The palatal dentition in squamate reptiles is poorly known compared to the marginal dentition. We surveyed species representing all squamate families for the occurrence of palatal teeth. For those exhibiting palatal teeth, we investigated palatal tooth arrangement, morphology, development, attachment, and replacement patterns using SEM, high-resolution X-ray computed tomography imaging, and histology. We found substantial variability in the presence of palatal teeth and in the arrangement and number of palatal tooth rows at familial, generic, and specific levels among nonophidian lizards. However, snakes exhibit much more uniformity in both these respects. These patterns of variability have implications for trade-offs between phylogenetic, functional, and developmental constraints.
When numerous, palatal teeth in most nonophidian lizards are either arranged in fields or loosely organized rows near the back of the oral cavity. In contrast, palatal teeth in snakes and in the “lizards” Shinisaurus and Lanthanotus always occur in well-organized single rows; in snakes, these rows extend to the front of the oral cavity. Palatal teeth are usually morphologically similar to the marginal teeth, although in some cases they appear simpler. Replacement palatal teeth develop within a fold of the dental lamina that originates from the oral epithelium. In species with palatal teeth arranged in single rows, the dental lamina occurs along the labial edge of the tooth row. In others, various modifications of the dental lamina were observed that accommodate tooth replacement across multiple tooth rows or fields. Ankylosis of palatal teeth involves attachment either to the ventral surface of the tooth-bearing element or within a resorbed cavity or gutter within the element, except in some iguanids in which teeth attach to a raised ridge on the surface of the palatal bone. Tooth replacement patterns on the palate generally mirror those on the dentigerous marginal elements (i.e., labial replacement of palatal versus lingual replacement of marginal teeth), and traditionally recognized modes of tooth replacement for the marginal teeth are also observed for the palatal teeth. The strong correspondence in morphology and replacement patterns observed between marginal and palatal dentitions supports hypothesized developmental homology between them.
The homology of dental tissues and tooth attachment patterns in snakes and other lizards is currently controversial. We observed plicidentine in both the marginal and the palatal teeth of varanoid lizards, but not in the teeth of snakes as has recently been proposed. In addition, snakes and the extinct mosasaurs have been suggested to share a thecodont mode of tooth implantation. Relevant to this issue, we observed implantation of palatal teeth within resorbed cavities of varying depths in the attachment tissue or underlying bone of virtually all squamate groups. Also, an extensive buildup of attachment tissue that surrounds the teeth and prevents contact of tooth bases with each other occurs not just in snakes and mosasaurs, but also in Shinisaurus and Lanthanotus. Although palatal tooth ankylosis often occurs within a cavity, various attachment modes were observed within such cavities, generally corresponding to the attachment modes present in the marginal teeth. We conclude that tooth attachment geometry reflects, at least partially, constraints imposed by attachment to substrates of varying shapes. In other words, nearly all taxa observed exhibited some degree of “socketing” of the teeth on the palate regardless of actual attachment mode. It remains a matter for further study whether such a condition corresponds to the true thecodonty exhibited by archosaurs and mammals, which may differ fundamentally from s