A description of dental microanatomy and implantation geometry in Chamaeleonidae

Chamaeleonidae is a clade of squamates distributed today across Africa and Madagascar. They exhibit an acrodont dentition wherein each tooth attaches to the apex of the jaw and is sutured directly to the supporting jawbone. Mediolateral contact between the tooth and the supporting jawbone is absent in conventional descriptions, which distinguishes acrodont dentitions from the pleurodont and thecodont tooth implantations seen in other reptiles. While squamates present a diverse array of tooth implantation forms, chamaeleonids uniquely display the development and retention of solely acrodont implantation throughout ontogeny. According to the historical understanding of dental tissue homology, acrodont chamaeleons are assumed to lack the complete set of periodontal tissues described in mammals, crocodilians, pleurodont iguanids, and agamids, among other amniotes. Here, chamaeleonid tooth implantation and attachment tissues are interpreted using thin-section histology and microcomputed tomography. In this study, we identify a previously unrecognized insertion of the labial side of each tooth within the supporting jawbone, in a manner that nevertheless coincides with the apical implantation of each tooth along the labial and lingual aspects of the supporting jawbone. This mediolateral contact, apparent through the appearance of dentine internal to the supporting bone in the transverse view, was previously assumed to be absent by definition in acrodonts. We interpreted Sharpey's fibres in the mineralized attachment tissue as the presence of calcified periodontal ligament, also previously undescribed in chamaeleons. Moreover, a unique interdigitation of dentine and alveolar bone is recognized in the form of a plicidentine-like condition of fluted projections of dentine through which growing jawbone interceded. Based on the results of our study, we define acrodont geometry as the implantation of the tooth onto the apex of the jaw, without the elevation of the labial and lingual pedicles as walls on either side of the tooth, though each side of the tooth may superficially insert within the jaw. Altogether, this work contributes to the trend in recent literature on amniote tooth anatomy towards describing a mammal-like tripartite periodontium in squamates, as opposed to the traditional interpretation of a single 'bone of attachment'.