How can we explain the morphological diversity of living organisms? Although genetics is the answer that typically springs to mind, it is not the only explanation. By combining observations of embryonic development, advanced microscopy, and cutting-edge computer modelling, a multi-disciplinary team demonstrates that the crocodile head scales emerge from the mechanics of growing tissues, rather than molecular genetics. The diversity of these head scales observed in different crocodilian species therefore arises from the evolution of mechanical parameters, such as the growth rate and stiffness of the skin. These results shed new light on the physical forces involved in the development and evolution of living forms.