One of my favorite Calvin and Hobbes comics concerned the visual record of the past: photographs and videos from before 1930.
When I was little, I thought of the same questions. Luckily, my dad explained technological history – and even bought me a film camera with black and white film. When I first became interested in paleontology, I knew that the black, gray, and brown fossils I examined were only the shadow of the original organism, but I thought we had no way of reconstructing original colors of many organisms. Color patterns, such as stripes on a shell or banding on a feather, have been noted, but is that evidence of original color? What about the diverse array of color found in insects?
Color in the fossil record
In 2008, researchers at Yale University decided to investigate the cause of color banding in an early Cretaceous feather from the Crato Formation in Brazil. Using a scanning electron microscope, they found masses of small (1-2 µm or 0.000079 inches) oval structures that were aligned with the barbs of the feather – but only in the dark areas. In the lighter bands of these feathers, no such structures existed.
When they compared these structures to a modern feather from a Red Wing Blackbird, they found that the modern feather also had these structures of similar size and shape in black areas. These structures in modern feathers are called melanosomes – small organelles which produce and store melanin, one of the most important and common pigments in the animal kingdom. Two types of melanin are found in birds – eumelanin, which produces dark brown and black color, and pheomelanin, which produces red and amber hues. Eumelanin and pheomelanin are found in distinctively shaped melanosomes, and the long oval structures identified in the fossil feather are very similar to eumelanosomes in modern birds.
This was the first evidence that color and color patterns may have a much larger fossil record than originally thought! A flurry of investigations soon began, and in 2010, the first color reconstruction of a dinosaur was published. Anchiornis huxleyi, a chicken-sized dinosaur, was mostly black with some white patterning with a red crown of feathers.
But what about other colors in the fossil record? Melanins produce colors from red to black (think the spectrum of human hair color) – but there are many other colors in the animal kingdom! Other pigments are responsible for brighter reds, such as those seen in Cardinals, yellows, and greens. However, a whole different type of color exists – that which is produced through physical structures (biophotonic nanostructures). Structural color produces some of the most difficult-to-make colors in nature – blues are not produced by any known pigment – along with the entire suite of other colors and also iridescence. In order to interact with light, which is made of nanometer scale waves, these structures have to be very small. Additionally, small changes in the chemistry of the structure can cause changes in the color produced because it changes how light interacts with the physical structure. Animals have developed several different types of these color producing structures, such as multilayer reflectors (two-dimensional layers) and photonic crystals (three dimension organic crystals), and like I briefly mentioned in my first post, structural colors can also preserved in the fossil record.*
During my first year in graduate school at Yale, I was fortunate to meet Dr. Maria McNamara, then a post-doc in my lab. She had been studying fossilized structural color in insects, and had found interesting patterns in regards to both the preserved structure and also the colors produced. First, she had investigated hundreds of colored fossil insects, and the only type of biophotonic structures she found were multilayer reflectors. Secondly, she found that the preservation of the biophotonic nanostructure was directly related to the preservation of the cuticle in beetles – if the cuticle was well preserved, so too was the structure. Well-preserved beetles that lacked a multilayer reflector thus lacked a multilayer reflector in life. Finally, she found that the observed colors in fossil beetles and moths were ‘redshifted’ (longer wavelengths) from what the thicknesses of the multilayer reflectors suggested, indicating that a chemical change had occurred which altered the produced color.
I began working with her on a project looking at the preservation potential of structural colors in beetles. We wanted to test whether temperature and pressure – two variables involved with fossilization – had any effect on the structure and its ability to produce color. To do this, we ran a series of experiments on two types of beetles, each with a different form of biophotonic nanostructure, that tested changes in response to temperatures from 25-270°C and pressures from 1-500 bar (1-500 times atmospheric air pressure at sea level) – in essence, we baked beetles! After the experiments, we calculated what color should be produced by analyzing images taken using transmission electron microscopy, which gave us the thickness of the layers in the multilayer reflectors, and the chemical composition of the beetles. Here is what we found:
What effect did this have on the color?
We found that as temperatures and pressures increased, the color became blueshifted (smaller wavelengths)- until temperatures were increased to 250°C. At that high temperature, no color was produced – no peak was measured at all (bottom right in the figure above) – even though a multilayer reflector was detected. But how did this compare to the fossils? Weren’t they all redshifted? Why wasn’t the multilayer reflector that was baked at the highest temperature producing color?
The bottom row in the above figure connects the results of the experiments to what Maria had seen in the fossils – overall, the color produced was more blue than what the analyses on the thickness and contrast (a proxy for chemical composition) of the structure. However, look at the differences between the black peaks (predicted) and the red peaks (observed) – the red line is in fact redshifted from the calculated color!
When molecules are exposed to temperatures they are not made to be in, they change to become more stable in that temperature. In our experiments, the high temperatures caused the molecules in the biophotonic nanostructure to change. Although the thickness of the different layers had become more compressed, causing the color to be made from smaller wave lengths (blueshifted), chemical changes to the proteins and lipids then redshifted the color. The final color was the result of both physical and chemical change! The beetle baked at 270° maintained a multilayer reflector, but the high temperature changed the chemistry so much so that light no longer interacted with the structure in a way that produced color. This explained why fossils could be black but retain a multilayer reflector – the structure was preserved, but chemical changes altered the organics so that it no longer produced any color! We also did the experiments on a beetle with a three dimensional photonic crystal, Pachyrrhynchus reticulatus, and found the same thing, even though there had been no fossil with this type of structural color identified at that point (but now there is!)
What does this study say about color in the fossil record? First, reconstructing color from fossils is more complicated than it might first appear. Even apparently well preserved multilayer reflectors may have undergone physical change due to pressures – thus changing how the structure interacts with light. How much of a change a multilayer reflector undergoes is very difficult to understand. However, analyzing the chemical composition – to determine how the chemistry changed – may allow us to determine how much chemical changes might be influencing color. Secondly, our experiments showed that the physical structure can survive despite being altered so that no color is produced. This means that structural color may have a much larger fossil record than we recognize now – we just have to start looking at black beetles! This will allow us to more fully explore the evolution of color in insects!
While the investigation into colors in the past is still in its infancy, we have already made great strides in reconstructing the appearance of ancient animals. Perhaps, with time, we will be able to better reconstruct ancient life in true color – and some of the most famous organisms given a full make over.
*For a wonderful summary of structural colors, see this article in the Yale Scientific Magazine about fossil weevil with preserved color (McNamara, Saranathan, Locatelli, et al. 2014).
Works cited and referenced: