Tooth histology, attachment, and replacement in the Ichthyopterygia reviewed in an evolutionary context

Abstract

Ichthyosaurs, an extinct group of Mesozoic marine diapsids, show a relatively small range of tooth crown morphologies. With few exceptions, members of the group bear a large number of conical teeth and show only minor heterodonty within a jaw. This uniformity in gross morphology masks a high degree of variation in both the quantity and arrangement of the mineralized tooth tissues. Here, we describe tooth tissue structure and distribution in derived ichthyosaurs. We synthesize these new observations with the historical literature, to map changes in the quantity and arrangement of tooth tissues. These changes affected tooth attachment, tooth replacement, plicidentine morphology, and the amount and distribution of cellular cementum. The amount of variation detected in features relating to ichthyosaurian dentition is not surprising given the geological longevity and morphological disparity of the group, but does emphasize the importance of extensive taxon sampling in studies of tooth histology and evolution. This study is important in that it incorporates morphological and histological information in a phylogenetic and developmental context, something that is rarely done for marine reptile dentitions.

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

“…(Figure 7A, 7B). This striking similarity between the cellular cementum of mammals and non-mammalian amniotes [8], [11], [24] and the incrementally banded layer of diadectids allow us to identify the latter as cellular cementum. The diadectid cellular cementum has a second layer external to the incremental bands that does not possess any clear incremental banding, and is characterized by high densities of Sharpey's fibers that are oriented nearly perpendicular to the apical-occlusal axis of the tooth root in longitudinal section (Figure 7C).…”

“…From a phylogenetic perspective; however, these three categories should not be used as separate character states because they are the results of the interplay between different amounts and arrangements of periodontal tissues that can vary significantly, even within a single jaw [8], [11], [34]. Instead, interpretations of amniote tooth implantation must consider the identities of the tissues that form the periodontium, and their histological properties [3], [8], [24], [34]. This new approach can then lead to testable hypotheses of homology.…”

“…More recent paleontological [2], [7], [8], [17]–[20] and developmental [16], [21]–[23] studies of amniotes have suggested that cementum, alveolar bone, and the periodontal ligament are present in some capacity even in non-mammalian and non-crocodilian amniotes (Figure 2A). Much of this recent work in dental histology, however, has been limited to studies of mammals, squamates [3], [7], extant crocodilians [11], [21], and ichthyosaurs [8], [24], and has not addressed the issue of homology of these tissues at the level of Amniota.…”

Mosasaurs and snakes have a periodontal ligament: timing and extent of calcification, not tissue complexity, determines tooth attachment mode in reptiles

“…Cementum, alveolar bone, and even a PDL have also been identified in numerous Paleozoic and Mesozoic amniotes (Maxwell et al. ; LeBlanc & Reisz, ; Pretto et al. ; Sassoon et al.…”

“…), (4) ichthyosaurs and pliosaurids (Maxwell et al. ; Sassoon et al. ), (5) non‐avian dinosaurs and toothed birds (Dumont et al.…”