Lots of fossils, such as the ichthyosaur shown here, are outlined or shellacked with a mysterious dark deposit. For a long time, scientists couldn't be sure what the material was or where it came from. Under the microscope, the material housed tiny egg-shaped structures that looked like melanosomes—the cell organelles that secrete pigments into an animal's skin. Other scientists thought the structures might be bacteria.
By studying the molecular composition of the pigments, the scientists in this study not only concluded that the deposits are pigment remains, but also determined what those pigments were. They say that three fossilized marine reptiles they studied—a 190-million-year-old ichthyosaur, an 86-million-year-old mosasaur, and a 55-million-year-old leatherback turtle—probably had blackish skin like the .
"This is the first time that we're reporting pigments, the animal's own biomolecules from reptile skin," says , a geologist at Lund University and the lead author on .
Previous studies relied on a visual identification of those egg-shaped melanosomes. Lindgren's team went a step further by analyzing the chemistry of the structures and pigments in the samples. The molecule that causes black coloring, called eumelanin, had degraded over time but remained largely intact. It was enough to provide the first unequivocal evidence of pigmentation in the skin of a fossilized animal, says Maria McNamara, a paleontologist with the University of Bristol who was not involved in the study.
Skin from a 55-million-year-old leatherback turtle (left), scales from an 85-million-year-old mosasaur (center), and tail fin of a 196-190-million-year-old ichthyosaur (right). Credit: Bo Pagh Schultz, Johan Lindgren and Johan A. Gren.
To identify the dark deposits, Lindgren's team fired a beam of ions at samples of the material. The ions broke up the material and sent fragments flying into a detector, which analyzed their chemical composition and confirmed that the dark deposits were eumelanin. Under the microscope, Lindgren's team showed that concentrations of eumelanin peaked in areas with the highest density of the tiny egg-shaped structures—suggesting the structures were indeed melanosomes, not bacterial cells.
Most studies up to now have tried to learn the coloration of ancient organisms by studying fossilized feathers, because feathers are tougher and more resistant to decay and their melanosomes are more densely packed than in skin. Lindgren's study opens the door to reconstruct coloration in a wider range of species, including nonfeathered dinosaurs.
The ichthyosaur fossil the scientists sampled. Credit: Benjamin P. Kear
That's important because an animal's color can say a lot about its behavior. Color can be used as camouflage, to signal to mates, or to flash a warning to potential aggressors. Lindgren's team hypothesizes that the black coloration helped ichthyosaurs, mosasaurs, and ancient leatherback turtles to absorb extra heat. That would be helpful if, like modern-day leatherbacks, the large reptiles ranged into the icy waters of the Arctic Circle.
The distribution of dark pigment around the fossil ichthyosaur suggested the animal was uniformly dark-colored. "That's pretty neat, because most marine animals have a dark back and a white belly," Lindgren says. "If you look at sperm whales today, they have a uniform coloration. Ichthyosaurs are also inferred deep divers, so it's an interesting similarity."
There appears to be a limit to how well scientists can reconstruct ancient coloration. Some non-melanic pigments (which can be responsible for red, yellow, and blue coloration) don't preserve as well as melanin-based pigments. Still, "the glass is half-full as opposed to half-empty," says Patrick Orr, who studies fossil preservation at University College Dublin. "We're now getting data that, a decade ago, would have been impossible."
Orr predicts that this work is just the tip of the iceberg for fossil pigmentation studies, which can now begin to link coloration patterns with different ecologies and chart how reptile color evolved over time.
Anne Schulp, who studies mosasaurs at the Naturalis Biodiversity Center in the Netherlands, called the new research exciting. "As a paleontologist at a museum, I'm always trying to take our visitors back on a trip through time, and the more details we have the better the story gets. We can now do a little paint job on the marine reptiles—a paint job that's actually based on real research."