Maria McNamara on Seeing the Ancient World in Color
Transcript
Oliver Strimpel
This is Geology Bites with Oliver Strimpel. Popular reconstructions of ancient environments, whether they be in natural history museum dioramas, in movies, or in books, present a world of color. But are those colors just fanciful renderings, perhaps based on the colors we see around us today? Or is there evidence in the fossil record that we can use to determine the actual color of plants and animals that lived in the geological past? Maria McNamara tries to answer this question by studying the fossil preservation of soft tissues such as skin, muscle, and internal organs. She does this by analyzing fossils that come from sedimentary deposits that contain extraordinarily well-preserved fossils. She also does lab experiments to investigate the processes of soft-tissue degradation and preservation. She is Professor of Paleontology at University College Cork in Ireland. Maria McNamara, welcome to Geology Bites.
Maria McNamara
Hi, Oliver; nice to be here.
Oliver Strimpel
It must require very special circumstances to preserve soft tissues in the fossil record, precisely because they are, indeed, soft. What kind of depositional setting allows this to happen?
Maria McNamara
There are some generic situations where preservation of these decay-prone soft parts of animals' bodies is more likely. For instance, restriction of oxygen tends to slow down decay, so places like stagnant lagoons are often associated with preservation of soft tissues. Also, places such as Deep Lakes, again because we can have very low oxygen in these settings, and places such as the continental shelves; areas far into the ocean, far from the edges of continents where the ocean deepens. And in those settings, it's not so much a lack of oxygen that's important, but rather the fact that we get lots of underwater earthquakes, and these generate great deposits of sand, silt, and mud, and these flow rapidly down the continental slope, and they cover any organisms living there. So, any organisms living there are smothered. So, in other words, where we have rapid burial. There are also a few other settings that are not quite so geographically widespread. They tend to be more restricted, but are associated with very, very good preservation. These would be things like tar pits, like the famous La Brea Tar Pits from California; and also amber deposits are associated with excellent preservation, because amber is… locks-out oxygen and it also has lots of antimicrobial compounds that slow down decay. So, there's actually a variety of different settings on land, in terrestrial environments, and also in the ocean, where soft parts can preserve.
Oliver Strimpel
There’re quite a few different settings, but how common is it for this exceptional preservation to prevail? How many such sites are there, roughly, around the world that we know of?
Maria McNamara
Well, it depends in part on how you define soft tissues, but if you use, let's say, our preferred definition, which is the tissues of organisms that are not biomineralization, so they lack a mineral component. So, we're excluding shells and teeth and bone, but we are including things like leaves and plants and the cuticle of insects. Well, then, we have on last count over 600 fossil deposits in our data set. So, there are hundreds of these fossil deposits that preserve leaves, and insects, feathers, and other tissues.
Oliver Strimpel
OK, so, let's talk about our topic today, which is whether we can tell what color plants and animals actually were when they were alive. This is probably too naive, but supposing you find a body fossil that has some green pigment in it. Would that green pigment be reflective of an original color, or would that almost certainly be some kind of alteration product or something that happened subsequent to preservation?
Maria McNamara
There is evidence for the survival of various pigments in the association with body fossils. So, sponges, for instance, fossil sponges, can preserve pink pigments, spiral boring pigments. Fossil crinoids can preserve quinones that generate purple and dark bluey-grey colors. And fossil vertebrates can preserve evidence of the pigment melanin. So, we can retain in body fossils evidence of pigments, and we can infer to some degree what colors they would have generated, but we must exercise caution because the pigments are altered during the fossilization process. It's difficult to work out exactly how much of the pigment was originally present.
Oliver Strimpel
What are the most common pigments that you can actually find that relate to color, and can we infer that that actual pigment confers that color if we see it in a fossil?
Maria McNamara
The only pigment for which we have evidence of is melanin. Melanin is really common and in vertebrates it's the pigment that colors our skin, our hair, and also the feathers of birds, for instance. We have evidence of melanin from many different types of fossils: fish and amphibians, reptiles, birds, mammals and some invertebrate animals (things like squid). But the pigment itself isn't preserved intact. Like I said earlier, the pigment itself, it becomes degraded, it breaks down into its constituent parts. So, in order to find the evidence, the chemical evidence for the pigment, you're often looking for scraps. Imagine someone's got a book and rips up all the pages into little pieces. You're just finding the little pieces. And so you're trying to gather evidence to suggest that these molecular fragments do indeed come from this particular pigment of interest. The interesting thing about melanin is that not only do we preserve chemical evidence that it was there, but we can also preserve physical traces of its presence, and that physical evidence comes in the form of tiny microstructures. They can be spherical. They can be elongated. They can even be disc-shaped. And these micro bodies are called melanosomes. We produce them. That's what our melanin looks like in the human body, across all vertebrates, actually. These wonderful little structures survive in many fossils, and actually that was the first evidence we had for the pigment melanin. With this pigment, we can look for morphological evidence using electron microscopes, and we can look for chemical evidence.
Oliver Strimpel
Can we infer the color of ancient life forms even when there are no pigments present?
Maria McNamara
Yes, we can. Many fossil insects, in particular, preserve evidence of color in hand specimens. So, when you look at the actual fossil, you can see color. So, you might see metallic blues, metallic greens, or yellows or reds. These colors are not generated by pigments. Instead, they're generated by highly ordered tissue nanostructures. They're called three-dimensional photonic crystals and two-dimensional and one-dimensional photonic crystals. These wonderfully ordered tissue architectures, these tissue structures are periodic, and that's really important. The regular structure scatters light of particular wavelengths. In modern animals, these generate the purest and brightest and most intense colors in the Animal Kingdom. And they actually preserve in fossils. So, we studied hundreds of fossil insects from fossil localities around Germany and the US that showed these metallic colors. And we were able to show, by using electron microscopes, and other techniques, that these metallic fossils do actually preserve these photonic crystals. So, they preserve these wonderful tissue structures that are gathering light and producing very bright colors that were presumably used for signaling of some sort in modern insects. These colors are used for display and in some cases, with metallic greens, they can actually be used for camouflage.
Oliver Strimpel
Wow! So, we're able to see in the fossil record features that are as small as the wavelength of light so they can do these physical optical type colors like scattering, diffraction, iridescence, and so on. That scale, in the hundreds of nanometer scale, can actually be preserved in the fossil record.
Maria McNamara
Yeah, to form the metallic colors that we see in leaf beetles and jewel beetles, in the fossils, just like the modern examples, the outer shell, or cuticle of the insect, contains very tiny layers. Imagine like a sandwich, but a sandwich with maybe 10 or 20 or 30 layers. Each layer might only be 50 to 80 nanometers thick. But what's critical is that your fossil isn't buried too much, because if you bury these fossils too deep, the cuticle becomes compressed, and this also becomes thermally altered and these processes destroy these photonic crystals. So, they tend to be preserved in the younger fossil biota, which haven't been through too much in terms of the rock cycle, if they haven't been buried too deep.
Oliver Strimpel
Did you say photonic crystals? Is that what you call these structures?
Maria McNamara
Yes, they're photonic structures because they interact with photons.
Oliver Strimpel
That is absolutely incredible. What kind of instruments do you use to see these structures?
Maria McNamara
You really need to use electron microscopes. You can only really see them in cross sections, so you've got to take little pieces of your fossil, and you've got to slice them up. And you can view them on-edge with the scanning electron microscope. Well, you get the best images of all with the transmission electron microscope. As any biologist will tell you, preparing tissue samples for transmission electron microscopy is quite a task in itself, because what you're effectively doing is you cut your sections as thin as a cell membrane, and you float them on a little bath of water, and you pick them up with an eyelash attached to a cocktail stick. It's really incredible kind of preparation work. You know, if you can do this kind of preparation work, you probably would have been a good surgeon in another life. And then you blast them with a beam of electrons, and then you get your nice images.
Oliver Strimpel
So, when you see these nanostructures or photonic crystals in these fossils, how do you infer what color that actually would have appeared as? Do you have to use analogues to modern insects or vertebrates?
Maria McNamara
The color produced by these nanostructures - there's actually a really simple formula to work it out. You have to look at the thickness of the structures, which is… or their periodicity, but for layers, it's thickness, and the refractive index of the material. We can estimate the refractive index, and so, literally, by plugging refractive index and the thickness of the layers into a formula, we can work out what color should be produced. We used this approach with our fossils. We guesstimated what the refractive index should be based on what the cuticle is composed of in modern insects. We put those values in, together with the measured thicknesses of the layers. To our absolute horror, the predicted color is not the color that the insect fossils have. And this tells us that the refractive index has changed. The chemistry of these fossils has changed during the fossilization process, and that affected the color which is produced.
Oliver Strimpel
OK, so just to clarify, the formula did indeed correctly predict the color of your fossil insects when you plugged in the refractive index of the actual, altered material in the fossil insects instead of the refractive index we see in the photonic structures in modern insects. So, as I mentioned in my introduction, you don't confine your research to analysis of the fossils themselves, but you perform experiments in your lab. What are you learning from those about reconstructing colors in the geological past?
Maria McNamara
So, we tend to work a lot with decay and with heat and pressure, because we know these occur to all fossils. When you take modern insects that have metallic colors and you subject them to high temperatures and pressures, we know from studying what happens to the color and what happens to their cuticles that: yes, the refractive index is changing, and there is a change to the thickness of the layers, too. But we can see from our experiments that temperature is the main culprit here. The pressure on its own doesn't really do much. The key factor in causing the color change during fossilization is actually temperature And, similarly, we've done a lot of work looking at how temperature and pressure affect the preservation of the pigment melanin. And we know, for instance, that if you take bird feathers and subject them to these high temperatures and pressures, that all evidence of every single pigment will be lost, except melanin. There's a preferential preservation of melanin in feathers in the fossil record. You know, we lose our carotenoids. We lose our porphyrins. We lose our pterins. We lose our psittacofulvins. So, we lose all of these other pigments, So, that's why we've got to be so careful when we look at fossil feathers and only find melanin. It doesn't mean that that was the only pigment originally present. Other pigments may have been present, too, and they may have modified the color, producing colors other than the normal oranges and browns and greys and blacks that we would normally associate with melanin. So, the experiments are really useful. They kind of tell us how confident we can be when we interpret fossil evidence for particular colors.
Oliver Strimpel
Your research concentrates on insects and vertebrates. Can you give us some examples of insects you've worked on and what you've been able to infer about their colors?
Maria McNamara
The first example that we focused on were some beautiful fossil moths from the Eocene of Germany, so they're about 50 million years old. People think of moths as these drab brown things that flutter around at night. The kind of pale inferior cousins of butterflies. But there are some day-flying moths nowadays, like the cinnabar moths, which are very brightly colored, with their reds and blacks. And it transpired that in the Eocene there were other very brightly colored day-flying moths. The particular moths which we looked at, today, they appear a yellow color that's actually partly an artifact. These fossils come from rocks that are very, very wet and if you allow them to dry out, the fossils curl up and shrivel and become irrevocably damaged. So, these fossils are all stored in water or in glycerine. And then they're stored in Glycerine. Glycerine has a different refractive index to air, so the fossils look yellow, but we did experiment. We took them out of the glycerine very, very briefly for a few seconds, and the fossil mots look blue. When we studied the wings, we could see that all their tiny, microscopic scales are preserved, and when we looked at them in really high level of detail, we could see that each scale preserves layers inside, and we were able to use our mathematical modelling to infer what color they would have had when they were alive. They would have had a beautiful, very bright iridescent green. That, incidentally, would have been pretty similar, if not identical to the green color that the modern representatives have, so these are a type of moth called the Forester moth, and these Forester moths in the Southern Hemisphere today, they're living examples have beautiful green colors. They actually have a dual purpose. They use them both for camouflage when they're just at rest, but when they're feeding on the host plants, the green color of the moth, it really contrasts with the color of their host flowers, so they're probably using the color for sexual display for mating signals as well. The other type of fossil insects that we've done some work on specific colors are some beetles and some very young fossil beetles. They're actually so young, we don't even really consider them true fossils. We call them sub-fossils because they're from the Pleistocene, so they're at most about 700,000 years old. And we studied some of these little beetles from glacial deposits from Northern Canada and also from Switzerland. And what's remarkable about the beetles from these two localities is that they preserve, not multilayer reflectors, but three-dimensional photonic crystals. So, where the tissue structures are ordered not simply in layers but as crystalline structures in three dimensions, so incredibly complex. And these are actually the most complex structures that are known in nature. And these little fossil beetles, when you look at them in your hands, they just look brown, because the scales are so tiny. But when you look at them with the microscope, you see beautiful colors. The insects from the Yukon of Canada have scales that appear red and yellow and green, and the scales of the Swiss insects are incredible turquoise, cyan, blue, and green color. Modern insects that have these scales with these mind-bogglingly complex structures, they use these colors for signaling to other members of the species at really close range, so when they're within, you know, a few millimeters to a centimeter or two of each other, they can perceive these colors. They perceive the individual scales, and they use them for mating signals. So, it's remarkable to find these, in these fossil deposits. We've only found them in very young fossils. We don't know why we haven't found them in older fossil deposits. Maybe it's because the structures are so small, they haven't been spotted yet, because you're not necessarily going to put every single brown insect you find under the microscope… but, maybe we should. Maybe some of these brown fossil insects actually have remarkable preservation of their scales. The other alternative, which I'm not sure I believe, but we have to consider it, is that they evolved relatively recently, so we haven't found them in all the fossils because they hadn't evolved yet, but my money is on it being a problem with preservation, or else a problem just with observation that we haven't observed them yet. But there's the potential for other sources of color to still be out there that we just haven't noticed, or we haven't got the analytical toolkit to find yet. I think that we'll be able to color-in a lot more fossil record in ten years’ time than we can do at the moment.
Oliver Strimpel
Can you give us some examples of your work on vertebrate color?
Maria McNamara
So, over the last ten years or so, we've done a lot of work on many different types of fossil vertebrates. We've looked at evidence for melanin and color in fish, in squid, in frogs, birds, mammals… you name it. One of the most exciting pieces we did was not focused exclusively on the pigment melanin, but it was actually a fossil snake from Spain from the Miocene - so, 10 million years old - that preserved evidence of many different color-producing mechanisms in its skin. So, this snake was a type of water snake that for one reason or another ended up dying and falling to the bottom of a very deep lake. And what's remarkable is that the entire skin of this snake is preserved in three dimensions. And it's not just that we preserve a thick layer of skin, we actually preserve all of the tissue components that we see in the skin of modern snakes, and even indeed in humans. So, we preserve our collagen fibers, and we preserve various pigment cells in the skin. Now, humans only have one type of pigment cell. We only have melanophores, so our skin can only generate at best brown colors. For snakes, just like amphibians and fish, and other reptiles, they have three types of pigment cell. They have the melanophores that contain melanin, but they also have xanthophores that contain carotenoids that make yellows, oranges, and reds, and they also have iridophores, a third type of pigment cell that has these little crystals inside that reflect light, so these help make colors bright and eye-catching. And what was remarkable was that our fossil snake, the skin, was so well-preserved at this micron level, that we could see all three types of pigment cells. And we could even, by sampling different parts of the body, we could see how the relative abundance of these different pigment cells varied from place to place. So, we figured that we could potentially infer what color the snake was, provided we had a good enough comparison with modern snakes. So, we looked at the skin of a whole bunch of modern snakes and lizards, and we characterized the color of the skin. And we looked at how many of the different pigment cell types were present in order to generate red skin, yellow skin, green skin, black skin, and so on. And then we were able to go back to the fossils and work out, well, by looking at the proportions of those different pigment cells present, what the colors were. So, we were able to actually infer the color of that fossil snake with remarkable accuracy because we weren't just basing the color reconstruction using melanin, but we have this melanin baseline color that's modified by having the xanthophores and iridophores. So, the snake was basically dark on top and sides with a very pale belly, and the top and sides were actually different shades of green. So, we have some very bright greens, some darker greens and some almost dark browns and blacks. So, we don't know how these different colors were organized on the snake, because to reconstruct the color pattern that accurately, you would have to sample the whole thing. We didn't want to destroy all the soft tissues. We want to leave some for future generations to study. But what we can say is that it did have these different colors. They may have been organized in stripes, maybe chevrons, maybe blotches. But the combination of colors indicates that they were used in these predominantly green colors for camouflage. That's supported by having the pale belly that's characteristic of a technique called countershading that modern animals use to help break up the body outline, and the fact that it had these patches of very bright colors - these would be consistent with some sort of element of sexual signaling or mating display, which is very common in some modern snakes in that same family. So that's one of the favorite pieces of work that I've done over the last few years on color, because I feel that it's the most accurate color reconstruction that we've got from the fossil record.
Oliver Strimpel
That is really incredible that you can get that level of detail. And how old was the snake?
Maria McNamara
So, the snake is Miocene. In age and it's about 10 million years old, and it’s from a site called Libros in Northeast Spain, which is very famous for preservation of its fossil frogs in particular.
Oliver Strimpel
Would that have been impossible, say, if it had been from the Cretaceous or earlier?
Maria McNamara
I don't think impossible, because we do have many fossils from the Mesozoic that have tissues preserved in the same way. The critical factor with this snake was not necessarily its youth in geological terms, but the way it was preserved. Instead of being preserved just as an organic film, like the Insects, it's actually preserved in three dimensions by the mineral calcium phosphate, so that the mineral has made a replica of the tissue in three dimensions at an incredible level of detail. We've known about this style of fossil preservation for over 30 years, so that, in itself, isn't new. But that mode of preservation, it also occurs in lots of other types of fossils from different time periods, so there is the potential to preserve in other fossils with this level of detail. I would not be surprised if somebody found more examples of this type of high-fidelity preservation of the skin.
Oliver Strimpel
So, in summary, do you think it's possible to say, based on your evidence, that the ancient world was every bit as colorful as the one we inhabit today, or maybe more colorful?
Maria McNamara
It all depends on who's looking at it, I guess. We have a perspective of humans. And you can detect three main channels of color, but some organisms, like certain types of shrimp, these have 12 different types of color-detecting cells, so it's all in the eye of the beholder. But from a human perspective, the types of coloration that we see in modern animals, the basic types of pigment cells, the basic types of color-producing nanostructure, everything we see today, is present in the fossil record. It's not present in every fossil, but that's almost certainly just due to the inadequacies of the fossil record and the vagaries of preservation. So, I think that the ancient world was as colorful, but you've got to remember that all we've studied is just a handful of fossils. We don't have anything that's truly representative of a population, or of a species. We're extrapolating from n = 1 in certain circumstances. What is remarkable is that that very small data set shows us that fossils were colorful, and they produced color in similar ways as modern animals. What would be really interesting to look at is how different was the ancient world. We already know from studying some fossil invertebrates, like sponges, that they produced pigments that we don't see today. So, perhaps other animals also produce colors that we don't see today and produce colors using different mechanisms that we don't see today. I think that's very likely, and that means that study of ancient color is very interesting. What has been selected over time? And why have certain pigments, and why have certain structures won out over others in the evolutionary arms race? Is it because they're better at generating certain types of visual signals? Are they better for camouflage? Are they better for mating displays? All of this we've got to work out.
Oliver Strimpel
What are you working on the moment? Do you have a particular research focus at the moment?
Maria McNamara
The big research focus at the moment is melanin. The initial drive for studying this pigment came from the urge to know about the color of fossil animals, but over the last few years we've come to realize that melanin is doing an awful lot more in our bodies than simply producing color. It's in all of our internal organs. It has a different chemistry in our organs. So, it has a physiological role. We suspect that this relates to metal sequestration, so the melanin in different organs is holding on to different metals - metals that might be useful for the function of different enzymes for speeding up different metabolic processes. Or maybe the melanin might be actually locking away metals, preventing them from accumulating to toxic levels in our bloodstream. So that's something which we're working on at the moment. We're working across many different types of vertebrates. We're actually working on humans as well. We're also really interested in the role of melanin and the evolution of melanin in feathers, and we recently published a paper in the journal Nature where we were looking at melanin, preserved in the feathers of pterosaurs, the flying cousins of dinosaurs. We were able to show that pterosaurs preserve different types of feathers, and the different feather types have melanosomes with different shapes. So, in other words, these pterosaurs were selecting for different colors in their different feather types. And that's a characteristic of modern birds. That was so exciting because it meant that this ability to tweak your color by changing the shape of your melanosomes must have already been present very, very early on, during feather evolution. So, it suggests that one of the important early functions of feathers was the production of color. Why that is involved and what was their function is one of the biggest questions in paleo- and evolutionary-biology. What were the early functions of feathers? What did they do before they were big enough and long enough and complex enough to allow animals to fly? For a long time, we've suspected that feathers were using insulation. But now we had the first concrete evidence that very early in their evolution history, feathers were being used for color. They were being used to generate visual signals. It puts color flat on the table in terms of understanding why that particular type of tissue structure evolved. So that's a very interesting question that we're going to have to look into some more.
Oliver Strimpel
Maria McNamara, thank you very much.
Maria McNamara
Thanks, Oliver.
Oliver Strimpel
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