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This is Episode 2 of Season 3 of the Knowable Magazine podcast. Listen to Episode 1: Scientists warned about climate change in 1965. Nothing was done.
Transcript
Charlotte Stoddart: When you think of a dinosaur, what do you imagine? Is it a hulking great Tyrannosaurus rex? Or a gentle but giant Diplodocus with a long neck? Or do you think of something birdlike? A chicken-sized creature with feathers?
I’m Charlotte Stoddart, and this is Knowable. This season we’ve decided to really focus tightly on just how and when ideas in science develop. In each episode, we’re investigating one text that’s changed the way we think. And in this episode, it’s the ancestry of birds by American paleontologist John Ostrom.
In this one-page paper, Ostrom makes the case that birds are the descendants of dinosaurs. Specifically, a group of dinosaurs called theropods, which includes large charismatic carnivores like T. rex, but also much smaller dinosaurs.
Ostrom’s paper was published in the journal Nature in 1973, and it spurred a revolution in our understanding of dinosaurs and birds. Ostrom wasn’t the first to suggest that birds evolved from dinosaurs, but he revived the idea and made a convincing case. The purpose of this paper, Ostrom wrote, is to present evidence pertaining to the immediate ancestry of birds. Evidence which has generally been ignored for the past 50 years. The evidence he’s talking about is Archaeopteryx. Archaeopteryx was a small, birdlike dinosaur. The first specimens were found in Germany in 1861 and known there as Urvogel, meaning first bird. Remarkably, next to the bones there were fossilized feathers, hints of a close relationship between dinosaurs and birds.
Ostrom called Archaeopteryx perhaps the most famous of all fossils, and said it’s almost universally recognized as one of the most important. So why were the similarities between Archaeopteryx and dinosaurs ignored for 50 years? What other ideas were there about bird evolution? And why does it matter?
These are all questions I put to paleontologist Julia Clarke, who agreed to help me understand the significance of Ostrom’s paper. Like Ostrom, she’s studied the Archaeopteryx fossils. Her research at the University of Texas at Austin focuses on the early evolution of birds and the origin of flight. Fundamental to Julia’s research questions is this idea that birds are the living descendants of dinosaurs. But when Ostrom wrote his paper, this was far from an established fact. There were various different theories about where birds came from.
Julia Clarke: People across the 20th century were even aligning birds with mammals. That was a little, retrospectively, a crazy notion. But there were a lot of different ideas about bird affinities. They were considered so different from everything else that they were difficult to place within the tree of life.
Charlotte Stoddart: The idea that birds are mammals made sense to some scientists. Because like mammals, birds are warm-blooded. And back then, people thought that all dinosaurs were cold-blooded. Another theory was that birds and crocodiles share an ancestor. So far correct. But that birds and crocodiles split off from this ancient reptilian ancestor before dinosaurs. And it’s this idea of a close relationship between birds and crocodiles that Ostrom is disagreeing with in the one-page paper, published in 1973. I called Julia to talk it over.
Julia Clarke: I think I got it here. Because I read that, back when I was in graduate school.
Charlotte Stoddart: Ostrom’s paper is a significant step in the quest to understand the origin of birds.
Julia Clarke: Oh, man. I mean this is a long history and it goes back a long ways.
Charlotte Stoddart: It goes back to the middle of the 19th century — a hundred years before Ostrom’s paper was published. When the idea of birds evolving from ancient reptiles, possibly dinosaurs, was first suggested, it was radical and controversial. Not least because the very idea of one species evolving from another was new and fiercely debated. Charles Darwin’s treatise On the Origin of Species by [Means of] Natural Selection, was published in 1859. Two years later, the first specimens of Archaeopteryx were discovered in Germany. One of those specimens ended up in London, where Darwin’s friend and champion Thomas Henry Huxley examined it. He made a connection between Archaeopteryx, which he classified as a kind of bird, and reptiles, including dinosaurs like Compsognathus.
Some people disagreed with the idea of an evolutionary relationship between these groups. They argued against the very possibility of evolution. Or they argued that any similarities between dinosaurs and birds were a coincidence, and the result of what’s called convergent evolution. Over the next hundred years, this convergent evolution argument persisted, while the idea that birds evolved from dinosaurs fell out of favor. I wanted to know why this idea was ignored and widely dismissed, as Ostrom put it. I asked Julia.
Julia Clarke: There was a very influential book that came out, very interesting book, in the early part of the 20th century by [Gerhard] Heilmann. Heilmann was a fascinating character. He wrote this whole book where he actually details a lot of the traits that Ostrom refers to here. But then concludes that in the absence of one trait that birds cannot conceivably be related to dinosaurs. Even though he beautifully details all of the traits they share with these dinosaurs, he then kind of rejects the notion. It’s a fascinating book, but what he says is on one level, he uses the clavicle as a touchstone, and he says, “In the absence of this, I can’t do this. I can’t make this relationship with dinosaurs.”
Charlotte Stoddart: The clavicle is what’s more commonly known as the wishbone. The Y-shaped bone that sits between the neck and the breast of a bird. It’s the one we sometimes snap and make a wish on. Heilmann thought that dinosaurs didn’t have clavicles. And he took this one piece of anatomy as evidence that birds and dinosaurs were not closely related, despite many other anatomical similarities. By the time Ostrom was writing, a few dinosaur clavicles had been found. But questioning Heilmann’s conclusion wasn’t going to be easy. Heilmann’s book was exceedingly influential.
In another paper, this one published in 1975 in the Annual Review of Earth and Planetary Sciences, Ostrom writes that, “The impact of Heilmann’s book cannot be exaggerated. … So thorough and extensive is the work that few have challenged any part of it and almost no one has raised the question of bird origins since its publication half a century ago. In fact, it has been widely accepted as the last word on the subject and virtually all subsequent works have simply reiterated Heilmann’s conclusions.”
But not Ostrom. Ostrom looked at the evidence and came to a different conclusion. Julia talked me through Ostrom’s evidence in detail.
Julia Clarke: What he did was take evidence, in some cases collected by others. He revisited fossils that had been looked at by others. And he offered concrete observations that could be replicated by others that underpinned his hypothesis.
Charlotte Stoddart: Right. And at the top of the paper, Ostrom writes that the critical evidence of bird ancestry is preserved in the four presently known specimens of Archaeopteryx. Could you introduce us to Archaeopteryx? When were the fossil specimens found? What did they look like? What kind of creature was it?
Julia Clarke: So the first fossil of Archaeopteryx was a single feather. And shortly thereafter there was the discovery of several key specimens — one known as the Berlin specimen nowadays, and one known as the London specimen.
Charlotte Stoddart: These were found in a mine in Germany, is that right?
Julia Clarke: Yes. And the mine was for limestone to make lithographic prints. And that’s why this was being excavated. And they’re like, “Boy, this fossil sure would mess up my nice clean printing of a lithograph.” And so they then went to other destinations. And those were the fossils like Archaeopteryx, and many, many others, including early pterosaurs and other things, that were eventually collected by museums around Europe. And so these two fossils, the London specimen and the Berlin specimen, were remarkably complete. So they preserved most of the skeleton, as well as feathers around the body. And the feathers were clearly associated with the forelimbs and with the tail. These penna or branched feathers that looked identical to those in living birds.
And there were other specimens, like the Eichstätt specimen that Ostrom worked on. And he’s like, “This is another specimen of Archaeopteryx.” There were other specimens that were coming out in different European collections, and Ostrom went and reviewed all of those fossils.
Charlotte Stoddart: What kind of animal was Archaeopteryx? How big was it? Do we know if it was able to fly? What do we know about Archaeopteryx?
Julia Clarke: Archaeopteryx would fit on my desk with ease. Archaeopteryx is not particularly large. Smaller than a turkey, bigger than a chicken. Of course it would’ve had this long, bony tail covered in feathers. But, yeah, pet-sized. And obviously humans did not have Archaeopteryx as a pet because we evolved much later.
Charlotte Stoddart: Why is Archaeopteryx so key to understanding the origin of birds?
Julia Clarke: So Archaeopteryx has a key role to play. And in some ways I would consider Archaeopteryx to be key in kind of a historical sense, in terms of being a catalyst for a paradigm shift.
Charlotte Stoddart: Does their historical importance come from the fact that they were the first fossils found of this type?
Julia Clarke: Yes.
Charlotte Stoddart: With feathers?
Julia Clarke: Yes. The timing of that discovery, and the fact that like a hundred years later they were still pretty much the only such fossils known. We now have tons of dinosaurs that are in the same size range that are covered in feathers. And so the uniqueness of Archaeopteryx is, it’s still a very important species known from all of these different fossils. But we have a much better sense of all of the diversity that’s around in the late Jurassic.
Charlotte Stoddart: Right. And do we know if Archaeopteryx could fly?
Julia Clarke: Oh, man, you’re asking all the tough questions. So first of all, with flight, we have to break it down. Some people want to talk about gliding flight, a behavior that, to the exclusion of active flight, is not known in living birds. So there are no birds that just glide, and do not also actively fly.
In Archaeopteryx, people have looked at the surface area of the feathers of the forelimbs to see whether if you modeled those, they could generate enough lift to keep this animal airborne. There’s been a lot of debates over that. And of course the surface area of the long tail, which could also potentially generate lift. So that’s one way people have gone about estimating, could this thing be airborne?
The second way is whether the feathers on the forelimb are asymmetrically veined. That is a trait that is only known in birds that use the forelimb in active flight. It does have asymmetrically veined forelimb feathers. We now have dinosaurs from China that have asymmetrically veined hindlimb feathers, for example, too. So what I like to say is that this animal is in some way airborne. What we disagree on is, what were the ways that Archaeopteryx used its forewing and its hindwing. And whether these are consistent with a phase in which there was solely gliding before active flight evolved.
Charlotte Stoddart: Whether or not Archaeopteryx could fly and how, to Ostrom it was clearly a dinosaur. A small dinosaur with feathers. As evidence, he lists the traits that Archaeopteryx specimens share with theropod dinosaurs: traits like the number of vertebrae, the structure of the forelimbs, and the shape of the pelvis. Ostrom addresses his paper to a British paleontologist called Walker, who had recently published his own paper proposing a close relationship between birds and crocodiles.
Julia Clarke: And Ostrom’s like, “Hey buddy, all the evidence has been here. We have a pretty good idea where they go and they’re going within Dinosauria.” Yes, extinct crocodiles are their closest living cousins, but there’s a lot of extinct diversity in there, a lot going on. And phylogenetics is what helped us untangle all of those complex relationships. And that wasn’t really a toolkit deployed to look at this group at the time. Or it was just starting to be used.
Charlotte Stoddart: What do you mean by a phylogenetic toolkit?
Julia Clarke: What I mean is the phylogenetics involves looking at atomizing different pieces of evidence. In this case from the anatomy of fossilize parts, which are like bones. And taking all of those traits and saying, “If I look at all of these traits and I estimate, what does the balance of evidence indicate about the relationships among these species?”
Charlotte Stoddart: Let’s pause, Julia, for just a moment to make sure we’ve got this. Because phylogenetics is an important research method in biology. It’s a way of reconstructing the evolutionary history of life on Earth, so that we can draw a diagram, called an evolutionary tree, to show how different species are related. To do this, scientists look at genetic data, if it’s available — which it isn’t for dinosaurs, because we don’t have dinosaur DNA. And they look at morphological traits, the structure of the skeleton, which is what Ostrom did. In the early 1980s, a thorough phylogenetic analysis gave weight to Ostrom’s theory that birds sit within the theropod dinosaur branch of the tree. Julia calls Ostrom a proto-phylogeneticist, because what he did was less detailed and systematic compared with modern phylogenetics.
Julia Clarke: He was not a phylogeneticist, but he’s thinking that way. So he’s thinking, he’s saying, “Hey, Walker, look at all of these traits.” Which he lists. He spends part of his one page listing those traits. And so he’s thinking about atomizing, articulating testable data points, that support his hypothesis. And so he isn’t a phylogeneticist in the practices that we consider that to be now. But he is thinking about these things.
Charlotte Stoddart: Right. And using this approach, he concludes that birds are descendants of theropod dinosaurs. And he uses a word that I’m not familiar with, coelurosaurian dinosaurs.
Julia Clarke: Coelurosaurian dinosaurs. That’s an important word: coelurosaurian. So, coelurosauria is a group within theropod dinosaurs. So he’s giving a specific address within theropod dinosaurs.
Charlotte Stoddart: OK. What I’m trying to understand is how, what he was doing, was different from his peers like Walker, who came to different conclusions from him.
Julia Clarke: So Ostrom’s contemporaries, they were thinking about traits. But they emphasized certain traits to the exclusion of others. For example, while Ostrom emphasized or lists in this paper a set of traits to consider, what we see is a lot more utilization in his contemporaries of the concept of a hypothetical ancestor, of data we have as yet not discovered. Whereas Ostrom was looking and saying, no, we have all of these discoveries, some of them made in the 19th century, that are very coherent with one hypothesis. And they are not coherent or consistent with another hypothesis that actually invokes hypothetical, largely undiscovered fossils that we don’t know of.
So I do think there was a difference there. But these alternative hypotheses, like that of Walker, stuck around for a long time. I remember going to a conference around 2000 and there were still people that could look at things that were bipedal, that were covered in pennate feathers and say, “These are secondarily flightless birds. These aren’t related to bird ancestry.” So that was remarkably persistent, the idea that birds could not be dinosaurs.
Charlotte Stoddart: Is that debate ongoing? Are there still scientists who doubt that birds originated from theropod dinosaurs?
Julia Clarke: I don’t go out of my way to find papers of this sort. If you’re going to overturn a hypothesis based on this many fossils and all these analyses, you have to have even more data to counter that hypothesis, to swing the other direction. And that has not happened.
Charlotte Stoddart: You mentioned that you read Ostrom’s paper in grad school. And I wondered if you remember what you thought of it when you read it back then.
Julia Clarke: So I read this paper, among a suite of papers by John Ostrom, gosh, right around my first year of graduate school. In fact, John Ostrom was in the office two doors down from the lab that I was in. So we would actually see and interact with John even though he was retired, or retiring, at that particular moment.
Charlotte Stoddart: And how important was this paper and the other ones that he wrote at this time for our understanding of dinosaurs and birds?
Julia Clarke: This work was key. Those were signal papers in sort of thinking about the traits that underpin the evolution of a complex behavior like active flight. And Ostrom really set that scene. But of course, I wasn’t around in the 1970s to be living through that part of the revolution.
Charlotte Stoddart: And you use that word revolution. So do you think, I mean, his papers really were revolutionary?
Julia Clarke: So I do describe, and when I teach dinosaur courses, I talk about the dinosaur revolution. And I consider him one of the key players in that revolution. Obviously, as noted in this paper, it has much earlier underpinnings. And there’s a fascinating earlier history to that revolution. All that said, yes, John Ostrom’s work was canonical, it was transformative in thinking about the origin of birds.
Charlotte Stoddart: How was it that our understanding of dinosaurs changed at that time?
Julia Clarke: So the big revolution that I would track back to that time was, if you nest birds within Dinosauria, which was the fundamental premise of this particular paper, and you nest them firmly within theropod dinosaurs, that means you have an origin of high metabolic rates of homeothermy, which is what we think of under the term warm-bloodedness, that occurs within one part of Dinosauria. And we don’t know how far that trait goes back, because we don’t know what are the fossil correlates that we can track to see if it occurs even earlier within Dinosauria. These new data kicked off a hunt for that evidence and for approaches to when you have the origin of warm-bloodedness within Dinosauria or higher metabolic rates. And that search is ongoing. There’s been a lot of breakthroughs in that area. So first of all, there’s how do birds evolve? Which Ostrom very much influenced.
But the other idea is that once you nest birds within dinosaurs, then that changes your whole understanding of dinosaurs. And that was revolutionary. Because before that, if you look at representations of dinosaurs going all the way back to the 19th century, they are sluggish, slithery things that you don’t anticipate had anything like sustained metabolic rates like those we see in living birds and mammals.
Charlotte Stoddart: At the end of Ostrom’s paper, there’s a really intriguing final paragraph. I’ll read it quickly. He talks about Archaeopteryx being descended from these theropod dinosaurs. And then he says, “The additional significance of this phylogeny is that ‘dinosaurs’ did not become extinct without descendants and I suggest that feathers, as thermal insulators, could be the primary reason for the success of dinosaurian descendants. Can it [be just] coincidental that mammals succeeded as therapsid descendants (at least partly) because of a comparable adaptation — perhaps acquired at about the same time?”
Julia Clarke: That’s so cool. So to break down that last paragraph: The first part is super key and was part of that revolution. The idea that Dinosauria did not go extinct, that dinosaurs are around today, but we call them birds. The second part of his statement is the hypothesis that you have body-covering innovations that are firmly linked to survivorship.
So I would say the present state of that hypothesis, and those in light of new data is that we have many forms that were, in fact, feather-covered that went extinct 66 million years ago. It isn’t like the non-feathered-versions species went extinct and the feathered versions persisted. There were many species that were feathered that also went extinct. So it doesn’t fully explain the pattern of survivorship and extinction within Dinosauria. Secondly, so he’s right about our earliest evidence now discovered after Ostrom is that mammalian hair is known from at least the same time period in the Jurassic.
So it’s not like those traits evolved then there’s a mass extinction event. There’s a lot of intervening evolution of haired and feathered forms, and then there’s an extinction event. We now know that lots of other dinosaurs had fuzzy stuff, kind of like hair, that presumably participated in their thermo-regulatory strategy. And that includes things that are related to Tyrannosaurus. And they clearly went extinct. So I will say that as a mechanism for survivorship, it is insufficient. However, he’s getting at things that we definitely have investigated further and think about. But I would say that we kind of know that to be an insufficient explanation.
Charlotte Stoddart: As well as arguing that birds evolved from dinosaurs, an argument that was ongoing for decades afterwards, Ostrom’s paper triggered many other questions. Questions about if and how Archaeopteryx could fly. Questions about how many different dinosaurs had feathers. And what kind of creatures dinosaurs were. And questions about the role that feathers might have played in the survival of some birdlike species. Researchers like Julia are still trying to answer these questions. And our understanding of birds and dinosaurs continues to evolve.
Thank you for listening to this episode of the Knowable Magazine podcast. If you want to dig more into the science, you’ll find a copy of the transcript along with links to the related papers at knowablemagazine.org/podcast. You’ll also find a link to sign up for the Knowable Magazine newsletter. Subscribe to that to stay in the know.
This season, we’re releasing a new podcast episode every fortnight. Up next: How the placebo effect has been used and understood since 1955, when Henry Beecher published a classic paper on the subject. And after that, Einstein’s concerns that quantum described a spooky world, and the experiments that set out to prove him wrong.
If you enjoyed listening to this episode, please help others find us by sharing it with your friends and family, or by leaving us a review.
This podcast is produced by Knowable Magazine, a journalistic publication that seeks to make scientific knowledge accessible to all. Knowable is published by Annual Reviews, a nonprofit publisher dedicated to synthesizing and integrating knowledge for the progress of science and the benefit of society.
This episode was written and produced by me, Charlotte Stoddart, with editorial contributions from Eva Emerson and Rosie Mestel. Thank you to my co-host, Adam Levy. And a special thank-you to Julia Clarke for taking the time to share her insight and expertise.
I’m Charlotte Stoddart, and this has been Knowable.
