Replacement tooth in mesosaurs and new data on dental microanatomy and microstructure.

The Permian mesosaurs are well known for being the earliest amniotes to exhibit adaptations for living in a marine environment (Irati-Whitehill Sea). In addition to their set of skeletal features associated with aquatic dwelling life, their dentition includes important characteristics related to feeding in this habitat, which is described in this work, based on the analysis of mesosaur specimens from the Lower Permian Irati Formation of Brazil. Mesosaurs have several slender, conical teeth bordered by enamel apicobasal ridges, a feature predominantly found in aquatic amniotes. Internally, the dentine walls are formed by the arrangement of layers of orthodentine and globular dentine. To prevent tooth loss, the basal area is equipped with plicidentine, a particular type of orthodentine, allied with cementum, alveolar bone trabeculae, and periodontal space that reinforces anchorage and provides some flexibility. The teeth are replaced in a labio-vertical path, and the dentition replaces alternately. This feature is regarded as plesiomorphic, and it ensures the oral cavity is supplied with enough teeth. However, these features do not the assessment of whether mesosaurs teeth were capable of piercing prey with resistant tegument. Instead, we interpret this adaptation as a mechanism for catching prey, at least in adults, and we endorse a possible ontogenetic dietary shift from small to large forms.