It’s probably hard to believe, but there was a time, not that long ago, when scientists thought dinosaurs were extinct. No, seriously! That was before paleontologists began to understand the impressive anatomical similarities between fossil dinos and living birds. The icing on the cake: a series of discoveries, starting in the 1990s, showing that some dinosaurs even sported feathers. It’s no longer even slightly controversial to claim that birds are descended from dinosaurs, and even that they are dinosaurs—the only branch of the family that survived a massive comet strike 65 million years ago.
With that relationship firmly established, scientists have moved on to looking at some more finely grained questions, and a new paper in Nature is casting light on one of them: since the feathers of modern birds are often intensely colorful, how much color did their extinct cousins display? The answer, it turns out, is probably a lot. Feathers, says Julia Clarke, of the University of Texas, Austin, one of the paper’s co-authors, were brightly colored from the time they first appeared in Maniraptoran dinosaurs, including oviraptors and dromaeosaurs.
But that’s only part of the story: the genes that control the colors of skin, hair and feathers are part of the body’s melanocortin system, which also influences metabolism, inflammation and sexual function. “We hypothesize,” says Clarke, “that what we’re seeing is a big physiological shift in dinosaurs, a change that has other implications than just the color of feathers.”
What Clarke and her colleagues are actually seeing is melanosomes, structures so tiny they can only be viewed with a scanning electron microscope. “They’re pushed into developing hair or skin,” Clarke says, “and we know that the shapes of melanosomes are related to the chemistry of pigment.”
If an individual’s melanosomes are round, he or she will tend to have red hair or red feathers; if they’re long and skinny, the color is black. Mixes of different-shaped melanosomes are associated with all the colors sported by birds or humans or other mammals. Turtles and lizards, by contrast, don’t show much melanosome variety, and their colors tend to be drab.
Melanosomes can be identified in fossils as well, and when Clarke and her colleagues looked at long-dead dinosaurs and pterosaurs (extinct flying reptiles that weren’t technically dinosaurs), they found a rich diversity of melanosomes in dinosaurs with true feathers, but not in species with the fuzzy filaments that preceded feathers. They don’t know exactly what the colors were, but that answer should come with more research.
They also can’t say why the changeover happened, says Clarke. “When you’re looking back 150 million years through a dirty lens, causality can be hard to get at. We’ve got a lot more to figure out.” But it does seem clear that bright coloration may have been a side effect of a major change in dinosaur metabolism—a change that ultimately allowed one branch of the dinosaur family to escape the bounds of gravity and take to the air.