Comparative morphology of the tongue and oral cavity in squamate reptiles, and the biomechanics of vomeronasal chemoreception

Vomerolfaction, mediated by paired vomeronasal organs (VNOs), is a highly important chemical sense in squamates (snakes and lizards). The VNOs of squamates are only accessible via ducts that open into the oral cavity, and snakes and lizards engage in tongue-flicking to gather chemicals from the environment and transport them into the oral cavity for delivery to these ducts. Unfortunately, the biomechanics of chemical delivery to the VNOs has been poorly understood. I conducted a comparative study of the morphology of the oral cavity and tongue that led to a robust hypothesis of chemical delivery in squamates. I found that regardless of tongue form, which is considerably variable, the mechanism of chemical delivery is fundamentally the same for most squamates. The sublingual plicae (raised structures on the floor of the mouth) act like pistons within a chamber (or chambers in squamates with forked tongues) created by the sublingual plicae and features of the palate. They apply pressure on the foretongue forcing the chemical laden fluid coating it around the margins of the foretongue towards the ducts of the VNOs. Given this mechanism, I also found features that would act to maintain the discrete nature of chemical signals from the left and right sides of the foretongue allowing for tropotaxis (movement of an animal in response to differential signal intensities from paired sensory organs), which has already been established as the major selective force behind the deeply forked tongue of snakes and some squamates. Lastly, my investigations of the ultrastructure of the surfaces of the foretongue involved in chemical gathering in squamates yielded two major findings. First, there is a high level of desmosome junctions between the layers of squamous epithelial cells making this tissue resistant to the shear stresses that the tongue encounters during chemosensory tongue-flicking. Second, I found a feature unique to snakes and anguimorph lizards, the microfacet (small bumps, 0.5 - \u3e 2.5 μm, formed by packets of lipid pushing up the surface of the epithelial cells). This synapomorphy supports an anguimorph origin of snakes, a long standing hypothesis that has encountered recent controversy.