The past is no longer mostly in black and white. Thanks to an exquisitely well-preserved fossil skin, a new study has reproduced a fossil vertebrate’s full range of colors. The specimen—a 10-million-year-old snake—had a back patterned in green and brown-to-black, with a lighter belly, the authors say. But not everyone is convinced it’s an accurate reproduction.
Past reconstructions of color from fossils, like those in dinosaur feathers, have focused on melanin, the pigment that gives your skin its hue and produces a variety of reds, browns, and blacks elsewhere in nature. The pigment doesn’t dissolve easily, so it sticks around in fossils, making it the easiest ancient color-producing pigment to find and study. “Any other color would have long since rotted away,” says Maria McNamara, a paleontologist at University College Cork in the United Kingdom and lead author of the study.
But in some organisms, colors are linked to physical structures that may themselves leave behind evidence. In reptiles, amphibians, and other groups, color mostly comes from chromatophores—cells within the skin that hold color-producing pigment molecules.
The snake specimen, which was discovered in the early 1900s years ago in northeastern Spain, was fossilized in a way that preserved the shapes and sizes of these pigment cells. The original tissues have been replaced with calcium phosphate, the tiny crystals of which reproduced the shape of the original tissue in fine detail. “The mode of preservation is breathtaking,” says Mary Schweitzer, an evolutionary biologist at North Carolina State University in Raleigh who wasn’t involved with the study.
Using a scanning electron microscope, McNamara and her team found nanometer-scale objects in the fossil skin that they say are the right shape, size, and location to be three kinds of pigment cells found in modern snakes: one whose pigments produce a range of browns and blacks, a second that makes yellows, oranges, and reds, and a third that can contribute a variety of different colors.
Taking samples from the back, belly, and side of the fossil skin, the team found different numbers of the three pigment cells at each location. After that, they began a process that’s a bit like reverse-engineering a painting by figuring out which paints the artist mixed. They first pooled together information about colors in related modern snake species in the same family, Colubridae (which contains 70% of all known snakes): specifically, which combinations of pigment cells produce which colors. Then they matched the samples from their fossil snake against these modern snake colors to reconstruct what the serpent may have looked like when it slithered around in the late Miocene.
The snake had greenish and brown-to-black regions on its back, and a pale belly, according to the study published today in Current Biology. Reconstructing these colors can reveal insights into the animals’ behavior, too. For example, the greens and brown/blacks of the fossil snake hint at a possible role in camouflage—and the lighter colored belly is probably a sign of countershading, a camouflage technique widely used in modern animals.
“This is a very cool first pass at this material,” Schweitzer says. “And I’d like to see more.” For example, she says that a chemical analysis—looking for actual pigment molecules—is necessary to show for sure that these are pigment cells.
Of course, making any reconstructions of something as complex as color from a 10-million-year-old fossil will involve some uncertainty. And according to Matthew Shawkey, an evolutionary biologist at Ghent University in Belgium, the authors’ claims of being able to identify these different color-producing cells fall flat. For example, cells that in modern reptiles hold melanin—melanophores—are smushed together in an amorphous layer, he says. But the objects in the fossil snake skin that the authors call melanophores are egg-shaped. “I’ve never seen a melanophore that looks like that,” Shawkey says.
The authors say that pigment cells may be preserved in other samples of this phosphatized skin—people just haven’t known to look for it. Chromatophores produce the colors of all kinds of vertebrates other than reptiles, including fish and amphibians—so the new technique could open a door into seeing the full colors of a wide variety of ancient creatures.
Certainly, the field needs to be able to move beyond melanin to reconstruct specimens’ color—and behavior—in any depth. Shawkey agrees—but, he says, “I don’t think this is the paper that’s going to move us forward in that direction.”