Histology of tooth attachment tissues and plicidentine inVaranus(Reptilia: Squamata), and a discussion of the evolution of amniote tooth attachment

Abstract

Few recent studies have examined the histological basis for tooth attachment in squamates. In the past few years, a surge of interest in this topic has led to the intriguing suggestion that the major tissues derived from the tooth germ (enamel, dentine, cementum and alveolar bone), are conservative and are present in all amniotes. In this study, we describe the histology and development of the tooth attachment complex in Varanus rudicollis, the rough-neck monitor. We provide the first published evidence for the role of cementum and alveolar bone in tooth attachment in varanoid lizards. In Varanus, cementum is deposited on the external surface of the tooth root as well as at the base of the tooth, where it plays a role in the attachment of the tooth to the jawbone. Alveolar bone is also involved in tooth ankylosis. Our results support the hypothesis that the major tooth germ tissues are found in all amniotes. We provide insights into the structure and development of plicidentine, defined as infolding of the dentine around the tooth base. This feature is unique to varanoids among extant tetrapods and is the third tissue implicated in tooth attachment in Varanus. Plicidentine develops asymmetrically along the labial-lingual axis of a tooth. Varanus is characterized by the presence of both primary and higher-order lamellae, which anastomose to form a honeycomb-like surface that then interacts with the more basal attachment tissues.

Dental histology of Coelophysis bauri and the evolution of tooth attachment tissues in early dinosaurs

“…The evolutionary relationships of dinosaurs and crocodilians suggest that their periodontal tissues are in fact homologous. These data lend added support to a growing consensus concerning the homology of amniote tooth attachment tissues: cementum, alveolar bone, and PDL did not evolve independently in mammals and crocodilians, but are in fact plesiomorphic features of all major amniote clades and can be traced back to the base of Amniota, if not to an even more inclusive clade of tetrapods (Caldwell et al, ; Budney et al, ; Maxwell et al, ; LeBlanc and Reisz, ). Dinosauria is a highly diverse and successful clade within Amniota and these findings begin to fill in a larger gap in our understanding of dental tissue evolution across amniotes.…”

“…This will help establish a baseline for comparison in future studies on dinosaur dentition by approximating the ancestral condition for the clade. Cementum, PDL, and alveolar bone may be homologous for all major amniote clades (Caldwell et al, ; Maxwell et al, ; LeBlanc and Reisz, ), but our histological examination of tooth attachment between C. bauri and A. mississippiensis suggests that the individual tissues themselves can vary greatly in different taxa, presumably to suit different functions. These differences may be diagnostic of individual taxa, but may also be phylogenetically significant.…”

Periodontal Ligament, Cementum, and Alveolar Bone in the Oldest Herbivorous Tetrapods, and Their Evolutionary Significance

“…Several authors have discovered more recently the presence of a tripartite periodontium, consisting of cementum, periodontal ligament and alveolar bone in fossil squamates and ichthyosaurs [2], [3], [8], [17]. Interestingly, recent studies of extant squamates have also shown that the stereotypically pleurodont mode of tooth implantations in Varanus and Iguana also consist of cementum and alveolar bone, albeit in different arrangements from those associated with a thecodont tooth implantation [7], [41]. These findings raise the possibility that the ability to produce the three periodontal tissues may be a synapomorphy of Amniota, or at least is a plesiomorphy for all amniote taxa that have been examined thus far.…”

“…Whereas many amniote taxa do not exhibit thecodonty, or a ligamentous tooth attachment (e.g. extant lepidosaurs), the alternate forms of tooth implantation are highly derived arrangements of alveolar bone and cementum [16], [41], rather than being plesiomorphic, as previously suggested [15]. The ways in which tooth implantations are defined for amniotes must be changed in order to reflect the homologies of the periodontal tissues [2], [3].…”

Precise occlusion and trophic niche differentiation indicate specialized feeding in Early Devonian jawed vertebrates

“…Plicidentine is currently unknown in ischnacanthiforms, and a histological analysis of the teeth would be required to determine whether these ridges are the result of infolding of the dentine in Euryacanthus. Plicidentine has been hypothesized to strengthen the attachment of the teeth to the tooth-bearing element of the jaw, particularly in active predators (Scanlon and Lee 2002;Modesto and Reisz 2008;Maxwell et al 2011aMaxwell et al , 2011bMeunier et al 2014). Euryacanthus rugosus was also one of the largest ischnacanthiform acanthodians from the Early Devonian MOTH locality, although compared with ischnacanthiforms from other localities and ages, it was a small fish.…”