Naked Bearded Dragons Reveal Where Vertebrates Got Their Coats

From scales to feathers to fur, vertebrates clothe themselves in a dazzling variety of textures and hues. But scientists have shown that many of those coverings emerge from the same anatomical hardware.

Biologists have long known that feathers and hairs both start as structures called placodes. In reptiles, however, biologists had found distinct skin areas that yielded scales but no placodes. The absence proved puzzling, since birds are more closely related to reptiles than to mammals. Had birds and mammals evolved placodes independently? Or had today’s reptiles discarded them?

Then University of Geneva biologist Michel Milinkovitch visited an Italian animal fair, found scaleless, “naked” bearded dragons for sale—and a third scenario emerged. When he compared the naked lizards and their scaly kin, he saw to his shock placode-like bumps dotting the skin of scaled embryos. Naked embryos, however, stayed smooth.

In 2016 Milinkovitch announced that the bumps were indeed placodes, placing the structures in reptiles’, birds’, and mammals’ common ancestors more than 300 million years ago. And since placodes buzz with genes that also sculpt teeth and fish scales, some scientists think that placodes arose in the earliest vertebrates—a “remarkable conservatism,” argues Université Grenoble Alpes biologist Danielle Dhouailly, going back 420 million years.

Milinkovitch also found that naked bearded dragons lack scales because they’re missing working copies of a gene crucial to placode formation. As he noted with a chuckle, that brings his discovery full circle: “This animal doesn’t have scales, because it cannot make what people thought didn’t exist in reptiles.”