In most vertebrates, skin appendages such as hair, feathers, or scales originate from placodes—small, specialized skin regions whose spatial organization is controlled by well-conserved genetic signals. Crocodiles are an exception: their head scales do not emerge from placodes but result from simple mechanical folding of the growing skin.

A new study from UNIGE shows that turtles employ both of these distinct processes to develop the scales on different parts of their heads. These findings suggest that the mechanical shaping of scales is an ancestral trait, shared with crocodiles and likely dinosaurs, but lost in birds.

From an evolutionary perspective, this discovery is significant. Tortoises and aquatic turtles (collectively known as Testudinata) are the closest living relatives of crocodiles and birds. The fact that turtles and crocodiles share a mechanical process for forming head scales suggests it originated in their common ancestor and was later lost in birds.