Camouflage—in particular, clever patterns of skin pigmentation—helps many modern animals hide from predators in plain sight. The same was true nearly 120 million years ago in the Cretaceous. Now, researchers studying that era have taken reconstruction of fossil remains a step further, using the pigmentation patterns preserved in the fossil of a small, horned dinosaur to find out where it most likely lived. The answer? A dimly lit, dense forest.
The study is “a very welcome and very clever addition to the really limited information we have on dinosaur color and coloration patterns,” says Anne Schulp, a vertebrate paleontologist at the Naturalis Biodiversity Center in Leiden, the Netherlands, who wasn’t involved in the research. “If there's one question that keeps on popping up amongst our visitors, it's the question on the color and color patterns dinosaurs had,” she adds.
It’s a question in which scientists, too, are keenly interested. And by tracing the remains of pigments in fossils, called melanosomes, scientists have in recent years begun to breathe new life into the dun-colored relicts, discovering the Technicolor hues in prehistoric birds’ wings and the clever shading that veiled ancient mosasaurs from predators. The pigmentation patterns on those Cretaceous marine reptiles followed a pattern called countershading, in which the animal’s back is dark and the belly is lighter. It’s a pattern found in many modern animals dwelling in land and sea. In water, the pale belly blends in with sunlight falling from above, making the animal invisible to predators below, whereas the dark back hides the animal from shallow predators by helping it blend in with the darker depths. On land, the pigmentation counteracts the effect of light falling on the animal: The lighter belly is cast into shadow while the darker back is lightened, giving it a flat appearance that makes it much harder to see.
Psittacosaurus, whose name means “parrot lizard,” was a smallish, horned dinosaur that ran about on two legs eating nuts and seeds in what is now northeastern China. It was found in the same rock strata as many feathered dinosaurs, although it didn’t have feathers itself. But it does have complex pigmentation, says Jakob Vinther, a paleontologist at the University of Bristol in the United Kingdom and lead author of the new study. When he first saw an exceptionally well-preserved specimen of Psittacosaurus in Germany in 2009, which included multiple areas of dark pigment, Vinther says his reaction was: “Holy cow, this thing has beautiful color patterns.” But it wasn’t until a few years later that he began to wonder whether it would be possible to use those patterns to learn something new about the dinosaur—not just what it looked like, but where it lived.
To do this, the researchers would have to first reconstruct a 3D model of the dinosaur in painstaking detail, from its precise body shape to its pigmented skin. Then, his team would have to observe the model in various types of lighting environments, to see what shadows were cast along its body. Because the light in an open savanna comes straight down and casts harsh shadows, animals there have sharp dark-to-light transitions in pigmentation, located at the top of their bodies. In forests, light is more diffuse, and pigmentation transitions tend to be farther down on the body and less sharp.
The team turned to paleoartist Robert Nicholls to bring the flat fossil to life. Nicholls first photographed the fossil from different angles, using cross-polarized filters to remove light reflections and capture any tiny traces of pigment. Then he took multiple measurements of its bones and soft tissue.
Nicholls says he took it all back to his studio and began to draw Psittacosaurus “from the inside out”—first its skeleton, then its muscles and sensory organs, and finally its skin. Getting the details right “required many hours of staring,” Nicholls says, as he and Vinther studied how to untangle the overlapping pigmentation patterns left in the various folds of the flattened fossil. Those drawings became the basis for an anatomically accurate, life-sized sculpture of the animal made of clay, Styrofoam, and wire mesh. The skin pigments took 3 weeks to paint.
The reconstruction itself was revealing: Psittacosaurus was indeed countershaded, with a lighter belly and darker back. But its chest was also darker than the abdomen, which confirms that the animal stood upright on its hind legs. The head was also heavily pigmented, and its horns were apparently made of a soft tissue, not hard keratin, which suggests that perhaps they were used for signaling rather than aggression, Vinther says.
The team also wanted to see how the light would react just with the animal’s body shape, so Nicholls made a second model devoid of all pigmentation, colored a simple matte gray. Then, they took a field trip to the University of Bristol Botanic Garden, where they placed both models against different types of Cretaceous plants. The reconstructed patterns closely matched the “optimal countershading” for the diffuse light under a forest canopy, the team reports online today in Current Biology. That corresponds well with evidence from previous paleobotanical studies of the region, which suggest that it was dotted with lakes surrounded by coniferous forests. “These color patterns are a testament to an arms race [between predator and prey] that took place 120 million years ago,” Vinther says.
“It’s impressive enough that Vinther et al. managed to recreate the color pattern of a long-extinct dinosaur,” says zoologist Hannah Rowland of the University of Cambridge in the United Kingdom, who was not involved in the study. “But to have also shown that this pattern of countershading is best suited to a closed, wooded-type habitat environment is a real advance.” She adds, however, that although there is good evidence that countershading acts as a defense mechanism, there will always be some uncertainty about interpreting countershading in dinosaurs, because we can’t present a model Psittacosaurus to their natural predators to see which type of pattern provides the best protection.
“I hope that now we can start to think more about the ecology, to get a better picture of these animals,” Vinther says. “Definitely, in paleontology, we have become more astute to the fact that we have more evidence in these fossils than we thought we had.”