Rhinokinetic snout of thamnophiine snakes

Abstract

Radiographic and cinegraphic behavioral data, combined with anatomical evidence, indicate that the snout in Nerodia and Thamnophis consists of four movable elements (1, premaxilla; 2, paired nasals; 3, right septomaxilla and vomer; and 4, left septomaxilla and vomer), a condition we refer to as rhinokinetic. In thamnophiine snakes, movements of the snout bones allow the teeth of the right and left sides to separate further and increase the effective stroke distance of each palatomaxillary cycle during swallowing. Histological and microdissectional analyses suggest that snout movement is keyed to the placement of the cartilaginous nasal septum and associated nasal capsules relative to the surrounding bones. The nasal septum separates the paired septomaxillae and is surrounded by loose connective tissues that extend ventrally between the vomers. The nasal capsules separate the nasal bones from the underlying septomaxillae, and also surround the anterior ends of the septomaxillae, providing a cartilaginous cushion between these bones and the premaxilla. The extraordinary rotations of the snout tip seen during swallowing in thamnophiine snakes are thus due to motion at the prokinetic joint between snout and braincase, and at all rhinokinetic joints connecting the four functional elements of the snout.