Through their studies of bones, fossils, and geology, paleontologists have uncovered the prehistoric worlds of Earth's past. We watch movies and TV shows of computer generated versions of long-extinct dinosaurs, fish, birds, and even mammals and it seems obvious how we know about the sizes and shapes of these animals...but how do we know what their colors were like? A new scientific field is emerging, called paleocolor (or palaeo colour, if you're British), in which scientists use fossils, chemistry, cellular biology and comparative biology to reconstruct the color patterns and related behaviors of animals long since passed.

Paleontologists have long been aware that many fossil feathers, scales, mollusk shells, and insects have clear light and dark color patterns. Some even have patterns of color or iridescence (a shimmery metallic sheen). Fossil mollusk shells, for example, are sometimes found with color patterns. Fossil insects are sometimes found with iridescence. It was originally thought that these colors were the product of bacteria, but recent discoveries have turned this idea on its head.

Fossil cephalopods (similar to today's octopus and squid) were discovered with dark ink-sac-shaped splotches. This dark fossil ink could be dissolved in water and then chemically and biologically analyzed (or even used to write). When chemically analyzed, we learned that fossil ink has the same infrared spectra and contains the same form of melanin (a dark pigment) as modern day cephalopod ink. It is organic pigments, such as melanin, that primarily determine the colors of today's animals. Likely it was organic pigments, and not bacteria, that determined the colors of animals before our time.

Melanin is located in melanosomes (cellular structures) that are themselves found in feathers, scales, hair, eyes or skin cells. These melanosomes can vary in shape from balls to long rods, and they can vary in how densely they are packed. Scientists looking at modern-day bird feathers found that melanosome shape determines the color shade of the feather (rod-shaped melanosomes are black to dark brown and ball-shaped melanosomes are reddish brown to buff yellow) and the density of the melanosomes determine how brightly colored the feather is. Iridescence results from a particular shape and arrangement of melanosomes that scatters light in just the right way. Taking what we learned about bird feather coloration, we later found that this pattern also holds in modern-day mammalian hair, frog skin, tortoise shells and mollusk shells.

When we now look at fossils with our new knowledge of melanosomes, we can find fossils with melanosome impressions or even some preserved three dimensionally intact melanosomes. These fossils all have their own burial history and vary in the pressures and temperatures that they were exposed to, and thus, in their decay processes. For this reason, we are limited in our confidence of how dense melanosomes were for a given fossil, and hence, the brightness that the original animal's color had (although we can infer how bright one part of the animal was compared to another part). However, scientists that artificially age materials believe that melanosome shape appears to preserve faithfully (ball-shaped melanosomes stay ball-shaped and rod-shaped melanosomes stay rod-shaped). This means that we can say with relative confidence what part of an animal's body was black or brown or yellow. Many new chemical methods have been recently developed to analyze fossil melanin characteristics and we are likely to see many new exciting discoveries about the colors of ancient animals over the new few years.

Melanin is not the only pigment that colors today's animals (although it is the only pigment in mammals). Many birds have additional pigments in their feathers. Amphibians, fish, reptiles, crustaceans and cephalopods have chromatophores that have more pigments. While scientists have learned a good deal about melanin, we know much less about these other pigments and their influences on the colors of now extinct animals. For one thing, melanin is preserved much more reliably than these other pigments are. Also, some of these pigments can be found in the decaying algae and leaves that these animals died in, so even when we do find some of these other pigments in a fossil, we cannot be sure if they were part of the animal or part of that animal's environment. Where we are limited in our color interpretations using chemical and biological techniques, we can use our knowledge of animal behavior to help fill in the gaps.

Animals today commonly use color for camouflage, startling or confusing predators, advertising their toxins to predators (called aposematism), mimicking other dangerous animals, displaying to potential mates and rivals, and communicating. It is likely that ancient animals used color in the same ways, and we can use our knowledge of present-day animals to fill in some of our gaps in knowledge about past animals. For example, one of the most common animal color patterns is countershading (where the top/back of an animal is darker than its underside). This color pattern is useful in animals that relate to predators in three dimensional space: If a predator is below you and sees your light-colored belly you can blend in with the sky; If a predator is above you and sees your dark-colored back you can blend in with the ground. Contrasting light and dark bands are also common camouflage in prey animals that live in dense vegetation. Among aquatic animals, brightly colored animals are more likely to live in a brightly colored coral reef (where they can blend in from a distance, but stand out to potential mates that are up close). Contrasting white and black markings are common in animals that live in social groups in open environments, and they use these markings to visually signal one another. Paleontologists work with collections of incomplete information about the subjects they study, so in this way they can use evidence of color to infer behavior and evidence of behavior to infer color.

We have a long way to go in the field of paleocolor, but the Jurassic Park sequels to come are likely going to become much more colorful.

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Further reading:

Vinther, J. A guide to the field of palaeo colour, Bioessays, 37, 643-656 (2015). DOI: 10.1002/bies.201500018.

Image Credits:

Images are all from the paper.