How Plants Powered Prehistoric Giants Millions Of Years Ago

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. When you close your eyes and imagine prehistoric life, what comes to mind? For me and probably for a lot of us, it’s the dinosaurs, the long-necked apatosaurus, the flying pterosaurs, that big-old toothy T-Rex. But what don’t get as much love are the organisms living alongside them and underfoot, the prehistoric plants. But according to my next guest, we need to stop throwing shade on plants because they are the unsung heroes of evolutionary history.

Here to tell us why is Riley Black, paleontologist and author of the new book When the Earth was Green– Plants, Animals, and Evolution’s Greatest Romance. Riley is based in Salt Lake City, Utah. Welcome back to Science Friday.

RILEY BLACK: Oh, always a joy to be here. Thank you.

FLORA LICHTMAN: Your book challenges us to set aside the dinosaurs, put on our green-tinted glasses, and focus instead on plants. Make the case.

RILEY BLACK: A dinosaur by itself, when you imagine something like an apatosaurus, they’re just existing in a void. Otherwise, they’re divorced from their ecological context. So all the food they need to eat, all the environments that allow them to live and thrive as they did for so long, we can’t understand any of that whatsoever. The shape of that animal is really shaped by the plants that it ate, the ginkgos and horsetails and conifers that were around at that time.

And, in a sense, this dinosaur was not a machine, but it evolved specifically to horse down as much vegetation as possible, to grow really quickly, to get out of danger from the predatory dinosaurs that are around at this time. So it’s this whole ecological dance back and forth between the animals and the plants that create so much of this wonderful biodiversity that we then go to the museums and see as fossilized bones. So, really, without the fossil plants, without the sense of ecology, we wouldn’t really understand very much at all about the prehistoric life that we’re so fascinated by.

FLORA LICHTMAN: Are plants understudied in paleontology too?

RILEY BLACK: Plants are certainly not as popular at the paleontology meetings I’ve attended so far. I see them pop up most often amongst paleomammalogists in terms of geochemical isotopes– so isotopes of carbon, for example, that are in the soil that get taken up by plants that then make their way into the herbivore skeletons so we can say something about what they ate many millions of years ago.

But most often, paleobotany is quite overlooked. We have so many books that are encyclopedias of prehistoric life that feature dinosaurs and saber-toothed cats and all these other favorites. We don’t have an equivalent like that for fossil plants whatsoever, and sometimes it’s a little bit challenging. When you’re looking at fossil plants, you’re often looking at just a piece of what that whole organism was. So we might only have the roots or the bark or a leaf or something of that nature, so it can make them a little bit challenging to study.

But it’s the same thing as when we look at plants and animals today, that the charismatic animals are usually going to get more attention than the flora that surrounds them. But what I love about paleobotany and botany in the modern world is that people who love plants love them dearly and see how thoroughly they’re intertwined with the rest of ecology.

FLORA LICHTMAN: Yeah, I was going to say, I mean, I think for plant lovers, like I can hear them in the background saying, plants are charismatic. What are you talking about?

RILEY BLACK: Oh, absolutely, and I feel very much the same. But I have to admit, most of my career so far as a science writer has been focused on the dinosaurs and the charismatic animals, and it really took recognizing that gap in my understanding, that gap in my knowledge that was only going as far as the bones were to start to look into this and realize how much plants have changed our planet and what life looks like– even the fact that, as we’re speaking right now, we’re breathing the air which is rich in oxygen, which is largely thanks to plants and to early photosynthesizers that set oxygen as this essential molecule in our atmosphere.

So from everything like that to the foods that we eat, what our bodies are comprised of, it’s all thanks to plants, and it’s really trying to bring them to the forefront. And writing this book was as much a learning experience as it was for me as expressing what I’ve come to understand about prehistoric life.

FLORA LICHTMAN: You call the relationship between plants and animals evolution’s greatest romance. I am leaning in. Will you explain that to me?

RILEY BLACK: So often, the language of paleontology focuses on basically colonialist and imperialist language. There’s a whole lot of invasion of the land, the colonization of new ecologies, how long the dinosaurs ruled or dominated the planet. And when I was looking at these ancient relationships between plants and animals and how biodiversity basically creates itself through these interactions, I wanted to put all that baggage aside and think about, well, what about community? What about connection? What about relationships? rather than focusing on arms races and nature red in tooth and claw, as been traditional since the 19th century. What if we thought about these things as the way in which organisms influence each other and have these intensely intimate relationships?

One of my favorites in this book involves a prehistoric bat in New Zealand that was a pollinator, much like bats on those islands are today. And we know from modern pollinators the close, close relationship that plants and, specifically, flowers have with their pollinators where, over time, some of them become so closely connected that they really almost can’t survive without the other. If one of them goes extinct, the other one is suddenly under all this evolutionary pressure to either adapt or go extinct themselves.

FLORA LICHTMAN: This is the romance part.

RILEY BLACK: Absolutely, yes. So it’s like some of it is almost quite literal in terms of these very, very close connections. But I wanted to at least shift the frame a little bit and talk about prehistoric life in more romantic and relationship terms and not about everything trying to eat each other constantly.

FLORA LICHTMAN: Although that’s fun too. If we had to pick an it girl, a charismatic megaflora or miniflora of the plant world, what would you pick?

RILEY BLACK: One of my absolute favorites are these trees called scale trees. Now, the first thing to understand about them is that when we say tree, we’re not talking about a natural group of organisms that they all go together. A tree is really a shape for a plant more than it is a natural grouping. So when I say scale trees, these plants are much more closely related to sort of mosses and liverworts and things like that today but at giant size. These were towering trees that grew about 300 million years ago during a time called the Carboniferous, and they were some of the first trees to form forests.

Prior to this point, plants were relatively low growing, and now you’re starting to get trees that could be more than a hundred feet tall. But it had this green coating to it. It could photosynthesize as well as the leaves on top, and we sometimes–

FLORA LICHTMAN: Oh, the bark could?

RILEY BLACK: Yes, absolutely. So almost the whole surface of this tree is capable of photosynthesis. And they were so new that microorganisms that break down plant material hadn’t yet evolved to do this very efficiently. So you have all these trees that are growing. They’re falling down in these swamps, and that’s where most of our coal comes from today. That’s why this period is called the Carboniferous is because these were the coal swamps of the primeval world that were really kind of founded by these plants that are very small today but in the ancient past were these huge trees that really got life going on land as our own ancestors were just starting to get comfy coming out of the water.

FLORA LICHTMAN: Plants seem delicate. How often do they survive as fossils? Do they need special conditions to preserve?

RILEY BLACK: So ever since the 19th century– geologist Charles Lyell was one of the first– recognized that the fossil record is really a fraction of a fraction of a fraction of everything that ever lived, for a variety of reasons. Plants do require some special circumstances, but so do bones, for that matter, or things that we think of as more resilient. So sometimes when you’re looking for fossils– and you look for fossil plants much the same way that you look for fossil animals. You just get your boots on, and you go hiking, and you hope you see something good. Sometimes you just get petrified wood, for example, parts of the logs or the branches, but we don’t get the leaves per se or the root systems. Or sometimes we get root casts. We get these basically indentations that were made by the roots of these plants in life– so not the organic material itself but the outline of what they once were. And sometimes we get fossil leaves. And sometimes you get really lucky and you get everything together, and you can see how it goes together.

So often over the past 15 years, as I’ve gone out with paleo crews, they’re usually groups that are looking for dinosaurs, and when we find a fossil plant, they’re not terribly interested because they’re looking for bones. But to a paleobotanist, that plant might give you so much context about what the climate was like, how much rainfall there was, what sorts of plants the herbivores in the area were eating. So part of it is the bias itself of the fossil record, and some of it is the bias of what we go looking for when we get out into the desert.

FLORA LICHTMAN: What we put our attention on.

RILEY BLACK: Absolutely.

FLORA LICHTMAN: You spend a chapter on herbivorous dinosaur digestion. Please take us on a trip through the innards of an apatosaurus.

RILEY BLACK: So if you envision this apatosaurus on this Jurassic landscape, if you’re thinking about grasses– and I’ll tell you to put those grasses aside because grasses didn’t exist yet. So this floodplain that’s covered in ferns, and there are horsetails growing along the margins of a stream, and there are ginkgo trees. So we have the one species alive today, but there used to be many more. So if you imagine a landscape like that, that kind of greenery around it.

And apatosaurus couldn’t chew. Their jaws are basically like a big set of cropping shears. They can open and close, but there’s no grinding or processing like a cow or a deer or something like that would do. And what this apatosaurus is doing is using that incredibly long and muscular neck to move its head side to side as it’s grazing along in the low-growing plants. It can raise its head up to browse amongst the ginkgo leaves and basically just cropping that vegetation and swallowing as much as it can. It kind of is like a dinosaur vacuum for plants, more or less. There’s very little processing. Just swallow as much as possible.

FLORA LICHTMAN: I’m imagining like that Edward Scissorhands scene.

RILEY BLACK: Absolutely, yes. You’re on the right track.

So as all that plant material basically arrives in the stomach, it’s like a big fermenting vat. And we don’t know exactly how dinosaurs digested so much plant material because much like for us and other living things alive today, plants are very, very hard to digest. You need a special microbial community that this dinosaur would have had to accumulate in its infancy, either directly from its mother, maybe eating dinosaur pats filled with vegetation left by other apatosaurus to get that microbiota to be able to break down all this plant material. And on its way out, because of all this fermentation, you would have had probably some sonorous dinosaur farts that were contributing methane and other greenhouse gases–

FLORA LICHTMAN: Sonorous. [LAUGHS]

RILEY BLACK: –to the atmosphere. There’s actually a paper on this suggesting that dinosaur methane emissions might have not controlled but influenced the prehistoric climate.

And when you think about just the journey that these plants have gone on, the anatomy of this animal, the anatomy of an apatosaurus, aside from getting big so that won’t be an attractive target for predators, is really all about eating as many plants as it possibly can because it can’t break them down the same way that a mammal does. So it’s just picking a spot where there’s plenty of greenery, moving that head and neck around to eat as much as possible, and then moving on to the next spot. So it’s kind of like an endless salad bar for some of these ancient critters.

FLORA LICHTMAN: Do they have multiple stomachs? What’s their strategy for getting energy out of these clippings?

RILEY BLACK: I would love to know more about how these dinosaurs actually digested their food. We don’t know for sure whether they had multiple stomachs. So far, there is no direct evidence for multiple stomachs like a cow has. Paleontologists suspect that many were what we call hindgut fermenters. So elephants, horses, some other animals do this today where everything goes through the esophagus into the stomach, and then it’s after the stomach– so as it starts to wind through the intestines that the retention time is increased that that basically flow of plant material is slowed down to try and extract as much nutrition as possible from it.

But who knows? We’ve found stranger things in the fossil record before. I hope, one of these days, somebody finds a sauropod stomach and we can work this out. But it is pretty neat that there are paleontologists who look at modern relatives of these ancient plants, who look at ginkgos and horsetails and ferns and stuff today that used to be thought as low nutritional value– like how could you grow a big dinosaur on these things? And it turns out that many of them are much more calorie packed and nutritious than we previously thought. It was just an assumption that we had made that they’re ancient plants, so they must not be very good for growing on.

But through some modern experiments using sheep guts and digestive enzymes and things of that nature, they’ve worked out that a lot of these plants could certainly allow dinosaurs 80 feet long, more than a hundred feet long, 70 tons or more to be walking around these landscapes.

FLORA LICHTMAN: You write that plants are the aliens in our backyards. Will you give me just like your top weirdest things about plants?

RILEY BLACK: One of my favorite things about plants– this is one of the first things I even learned about them as a kid– involves phototropism. We see this all the time, especially if you have houseplants, that they tend to bend towards the light. And up until very, very recently, up until about two years ago, we didn’t know how plants did this, this basic thing that’s so essential for their survival to photosynthesize.

And it turns out, based on these lab experiments in which there was a plant that had a mutation that caused some of its cells inside of it to flood with water that it couldn’t really track where the sun was coming from. So researchers looked at, OK, what’s going on in our nonmutated plants? And they realized that as light hits the plant’s surface, the light scatters through its cells, and basically through the coordination of realizing where the light and shadow is, that is what allows the plant to bend towards it. So plants aren’t just these static things, but they’re constantly responding to the environment around them at all times.

FLORA LICHTMAN: Are there places we can go today to go back in time to get a sense of what prehistoric plant life was like?

RILEY BLACK: I’m so glad that you asked this. I’m so glad that we’re talking about this in the springtime, as we’re starting to get some of the first flowers of the year area up here in the Northern Hemisphere coming out. If you pass by a magnolia tree, magnolias have been around for about 125 million years, and magnolias that would be recognizable as more or less modern were around at the end of the Cretaceous. So T-Rex and triceratops and so many of our favorites would have been able to smell the flowers a bit that they were growing at that time. So we can still see some of these ancient remnants.

In fact, one of my favorites, this plant called metasequoia. It’s a redwood tree. It’s a conifer. And it was first described from fossils, and it was thought to be totally extinct until in the early part of the 20th century, a forestry official in China was going through a particular patch of woodland and recognized that these are metasequoia trees. They were still living. This is the equivalent of finding a triceratops still walking around. So we have a few plants today that are still growing that look very much like their prehistoric counterparts.

If you want to get more of a sense of what certain prehistoric forests are like, one of my favorite spots for that is the Gulf Coast of the Southern United States and that those coastal floodplain swamps with lots of conifers just so rich with life, they really closely resemble some ancient habitats, not just from the time of the dinosaurs but times after the asteroid impact that ended the age of dinosaurs as well. They’re really close to this time period called the Eocene, around 50 million years ago, where plant life was growing in lush new ways. We’re starting to get the earliest bats, but there are a lot of strange mammals still moving around, this kind of mix of the very ancient and more modern living together. So once you know what to look for, once you take a moment and think about the plants that surround you all the time and where they came from, you can start to see these prehistoric connections.

FLORA LICHTMAN: Super fascinating. Thank you, Riley.

RILEY BLACK: Oh, it’s always a pleasure. Thank you so much.

FLORA LICHTMAN: Riley Black is a paleontologist and author of the new book When the Earth Was Green– Plants, Animals, and Evolution’s Greatest Romance. She’s based in Salt Lake City, Utah.

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