Abstract

Various lineages of amniotes display keratinised skin appendages (feathers, hairs, and scales) that differentiate in the embryo from genetically-controlled developmental units whose spatial organisation is patterned by reaction-diffusion mechanisms (RDMs). Using customised robotic high-resolution 3D geometry and texture reconstruction hardware and software, as well as developmental biology techniques, we recently showed that face and jaws scales of crocodiles are random polygonal domains of highly keratinised skin, rather than genetically controlled elements, and emerge from a physical self-organising stochastic process distinct from RDMs: cracking of the developing skin in a stress field. In addition, using molecular and electrophysiological analyses, we demonstrated that the small integumentary sensory organs (ISOs) distributed on postcranial and/or cranial scales of crocodylians, are not only exquisite mechanosensors, but constitute multi-sensorial micro-organs innervated by multiple pools of sensory neurons with mechano-, thermo-, and chemosensory functions. Hence, the ancestral vertebrate diffused sensory system of the skin was transformed in the crocodylian lineage into an array of discrete multi-sensory micro-organs with no equivalent in other vertebrates. These two profoundly modified developmental processes (cracking of the skin and discretisation of sensory expression sites) allowed crocodylians to develop a highly-armoured, yet very sensitive, skin.