. It would buy you more time.

Oliver Strimpel: You mentioned the kinetic skull. That is a kind of amazing mechanism that lets the upper beak move independently of the rest of the head. Could you describe a little bit how that actually works, and how that really works in a bird's favor?

Steve Brusatte: It's incredible. The head of a bird is so different from our head. We have basically the skull: the cranium and the lower jaw. Now, our skull is made up of many different bones, but they fuse together into one, and by the time we're a toddler, it's all getting fused up. But birds have a very different skull. It's very lightly constructed. There are lots of different bones, and those bones are generally more loosely connected to each other. Yes, the bones surrounding the brain are really tightly fused up, but the bones of the lower jaw and the palate and the beak, and those around the eye, are all separate systems that are held together by very mobile joints, and a lot of ligaments and tendons and muscles.

And there's a bone at the back end of the upper skull. It's called the quadrate bone. We have a remnant of it. The remnant of the quadrate bone in us is actually in our ear, because mammals have been so modified. But birds, and also dinosaurs and reptiles today, have this quadrate bone that forms the joint with the lower jaw. In birds, that bone is really mobile. It's basically a hinge. It can move back and forth, kind of like a door hinge. And it connects to both the side of the face and the palate. Both the side of the face and the palate are basically push rods, so when the quadrate swivels, that motion is transmitted to those push rods. They push on the beak, and the beak is hinged relative to the rest of the skull. So the quadrate swivels, pushes the push rods, and that hinges the beak up and down. It's just a very mobile system.

And it means that birds can use their beaks in a really sophisticated way. Birds have hands, but the hand of a bird is not like our hand. It doesn't have separate fingers that can grab things and manipulate things. The hand of a bird is all fused up and supports the primary feathers of the wing. So the hand has to be rigid and strong to be part of the airfoil. So what do birds do to compensate? They use that beak. I know "manipulate" is a word that means using hands, but they manipulate things with their beaks. They have fine control over their beak. You see birds eating with their beaks, of course, but you see birds preening themselves, cleaning their feathers. You see birds picking up their babies with their beaks. You see birds doing all kinds of things with their beaks.

Oliver Strimpel: What about the digestive system?

Steve Brusatte: The digestive system of a bird is very different from our digestive system. It has the same basic components, an esophagus and a stomach and intestines and so on. But the esophagus of a bird has what's called a crop, and that's a pouch in the throat, really, that stores food. So a bird can eat a lot of food without passing all of it through its digestive system at once. And then the stomach of birds is actually divided into two parts. One of the parts of the stomach is like our stomach; it has chemicals for breaking down food. But the other part, the gizzard, also breaks down food during digestion, but not chemically. It does it mechanically, because gizzards often have these stones in them called gastroliths, and they basically are a bunch of roughage that just breaks up what the bird eats.

The reason why this is so important, having that crop to store extra food and regulate the pace of digestion, and having the gizzard to grind stuff up, is that, first of all, it helps compensate for birds not having teeth. Birds cannot chew their food in their mouth like we can. They basically use the gizzard to do it. But birds also have to have a lightweight digestive system because of flight. And birds are very active and energetic, and they're warm-blooded, and they need a lot of food. So how do you compensate? Well, you have this digestive system that's highly regulated by the crop, where the gizzard can really break down food. That's what modern birds do.

Now, we don't know exactly how that digestive system evolved, but there are a few fossils buried by some of those volcanoes in China that actually preserve some of the digestive system. And we can see that some of those features, especially the crop, were evolving at least by the Cretaceous in some of these early birds. So it wasn't something that just evolved in modern birds. It's something that goes back quite far in their fossil ancestors.

Oliver Strimpel: And then there's the growth and metabolism. The birds that survived went from hatchling to adult in a single year. How do we actually know that? And what does it tell us about how they were living?

Steve Brusatte: It's really remarkable. Birds today grow super, super fast, much faster than us. In many birds, the bird is born and grows from a hatchling into an adult within a few weeks, maybe a few months at most. And this is why we don't often see baby birds. This is enabled by the high metabolism of birds. Birds are warm-blooded like us. The earliest birds, like Archaeopteryx, that still had teeth and claws, did not grow that fast. They grew more like dinosaurs, like raptor dinosaurs, and even many other early birds did not grow that fast.

How do we know it? Because dinosaurs have growth rings in their bones, like tree trunks do, one ring laid down every year. So you can cut open a dinosaur bone, count the rings, and see how old it was. And that tells us that a lot of these early birds still took many years to become adults. But there are some Cretaceous birds that were living right at the end of the age of dinosaurs, right before the asteroid, at the same time as T. rex. These birds had beaks, and these birds could grow super fast. They have no growth rings at all. That means that they must have gone from a hatchling to an adult within a year. Some of these birds are birds like Vegavis, which lived in Antarctica right at the end of the Cretaceous, or Asteriornis, which lived in Europe. So these keystone features of modern birds didn't evolve after the asteroid. They didn't evolve because of the asteroid. They were there before, and they probably enabled modern-style birds to get through the asteroid.

Oliver Strimpel: One of the interesting things that you hinted at then, with the work in Antarctica and Vegavis, is that some of these features that seemed to be nice to have before the asteroid turned out to be must-haves after it arrived and the conditions got so extreme. And there's evidence for that in the late Cretaceous rocks in Antarctica, where it was cold and dark for months at a time. When teams have searched for fossils in the sedimentary rocks from that period there, they've only found modern-style birds and no primitive long-tailed forms. So would it be fair to say that the body plan that would survive the asteroid was actually already winning at the high latitudes before the asteroid hit? And so in some ways those birds were actually pre-adapted to the environmental extremes that were to follow?

Steve Brusatte: That seems to be a very plausible scenario. The evidence comes from these fossils from Antarctica discovered by Matt Lamanna, Julia Clarke and their team of really intrepid paleontologists. I mean, they go down to Antarctica, they camp amid the snow, they're in their super-heavy-duty Gore-Tex field gear, and they are looking for fossils in frozen rock. These fossils are important because they come from the few million years before the asteroid. And some of these fossils are birds. All the bird fossils found down there are of modern-style birds. It doesn't seem like more primeval birds survived at the high latitudes, or if they did, they must not have been very common.

Now, back in the Cretaceous, there wouldn't have been ice caps at the poles. The Earth was much warmer, but still those areas would have been dark for about half the year, because the orbit around the sun was pretty much the same then. And there would have been parts of the year when it would have been freezing at the poles, even if they weren't covered in ice. So to survive there, and especially to live there year-round, which we don't know for sure, though there is evidence that some of these dinosaurs did nest in the polar areas, which would mean they probably spent a lot of time there, you would have to be hardy. You'd have to be able to deal with the colder temperatures and deal with the dark. And probably that really fast metabolism and fast growth of birds like Vegavis from Antarctica helped underpin their survival at the high latitudes.

And then when the asteroid hit, all the soot from the fires and the dirt and the dust and the grime from the collision went up into the atmosphere and blocked out the sun for several years. It was a global nuclear winter. The Earth went dark and cold, maybe for a few years, maybe up to a decade. We don't know for sure. But we do know that there was a mass die-off of plants. We see this big fungal spike in the fossil record, where mushrooms and other fungi took over the world as they encrusted all the dying plants. And then a bit later we see a big spike in ferns, which, as we know from modern disasters, are a recovery flora, the first plants that come back. So we know that there was this long-term ecological upheaval, and if you were already living in the polar areas, if you were already dealing with dark and cold, that would have helped you when the Earth, just by chance, turned dark and cold after the asteroid. So we think that was integral to these birds surviving, and part of the reason why they were the only birds to survive.

Oliver Strimpel: I wonder if you could spell out a bit more how each of these characteristics that we've noticed in modern birds, and that distinguish them from some of the birds that didn't make it through, became so useful in the aftermath of the cataclysm.

Steve Brusatte: It really was a cataclysm. This was a six-mile-wide, or 10-kilometer-wide, rock that smashed into the Earth. It released more energy than a billion nuclear bombs put together. It triggered earthquakes and wildfires and tsunamis. We see evidence for all that in the geological record. And then of course, over the next few months and years, the Earth went dark and cold with that nuclear winter, and 75% of species, more or less, went extinct. Basically anything bigger than a husky dog that lived on land died. And it makes sense why. If you were bigger, you would need more food, and food would have gotten really scarce really quickly. But also, the bigger you were, the harder it was to hide, especially during those first few hours and days when there were all those tsunamis and wildfires. And of course, bigger things usually take longer to grow. For many dinosaurs, it took many years, even decades, to go from a baby to an adult. That's a long time to get through if the Earth is chaotic. So I think we can see why T. rex, Triceratops, the long-necked dinosaurs, those canonical dinosaurs, died. They were simply too big. And other big stuff died too.

So the first rule of survival is just to be small. Smaller things generally need less food, they can hide more easily, they can dig burrows or just shelter themselves away, and often they grow faster from a baby into an adult. Birds met those criteria, but so did many of the birds that still had teeth and claws and long tails. So why did they die? Well, it seems like the modern-style birds had the beaks, so they could eat seeds, some of the last surviving food, and they grew super fast, so they could turn over the generations quickly after the asteroid. They were accustomed to living at those high latitudes, so they could deal with darkness and cold. They were pretty small, even for birds. I mean, most of the ones that survived you could hold in your hands. And there are many other reasons too. But we think generally it comes down to having a good hand of cards.

Because when that asteroid hit, there was no time for organisms to adapt through the normal processes of natural selection. That would come in the aftermath, as the Earth healed. But you had to face that asteroid with whatever hand of cards you were dealt. The deck was frozen, and dinosaurs like T. rex and Triceratops had the dead man's hand, but modern-style birds had a good hand of cards. Even if you have a good hand of cards, you're still dealing with the other players at the table. You still need luck on your side, and timing. But if you could get through that extinction, if you could endure, just imagine what it would be like on the other side. You're now past the Cretaceous. You're into the next interval of time, what we call the Paleocene, and there's no T. rex, no Triceratops; 75% of all species are gone. Opportunities abound, and evolution then went into overdrive.

Oliver Strimpel: Right, I guess tons of ecological niches were just there for the taking, particularly when the forests came back, I imagine.

Steve Brusatte: Absolutely. The forests would have collapsed, and we have evidence for this in the fossil record. Again, we see the spike in fungus, then we see the ferns come back. And it probably took centuries, maybe even millennia, for the really big forests to grow back. If there's no sunlight or very little sunlight, plants wouldn't be able to photosynthesize, especially bigger plants that needed more light. So it would have been an ecological cascade, a collapse. But the Earth did recover. Seeds did remain in the soil. Those trees could grow again, and they did. And by a few thousand years out, in the fossil record, we see communities coming back. It was a pretty quick recovery, all things considered, at least compared to some of the other mass extinctions of Earth history that were less sudden, less of a punch to the gut like that asteroid, and had more prolonged causes and recoveries.

Oliver Strimpel: As is pretty clear from our conversation so far, you're decisively in favor of the impact theory of extinction: that the asteroid alone did it, and did it rapidly. But the Deccan Traps were erupting at the same time, and other mass extinctions, like the catastrophic end-Permian and end-Triassic ones, are convincingly tied to flood basalts of the type of the Deccan Traps. So why are you so confident that the asteroid is to blame this time, rather than volcanism or some combination?

Steve Brusatte: Well, it comes down to the evidence we have from the fossils, and some of this evidence is evidence I've collected myself with big teams. I've worked a lot in New Mexico. We have these big teams that look at what was happening right before the asteroid and then what happened afterwards. And what we see in New Mexico and Montana and the Dakotas is a thriving community of dinosaurs right up to the extinction. We can very accurately date the fossils showing this group of dinosaurs in New Mexico. We have meat-eating dinosaurs, big ones and small ones. We have some of the biggest long-necked dinosaurs that ever lived. There are horned dinosaurs and duck-billed dinosaurs. They were there right before the asteroid. The Deccan volcanoes had already started to erupt by that time. Now, the biggest pulse of the eruption was around the time of the asteroid. But if the volcanoes were slowly changing the environment, we wouldn't expect to see dinosaurs thriving so much, not just in New Mexico, but also in northern North America, where there are also very well-dated fossils. We would see some kind of decline, and we just don't see that. So all the evidence that we have really shows that the extinction was sudden. Dinosaurs had been around and thriving for over 150 million years. There was no sign anything was seriously amiss.

Now, there are a couple of caveats there. We don't have great fossils of the very last dinosaurs from outside North America. We're getting better ones, especially from Europe and from South America and from parts of China. But still, as far as really accurately dated dinosaurs from right before the asteroid and during the time of the volcanism go, it's mostly a North American story. So it is possible that in other parts of the world dinosaurs were in a bit of decline. But I don't think so, because of the evidence we have so far from these sites, especially in Europe. Europe was a series of islands at the time. You had a whole bunch of dinosaurs very different from North America's, dwarf dinosaurs, small dinosaurs, bizarre dinosaurs living on these islands. They were very diverse too, it seems, right to the end. But still, we need better fossils from other parts of the world.

And the second thing is that the Deccan volcanoes had been erupting, but it does seem like the big pulse of eruption was right around the time of the asteroid. Some geophysicists have even proposed that the asteroid punched that volcanic system into overdrive, which is possible. But regardless of that, it does mean that it's hard to get the fine-tuned details right of what's the asteroid and what's the big pulse of volcanism. It's very possible that the Deccan volcanoes were affecting dinosaur ecosystems during the very end of the Cretaceous, in ways we don't quite understand from the fossil record. Maybe the resolution of the fossil record isn't good enough to show changes that were happening over millennia or centuries because of those volcanoes, changes that maybe made them more vulnerable to the asteroid. So I want to keep an open mind about these things. But by and large, I am pretty confident that if that asteroid had not hit, the dinosaurs would have just sailed through and continued to change and continued to evolve, not only in the form of modern birds, but in the form of tyrannosaurs and horned dinosaurs and duck-billed dinosaurs.

Oliver Strimpel: There's a striking finding about what happened to brains after the asteroid. Surviving birds underwent a dramatic increase in relative brain size. Meanwhile, mammals went the other way. Could you tell us that story? And does that actually call into question the commonly held picture that mammals sort of won out after the end-Cretaceous extinction, that we live in the age of the mammals?

Steve Brusatte: Birds survived the extinction, at least modern-style birds did. There were also some small, furry little critters that could eat lots of different foods and hide away easily and dig burrows and grow fast and reproduce fast. And those were our ancestors. As the Paleocene dawned, these surviving birds and mammals would battle for evolutionary supremacy. Maybe that's a little bit dramatic, but they both adapted as the Earth recovered. And we like to think of today as being the age of mammals. And yes, mammals are very diverse: we have bats and whales and elephants and dogs and cats and humans and all kinds of mammals, an incredible diversity of size and shape and diet and behavior. But there are about 6,500 species of mammals today. There are at least 10,000 species of birds, probably quite a bit more. So there are more or less double the number of bird species as mammals today. So in that sense, the age of dinosaurs continues.

But when it comes down to it, both birds and mammals survived. Both birds and mammals are pretty smart, and intelligence has had something to do with their survival and adaptation over the years. But they responded in very different ways, in terms of their brains and intelligence, to the asteroid. When the asteroid hit, you had birds that were already flying, and those birds already had pretty big brains relative to their bodies, which they inherited from their raptor dinosaur ancestors. You had mammals living when the asteroid hit that also had pretty big brains. But those mammals were all small. There was never a mammal that lived with the dinosaurs that was bigger than a house cat, as far as we know.

Then the asteroid hit, and T. rex and Triceratops and their kin were wiped out. There were job openings at the top of the food chain. And what did mammals do? Mammals explosively diversified. A lot of new species evolved from those survivors, and they became big fast. Within 200,000 years of the extinction, just in New Mexico alone, we have mammals the size of pigs. Remember, they had never gotten bigger than a house cat for the previous 150 million years. So within 200,000 or so years, mammals the size of pigs. Within a million years, mammals the size of cattle. How were they able to do this? Probably because they could give live birth to bigger babies that developed in the safety of the womb. Some of these surviving mammals were placental mammals like us that could give birth to big babies.

Now, they got so big so fast that their brains actually did not keep pace. The brain relative to body size actually decreased. To be really glib about it, mammals got dumber for a little while after the extinction. The birds were different, and some of the birds that survived actually got smaller. As their bodies got smaller, their brains proportionately got bigger. So they became quite smart. And for many millions of years, maybe even tens of millions of years after the asteroid, it was birds, eventually birds like crows and parrots, that were probably the biggest-brained animals, maybe even until quite recently, when some apes came down from the trees and started to walk on their hind legs.

But mammals would eventually catch up. Beginning about 10 million years after the asteroid, mammal brain size started to increase. So really, for most of the last 50 or so million years, birds and mammals have both been intelligent, warm-blooded animals coexisting. In many ways it is the age of mammals, but it's also the age of birds, and therefore the age of dinosaurs does continue a little bit.

Oliver Strimpel: Steve Brusatte, thank you very much.

Steve Brusatte: My pleasure.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.