To the untrained eye, most fossils don’t appear to be bursting with color. The first scientific analysis of fossil color was published only a decade ago, and until recently, determining the color palette of the prehistoric world seemed an insurmountable task.
Maria McNamara, a paleontologist at University College Cork in Ireland, is trying to piece together the fossil evidence to paint a colorful picture of the past. When people think of paleontology, they often think of hard teeth and bone, but the softer parts of animals, like skin, muscle tissue and internal organs, can be preserved in the fossil record, too. It's much rarer, of course, because the squishy stuff usually rots away, but soft tissues are exactly the kind of specimens McNamara is looking for. She studies tissues from insects and vertebrates in order to envision what these critters looked like and how they interacted with their environments—what their predators were, where they lived, what their mating habits may have been and more.
McNamara will be discussing her work to find the color remnants in fossils at the Smithsonian's National Museum of Natural History’s "Life’s Greatest Hits: Key Events in Evolution" symposium on Friday, March 29, in Washington DC. Ahead of her talk, Smithsonian.com spoke to McNamara to learn more about the colors of the ancient world.
Scientifically speaking, what is color, and how is it measured?
Color is simply visible light. Anything that scatters energy between the wavelengths of 400 and 700 nanometers is what scientists call visible light. The human eye is trained to perceive subtle differences in energy within that window. Other animals can see color beyond that window. For instance, birds have sensitivities to the ultraviolet light, so they can perceive shorter wavelengths of energy. Many insects can also see ultraviolet light and potentially in the infrared, which has longer wavelengths. What you call color really depends on what kind of animal you are.
To put it in its simplest terms, color is a form of energy we can perceive, and different wavelengths create different colors.
In what ways does color develop in nature?
Color can be produced in two different ways. Many modern organisms, including animals, produce color using pigments. Pigments are chemicals that selectively absorb light of specific wavelengths. For example, plants' leaves look green because the molecules in chlorophyll inside the leaves absorb all of the wavelengths in the red and the blue part of the spectrum, and they're reflecting the greens and yellows that we can see.
The most common pigment in plants is chlorophyll, but in animals, some of the most common pigments are melanins. They produce the color of our hair. They produce the brown colors in fungi, for instance, and the dark-hued colors of bird feathers.
We also have common pigments called carotenoids, and these are produced exclusively by plants. But many animals ingest carotenoids in their diet and they use them to color their tissues. So, for instance, the red color of a cardinal, which are common on the east coast of the United States, is produced by carotenoids, which the birds take in their diet of fruit and berries. The pink feathers of flamingos are derived from carotenoids in the algae that tiny shrimp eat, which is the birds’ favorite meal.
But there’s actually this whole different way of producing color, and that’s called structural color. Structural color doesn't use pigments at all and instead uses very ornate tissue structures at the nanoscale. Basically some animals’ tissues will fold into highly complex structures at the nanometer level—or in other words, at same scale as the wavelength of light. Those structures affect the way light passes through biological tissues, so they can essentially filter out certain wavelengths and produce really strong colors. And actually structural colors are the brightest and the most intense colors that we get in nature.
What different types of color, or different structures that produce color, do you look for when you study these fossils?
When I started studying color, I was working with the structural color in fossil insects. I started off looking at these metallic insects. They showed bright blues, reds, greens and yellows, but no one had ever really studied what was producing these colors—there was just a single study of a fragment of one piece of beetle.
So I studied some 600 of these insects from many different fossil localities, and together with some collaborators, we got permission to take samples of the tiny fossils. When we did this, regardless of what species we were looking at, all of these structures in these colored insects were produced by a structure called a multilayer reflector. Microscopically, it basically looks like a sandwich with lots of really thin layers, maybe only 100 nanometers thick. Many modern insects have these in their outer shell. The more layers there are, the brighter the color that's scattered.
We were interested in finding out why we weren't finding other structures, such as three-dimensional photonic crystals, which are tiny, complex, layered structures that interfere with light particles called photons. The structures might be twisted into a diamond structure, a cubic structure, a hexagonal structure and even more complex structures. Many modern insects and butterflies display this. For example, the modern Morpho butterfly is this fabulous blue tropical butterfly with scales that contain 3D photonic crystals. So we wondered, “why did we never find these in the fossil record?”
Why do you think you were only seeing multilayer reflector structures in the fossils while other color-producing structures exist in modern insects?
We did some experimental fossilization, which is called taphonomy. We replicated the aspects of the fossilization process by allowing both multilayer reflectors and 3D photonic crystals to degrade in the lab. Both of them survived the experiment, which told us that these 3D photonic crystals had the same fossilization potential as the multilayer reflectors—so they must be in the fossil record somewhere.
We started looking a few years ago, and we did report the first case of 3D photonic crystals in fossil insects. The example where we did find them in the field is very small, so in many cases they might just be overlooked.
Can color change in the fossilization process?
The question we encounter is whether the preserved color is the real color. We initially studied the structure's chemistry by assuming that it’s the same as modern insects—or in other words, we assumed it would bend light the same. But when we input those values into our computer models, they didn't work. The models told us that the colors of our fossils actually had changed during the fossilization.
With our experiments we were able to work out that the change was due to excess pressure and, more importantly, steady temperature. Temperature, we found, really drives color change of these structural colors because the physical structure shrinks.
When studying the color of extinct plants and animals, what species leave behind the best evidence?
It's not a case of particular species, it's a case of getting things preserved in the right way.
Most of the studies that have been done so far have been done on feathers, either feathers in birds or dinosaurs, and they've all been preserved as carbonation compressions: fossils formed in sedimentary rock under immense pressure. This is problematic because you don't preserve the parts of the feather that are responsible for the non-melanin colors.
In extant birds, melanin is almost ubiquitous, and the effects of melanin are modified by the presence of other pigments. So if you take again the red feathers of a cardinal, they look red but inside, they contain carotenoids and also melanosomes. If that bird feather goes through fossilization, the carotenoids will degrade and all you'd be left are melanosomes, [and you wouldn’t know the cardinal was red].
There's a very real danger that a lot of the reconstructions we’ve been looking at of fossil birds and feathered dinosaurs may not be representative of the colors of the organisms as we might think. If you find evidence of melanin in fossils, it might be indicative of patterning, but not of the actual hue. So we argue then that these carbonation fossils are probably not ideal for studies of fossil color.
What types of fossils preserve color best?
We think we should be looking for fossils preserved in the mineral calcium phosphate. That was the case with the snake that we studied in 2016. The colors of the snake are preserved; the whole skin of the snake is preserved in calcium phosphate. The beauty of calcium phosphate is that it preserves everything. The entire pigments of the skin are preserved, including the three types of pigments that produce color in modern reptiles. It preserves structural color: red and yellow, and the dark color.
Those kinds of fossils where you've locked everything in calcium phosphate, they're actually a much better target for studies of fossil color than carbonation compression.
So what color were the dinosaurs?
We have various feathered dinosaurs that we have melanin in these color patterns for, and in modern birds, melanin coloration is modified by other pigments. These other pigments aren't preserved as fossils, so we cannot be sure for now.
If we found dinosaur skin that was really well preserved, we would have a good chance of reconstructing color in more detail. The problem is that most dinosaur skin is preserved as impressions. There are a number of examples where you actually retain a thin organic or mineralized film, but even though a few have been studied, none have actually yielded details of the pigments.
Today, we often see bright colors as toxic warnings to predators or as a lavish display to attract a mate, or other more subtle colors to serve as camouflage. What purpose did color serve for the first colorful animals?
Lots of dinosaurs we see have countershading, which is when the back and sides are darker in color and the belly is a paler color. This is a strategy used by many modern animals to help break up the body outline in strong light environments [and provide camouflage].
In a feathered dinosaur we studied, the tail has very striking banding on it. That type of banding is very common in animals today, and when it occurs on other areas of the body, it is typically used for camouflage. But in this specific dinosaur, it is localized to the tail. So that high color contrast in the tail in modern animals is often used in sexual signaling, so for mating displays.
The fossil snake that we studied was almost certainly using color for camouflage. It had quite striking blotches along its length, and those blotches probably served again as disruptive camouflage, to break up the body outline in strong light.
The fossil moth and some fossil insects we studied with structural colors—we got the sense that their colors served a dual function because they had a very striking green color. Such a color is cryptic when the insect is hiding in vegetation, but when these butterflies would have been feeding on the host plants, there would have been a sharp color contrast with the petals of the flower. Many insects use this as a warning signal to advertise that a predator is near.
What new tools do we have to study soft tissues, and what can we learn that we have not been able to learn from fossils up to this point?
Ten years ago, the whole notion that fossils could preserve color was hardly on the radar—there was only one study out. Twelve years ago, no one would even know that this was possible.
There are several mass spectrometry techniques that look at the molecular fragments on the surface of your material, but not all fragments are diagnostic. There are chemical techniques that produce unique fragments of the melanin molecules so you can't confuse them with anything else. People are also looking at the inorganic chemistry of fossils and trying to recover supporting evidence of color.
So it's really important to consider the taphonomy, the tissue chemistry and the evidence of color, and one really nice way of teasing out the biology from the effects of fossilization is to do experiments.
The symposium “Life’s Greatest Hits: Key Events in Evolution” on March 29, 2019 takes place from 10 a.m. to 4:30 p.m. at the National Museum of Natural History and features 10 internationally acclaimed evolutionary biologists and paleontologists. Ticket are available here.