The wide gape of snakes: A comparison of the developing mandibular symphysis in sauropsids

The origin and evolution of snakes has been marked by the acquisition of many morphological and functional novelties, one of which is the possession of a highly kinetic skull allowing for the consumption of prey that are often larger than their head diameter. One feature of the iconic wide gape of macrostomate (large-mouthed) snakes is due to changes in the rostral midline where the left and right hemi-mandible come together. Across vertebrates, the two sides of the lower jaw are held together by the mandibular symphysis. In snakes, the two halves of the lower jaw do not fuse and the symphysis remains free, facilitating gape expansion. The symphysis has previously been explored in lizards and crocodiles, where ligamentous fibres and cartilages span the joint. Here, we compared the anatomy of the forming 'free' mandibular symphysis in the corn snake (Pantherophis guttatus) to symphysis development in two lizards, the veiled chameleon (Chamaeleo calyptratus) and the ocelot gecko (Paroedura picta), and an outgroup sauropsid, the chicken (Gallus gallus domesticus). Microcomputed tomography imaging, whole-mount skeletal staining and histology staining confirmed the absence of bone and cartilage fusion at the mandibular symphysis in the corn snake during development, in contrast to the complete fusion of cartilage, but not bone, in both lizards and the fusion of the bone in the chick. Trichrome staining under circular polarised light and whole fast green staining highlighted that, while the symphyseal region was populated by a dense network of collagen fibres, the snake hemi-mandibles were not connected across the rostral region by this fibrous network. Instead, collagen fibres extended backwards and around the snake mental groove to an intermandibular nodule. This nodule attached to the midline dorsally, allowing integration of the movement of the soft and hard tissues. Our analysis highlights the adaptations required to allow extreme lower jaw mobility and independence of the two sides of the jaw as found in macrostomate snakes.