Histological and developmental insights into the herbivorous dentition of tapinocephalid therapsids

Abstract

Tapinocephalids were one of the earliest therapsid clades to evolve herbivory. In acquiring derived tooth-to-tooth occlusion by means of an exaggerated heel and talon crown morphology, members of this family have long been considered herbivorous, yet little work has been done to describe their dentition. Given the early occurrence of this clade and their acquisition of a dentition with several derived features, tapinocephalids serve as an important clade in understanding adaptations to herbivory as well as macroevolutionary patterns of dental trait acquisition. Here we describe the histology of tapinocephalid jaws and incisors to assess adaptations to herbivory. Our results yield new dental characters for tapinocephalids including a peculiar enamel structure and reduced enamel deposition on the occlusal surface. These traits are convergent with other specialized herbivorous dentitions like those found in ornithischian dinosaurs and ungulates. Furthermore, these results demonstrate that while acquiring some specializations, tapinocephalids also retained plesiomorphic traits like alternate, continuous replacement. We interpret these findings as an example of how different combinations of traits can facilitate a derived and specialized dentition and then discuss their implications in the acquisition of a mammal-like dentition.

Convergent dental adaptations in the serrations of hypercarnivorous synapsids and dinosaurs

“…This unexpected tissue arrangement highlights the oftencryptic complexity of many non-mammalian synapsid teeth. Mammalian dentitions are often regarded as complex and highly specialized [49][50][51], but continued histological research into fossil non-mammalian dentitions is revealing novel patterns in synapsid dental evolution [24,42,52,53]. Here, we have demonstrated that some non-mammalian synapsids evolved dental morphologies that may not be present in extant mammals and were previously unknown from the synapsid lineage.…”

“…The reduced number of denticles and their irregular sizes in Dimetrodon may have limited the strength and/or durability of an individual tooth or serration. However, Dimetrodon and other sphenacodontid 'pelycosaurs' underwent remarkably frequent tooth replacement [41,42] and this perhaps reduced the selective pressures on any given individual tooth to build durable serrations. While this rapid replacement is similar to theropod dinosaurs [43], this is in contrast to gorgonopsians, which did replace their teeth multiple times, but at a reduced rate and perhaps determinately [41,42,44] (table 1).…”

“…However, Dimetrodon and other sphenacodontid 'pelycosaurs' underwent remarkably frequent tooth replacement [41,42] and this perhaps reduced the selective pressures on any given individual tooth to build durable serrations. While this rapid replacement is similar to theropod dinosaurs [43], this is in contrast to gorgonopsians, which did replace their teeth multiple times, but at a reduced rate and perhaps determinately [41,42,44] (table 1). The convergent sabre-toothed morphologies in gorgonopsians and S. fatalis are superficially similar; however, the tissue compositions of their respective serrations are completely different.…”

Thecodont tooth attachment and replacement in bolosaurid parareptiles

“…Bolosaurids share the same implantation and attachment features with early Permian diadectids (LeBlanc & Reisz, 2013). Many other middle-tolate Permian amniotes, particularly therapsids, show similar thecodont implantation to bolosaurids, but the surrounding soft tissues do not mineralize, leaving a periodontal ligament space between the tooth root and the alveolar bone (Rybczynski & Reisz, 2001;LeBlanc et al, 2018;Whitney & Sidor, 2020). In bolosaurids, the teeth are firmly attached to the jawbone through the presence of alveolar bone.…”

“…The first occurs in modern mammals and crocodilians, wherein a replacement tooth develops from a dental lamina that is dissociated from the oral epithelium and is buried deep within the jaw, almost underneath the functional teeth (Gordon Edmund, Edmund & Museum, 1960). The second replacement type is found in most other thecodont amniotes, where the dental lamina is still located along the lingual margins of the jaws, an ancestral feature for amniotes (Gordon Edmund, Edmund & Museum, 1960;LeBlanc et al, 2017;Leblanc, Mohr & Caldwell, 2019;Whitney & Sidor, 2020). Because new teeth still form far away from the pulp cavity of the preceding functional teeth, they first form resorption pits along the lingual surfaces of the teeth that are visible externally in fossils and skeletons.…”