A Reptile’s Baffling Backfin And The Math Of Dashing Dinos

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Brace yourself. New animal appendage just dropped. Paleontologists have identified an ancient reptile with a towering crest on its back that is not made of skin or scales or feathers or antler, but something else entirely that we have never seen before. The small creature sporting this befuddling back fin is named the Mirasaura and lived during the middle Triassic period, about 250 million years ago, just before dinosaurs evolved.

Here to discuss this novel skin gear and what it tells us about the evolution of dinosaurs, birds, and feathers is Dr. Richard Prum, Professor of Ecology and Evolutionary Biology and Head Curator of Ornithology at the Peabody Museum of Natural History at Yale University. He’s based in New Haven, Connecticut. Rick, welcome back to Science Friday.

RICHARD PRUM: Thank you. It’s a pleasure to be here.

FLORA LICHTMAN: New flesh thing. That seems big.

RICHARD PRUM: Yeah. Well, in the skin world, this is a big deal. These structures are just so weird compared to anything we’re familiar with among extant animals.

FLORA LICHTMAN: Well, tell me more. Why are they weird?

RICHARD PRUM: First of all, the structures we’re talking about are part of a group of anatomical parts that are things that grow out of the skin. These integumentary appendages include hair and feathers and scales. Also antlers and horns, claw and nails, that whole group of things.

So that’s already really diverse. And these are really extraordinary in the fact that they’re flat like a feather, but unbranched and contiguous like a sheet of tissue. And they extend off the back of this tiny little lizard-like reptile. And the longest ones are nearly the length of the body of the animal. So these would have been very weird [LAUGHS] to encounter in life, and equally strange in fossils.

FLORA LICHTMAN: Do we know what their texture was like? Are they stiff? Are they soft?

RICHARD PRUM: That’s really interesting. The authors of this new paper support the idea that they’re stiff, that they’re solid, and if you will, keratinized like a horn or a nail or a feather. But it’s interesting. I think that we still aren’t exactly sure. These structures are kind of wrinkled in their preservation, and those wrinkles seem like they could be artifacts. So they might have been softer and fleshy, but they have a central shaft with a blade of tissue extending up from the back.

FLORA LICHTMAN: A central shaft with a blade of tissue extend– I’m having a hard time visualizing that.

RICHARD PRUM: Actually, it looks a lot like an unbranched fern frond. And that is actually one of the things that people thought this might be early on in the history of this fossil.

FLORA LICHTMAN: How do we know this is actually a completely new kind of skin organ?

RICHARD PRUM: So one of the reasons why this is so exciting is that there is the possibility, and actually, previous suggestion that these kinds of structures might be homologous with feathers. That is, early representatives of that appendage.

So the way in which we actually would study that is both to describe the anatomy of these materials, but also to make sure that we have the phylogeny right. That is that we’ve plugged these animals in the right branch on the tree of life.

So one of the extraordinary things about this paper is that they’ve done really careful phylogenetic work. They took these beautifully preserved but crushed, if you will, flattened fossils, and did advanced tomography on them, CAT scans, and then reconstructed the skull. And using that information, we’re able to find out where it belongs in the tree of life. And that, of course, is important, because that is really going to affect what you conclude about how these things may have evolved.

FLORA LICHTMAN: And where does it fall, and what does it mean for whether it’s a proto feather or not?

RICHARD PRUM: Well, it turns out that they have identified, I think, really robustly and for the first time that this Mirasaura is in an early diverging branch in the portion of the tree of life we call reptiles. It’s outside of what we would call the crown clade, the group of reptiles that includes all the extant organisms that we know, birds and crocodiles and lizards and snakes, turtles. It’s outside of that group. And that implies, since it’s so distant from birds, that this is an evolutionarily independent event.

FLORA LICHTMAN: Hmm. Does it tell us anything about the story of skin?

RICHARD PRUM: Yeah. Well, you know, what I think is most exciting about this research is that it’s a turn away from some paleontological work in recent years. Going back 20, 25 years ago, there was very exciting developments in understanding the evolutionary origin of feathers. People have discovered other kinds of reptiles with interesting things growing on their skin.

And the effort has usually been to associate them with feathers and to push back the origin of feathers into earlier and earlier lineages. And so what this work shows is that feathers aren’t the only complicated thing growing out of the skin of reptiles.

FLORA LICHTMAN: [LAUGHS]

RICHARD PRUM: They have independently evolved a really weird, large, blade-like structure. And so what it teaches us is that the history of the evolution of reptile skin– indeed, amniote skin– is complicated, right? I think it’s a move toward recognizing that these things are not feathers, but have evolved in a complicated way, independently of feathers. And it implies that there’s something special about reptile skin that’s fostered this diversity.

FLORA LICHTMAN: Hmm, that’s fascinating. I mean, do you think that paleontologists are going to revisit fossils and wonder whether they made the wrong assumption about feathers after seeing this?

RICHARD PRUM: Well, this is always being a tug of war intellectually, different people publishing different ideas. Yeah, I hope that we get a movement toward that. And one of the ways in which that happens is really looking at not only the unusual structures, but all the lineages we have between birds and these other reptiles that don’t have weird appendages on the skin, that are just scaly, right? So to me, I think feathers originated in theropod dinosaurs, the lineage of bipedal, mostly meat-eating dinosaurs to which birds belong, and that these other things are really additional chapters in the complex history of reptile skin.

FLORA LICHTMAN: As always, the story is more complicated than we thought.

RICHARD PRUM: Well, you know, [LAUGHS] that’s what makes it fun. And I mean, one of the fascinating things about this is that these specimens have been sitting in a collection. They were actually all collected, I think, on the same day in the 1930s, an extraordinary but small place in eastern France, and misidentified as plants, as potentially fish. And it was only recently when that private collection became part of an established museum that the research team realized, wow, these are really weird and deserve focused attention.

FLORA LICHTMAN: You’ve written about the evolution of beauty. Does this fit into that story?

RICHARD PRUM: Well, these things are so unusual. And that’s what– paleontology often requires a kind of fantasy, functional biology, where you go, wow, what were these for? And of course, a lot of that’s pretty speculative.

Obviously if they were hard and rigid versus soft and fleshy, that could really affect what they might have been doing, right? But they’re certainly, without a doubt, conspicuous.

And these authors claim that they were immobile. That is, that they stuck up in this sort of– above the plane of the body permanently. The study also shows that these structures were melanized. That is, that they had at least some black or red-brown pigments in them. So we know they were colorful or colored. Were they patterned, spots and dots and stripes? We don’t that yet.

So they could have been– I don’t know– thermoregulation. They could have been useful in communication, et cetera. So it’s possible that these things evolved for social sexual display, which means that they could be– their function could be in essentially some variety of reptilian beauty.

FLORA LICHTMAN: Reptilian beauty. That is the perfect place to land. Thanks, Richard.

RICHARD PRUM: Thank you very much, Flora. A pleasure to be here.

FLORA LICHTMAN: Dr. Richard Prum, Professor of Ecology and Evolutionary Biology and Head Curator of Ornithology at the Peabody Museum of Natural History at Yale University in New Haven, Connecticut. To close out the hour, moving from ancient reptile aesthetics to ancient reptile athletics. How fast could dinosaurs run?

It turns out the equation scientists have been using for five decades to estimate dinosaur speeds, eh, maybe isn’t so accurate. Here to tell us what this could mean for Velociraptor velocities, T. rex tempos, Spinosaurus speeds– you get where I’m going– is Dr. Peter Falkingham, Professor of Palaeobiology at Liverpool John Moores University in England. Peter, welcome to Science Friday.

PETER FALKINGHAM: Hi. Thanks for having me.

FLORA LICHTMAN: We’re going to get into the details of this study in a second, but I’m going to start with the question that I think we all want to know, which is, according to these new calculations, were dinosaurs faster or slower than we previously thought?

PETER FALKINGHAM: [LAUGHS] This doesn’t change maximum speeds, which we get from other means. What it changes is how reliable we think the speeds are from trackways. And in that regard, mm, most of the trackways, the animal was probably moving slower than we thought.

FLORA LICHTMAN: I love that outcome. Why did you take this question up?

PETER FALKINGHAM: One of the things we see in paleontology is people really trying to make it more quantitative, more computational. But one of the side effects of that is that people like to get numbers out of equations. And this is an equation that has been around for five decades, more or less. And people like to use it.

But the problem was it was becoming more and more common that people were reporting speeds from trackways to the nearest centimeter per second. They’d be saying it was moving 7.56 meters per second. So that kind of accuracy just isn’t really feasible or useful.

FLORA LICHTMAN: And when you say trackway, do you mean people saw fossilized prints and they were like, OK, based on these prints, we think the dinosaur’s moving at this speed?

PETER FALKINGHAM: Yeah, exactly that. So if you think about when you walk versus when you run, you take shorter strides when you walk than when you run. And the equation is based on watching mainly mammals– there are some birds in there– running and walking and measuring the stride length and relating that to the speed they’re moving at. So in theory, you can look at a trackway, measure the stride distance, and calculate the speed.

FLORA LICHTMAN: And what’s wrong with the old equation?

PETER FALKINGHAM: There’s nothing wrong with it as such. It was never meant to be a precise tool. So McNeill Alexander is this absolute giant in the field. He wrote some amazing papers. And this is one of those things where for what he was doing, it was good enough.

But you look at the original graph, and there’s these pretty big error bars on it. And he himself in 2006 wrote a paper where he commented, people are using this wrong. It’s not for specific values. It’s for, this animal was moving very quickly. This animal was moving slowly. And in that regard, it’s still right. It’s just, you can’t say this thing was moving at 5.632 meters per second.

FLORA LICHTMAN: Well, let’s talk about your study. Explain how you used videos of Guinea fowl to figure out if the calculations for small theropod dinosaur speed might be off.

PETER FALKINGHAM: Right. So my whole career, really, is based around figuring out how dinosaur footprints are made. And so 10 years ago, a little more than that now, we collected a whole bunch of data of Guinea fowl moving over clay, mud, sand, that kind of thing, to look at how individual footprints were made.

But we have this vast data set where we’ve collected slow motion video of Guinea fowl, small theropods, small modern theropods running over soft substrates, and we collected all the data of the trackways afterwards. So all the data was there to say, OK, how accurate is this equation? How well does it work when we have a bird, we can watch it move, and we can look at the tracks in the same way we would if we found these as fossil tracks?

FLORA LICHTMAN: And what did you find?

PETER FALKINGHAM: Well, we plotted them on the graph and then plotted the original Alexander’s equation and a couple of variants of that on the graph. And our data were nowhere near the lines produced by the equations.

FLORA LICHTMAN: [LAUGHS]

PETER FALKINGHAM: So the Guinea fowl, they were about four times slower than the equation predicted.

FLORA LICHTMAN: Really? Wow. Much slower.

PETER FALKINGHAM: Yeah, much slower. But maybe that’s to be expected. So as you get to slower speeds, the equation doesn’t really work as well. So if you think about– I gave the example earlier.

If you’re walking versus if you’re running, you take longer strides when you’re running. You move faster by moving your legs quicker and moving them further. But if you’re walking through the kind of deep mud where you’re leaving substantial footprints, that equation sort of breaks apart, and you start taking longer strides to get your foot out of the mud and avoid getting stuck in the mud without really increasing your speed.

FLORA LICHTMAN: You know, one thing that’s confusing to me about this equation in general is like if you’ve been to the beach, you’ve seen these tiny little plovers running around, right? And they’re running very fast, even though they seem to have– they have a very short stride length. And then you’ll see a seagull running, and it’s much slower, even though the stride length is longer. What am I missing?

PETER FALKINGHAM: [LAUGHS] Yeah. So this is another aspect to it, is this is an equation that was derived from elephants and rhinos and cheetahs and dogs and cats and people and ostrich, and all mushed into one big data set. But as you’ve just pointed out, animals move differently. So your plover and seagull, they’re going to have very different leg proportions at the thigh and at the shin, and that’s going to change the stride length that comes about when they’re swinging the leg in the same way, if that makes sense.

FLORA LICHTMAN: So is there a way to figure out exactly how fast a dinosaur could run?

PETER FALKINGHAM: [LAUGHS] No.

[LAUGHTER]

FLORA LICHTMAN: No. [LAUGHS]

PETER FALKINGHAM: So what we can do– there’s two ways to get at dinosaur speed. There’s tracks, which I’m biased towards. That’s my area of research. And that’s the ground up. That’s direct evidence of motion. It’s literally left by an animal moving, right?

And the other side is the bones, the skeletons, the things that we see most of in the museums. So people can use computational models. They can apply muscles to digital replicas of the skeletons, get the computer to figure out how fast it could have run.

There’s problems with both. So the problems with the musculoskeletal models is you can’t actually prove you’re right or wrong. Your computer can say it could move at this speed, but how do you justify that?

Normally, I would say the way you validate that is with the tracks. Do the two sources of information match? But when we’re talking about maximum speeds, we have this problem that is, animals don’t move at their maximum speed very often, and they leave tracks at their maximum speed even less. I know we are an edge case as humans, but if you imagine how many times you’ve sprinted at your maximum speed today, it’s probably not many times, right?

FLORA LICHTMAN: It’s very rare for me, yes.

PETER FALKINGHAM: Yes. Yes. Me too. And you also probably haven’t sprinted at your maximum speed over the kind of soft substrate that would leave behind recognizable footprints.

FLORA LICHTMAN: Never would I sprint on a soft substrate.

[LAUGHTER]

PETER FALKINGHAM: So fossilized trackways, unless we have a really freak occurrence, are really unlikely to preserve the maximum speed of any animal, really. I would say the best way to approach that is to use the computational models not to look for the maximum speed, but to look for maybe the most efficient speed or the speed that involves the least metabolic cost. And then do the trackways line up with that kind of thing as a sort of validation for our models.

FLORA LICHTMAN: The mall walking speed for dinosaurs.

PETER FALKINGHAM: Yeah, exactly.

FLORA LICHTMAN: I’m sure just how you’d put it. One more question. Why are we so obsessed with how fast animals can run anyway? Why do we care about this question, do you think?

PETER FALKINGHAM: Ah, that’s a great question. [LAUGHS] I don’t know, but we do, don’t we? In the same way that we care about which dinosaur was the biggest and which one was the heaviest. But there is a genuine scientific reason for it, and that is when you find the extremes of what animals can do, that can tell you something about what’s possible and how the systems work.

And that’s one of the reasons dinosaurs are important, is because they expand so far beyond the animals alive today. Our sauropods were so much bigger than elephants, and T. rex was an eight-ton biped. We have nothing like that. We have no eight-ton, two-legged animals today. And so by looking at these animals, that tells us what’s possible.

FLORA LICHTMAN: Hmm. This was delightful. Thank you, Peter.

PETER FALKINGHAM: My pleasure.

FLORA LICHTMAN: Dr. Peter Falkingham, Professor of Palaeobiology at Liverpool John Moores University in England.

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