Despite its use as a diagnostic taxonomic feature, the occurrence and distribution of plicidentine in the teeth of squamate reptiles is unclear. This appears to be due to several factors: the various kinds of folding, wrinkling, and striation that occur within different dental tissues; difficulty of interpreting conditions in poorly preserved extinct taxa; and incomplete knowledge of tooth development. We investigated tooth development and morphology in extant and fossil squamate reptiles using skeletal preparations, histological sections, and CT-scanning data. Among squamates, we found plicidentine only in the teeth of varanoid lizards and note that much more anatomical complexity exists than previously thought in the dental and attachment tissues of these groups. Degree of development of plicidentine is variable within varanoids, with the strongest development occurring in some species of Varanus. In contrast to some reports, we found no evidence for the occurrence of plicidentine in the teeth of mosasauroid lizards or snakes. Some mosasaurs exhibit raised ridges on the enamel surface that extend from the base to the tip of the tooth, as well as occasional striation of the tooth bases built from bone of attachment; neither of these features is considered homologous to plicidentine infolding. Some snakes exhibit weak wrinkling of the tooth base that corresponds closely to the pattern of wrinkling in the overlying bone of attachment. This condition occurs infrequently in snakes, and details of tooth development and attachment also do not support its homology with plicidentine. Our results indicate that plicidentine is best interpreted as a synapomorphy of Varanoidea.

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