When we try to picture the ancient world, one thing usually stops us: No matter how much we know about the shape of creatures past, their color is a mystery. Now, the reconstruction of green and black markings on a 10-million-year-old snake opens the door to discovering how other animal species looked as well.
Logic tells us that some animals must have used camouflage, and the use of red as a signal of danger has probably been around for a long time. Nevertheless, if we had not seen them in the flesh, who would know what colors adorn the peacock's tail or squirrel's fur?
In Current Biology, paleontologists present evidence for the color scheme sported by a snake from the Colubridae family. The discovery was made possible because when the specimen died, it became fossilized in calcium phosphate.
The snake's head was lost, preventing determination of the species, and the pigments that made it green have long since dissolved. Nevertheless, the calcium phosphate preserved the skin in such detail that we can see the shape of the pigment cells. Since each different cell type has a specific shape, comparison with modern equivalents allows us to determine what color scheme the long-gone colubrid adopted.
“When you get fossil tissues preserved with this kind of detail, you're just gobsmacked when you're looking at it under the microscope,” said first author Dr. Maria McNamara of University College Cork in a statement.
The snake was dug up in a mining operation a century ago in the Libros Gypsum in northeastern Spain. McNamara came across it while working on her Ph.D., and years later examined the fossil under a scanning electron microscope, allowing her to see the pigment cells in such detail that she could match them to modern equivalents.
To the naked eye, no color remains. Under a microscope, however, the shapes of the cells reveal all. McNamara et al./Current Biology 2016
The cells are categorized as melanophores, which once contained melanin, and xanthophores that held red or yellow carotenoids and pteridines. A third type of cell – iridophores – did not hold pigments, but contained structures spaced to reflect light of particular wavelengths, producing iridescence like that of copepods or some butterflies.
By mapping the location of these cells on the snake's skin, McNamara was able to reconstruct the coloring shown above.
Melanin is more stable than other pigments, allowing scientists to sometimes detect black and brown coloring on extinct species, even in the absence of preservation such as this. Structural coloration, as represented by the iridophores, might be expected to be preserved better, but like blue or green pigments, it has not previously been observed in fossils.
Skins preserved in calcium phosphate are rare, but not unknown, and the authors suggest those samples that exist to be reanalyzed. “Now that we know that color can be preserved even for tissues that are mineralized, it's very exciting,” McNamara said.
Knowledge of an animal's coloring can also help reveal their behavior. These markings would have served as camouflage in sun-dappled forests, suggesting it was an ambush predator, but that it also maintained some additional color to attract mates or repel rivals.