Thecodont tooth attachment and replacement in bolosaurid parareptiles

Adam J Snyder; Aaron R H LeBlanc; Chen Jun; Joseph J Bevitt; Robert R Reisz

doi:10.7717/peerj.9168

Thecodont tooth attachment and replacement in bolosaurid parareptiles

PeerJ. 2020 May 13:8:e9168. doi: 10.7717/peerj.9168. eCollection 2020.

Authors

Adam J Snyder¹, Aaron R H LeBlanc², Chen Jun^{3

4}, Joseph J Bevitt⁵, Robert R Reisz^{1

3}

Affiliations

¹ Department of Biology, University of Toronto Mississauga, Mississauga, Ontario, Canada.
² Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada.
³ International Center of Future Science, Dinosaur Evolution Research Center, Jilin University, Changchun, China.
⁴ Lab for Evolution of Past Life and Environment in Northeast Asia, Jilin University, Ministry of Education, Changchun, China.
⁵ Australian Centre for Neutron Scattering, Sydney, Australia.

Abstract

Permian bolosaurid parareptiles are well-known for having complex tooth crowns and complete tooth rows in the jaws, in contrast to the comparatively simple teeth and frequent replacement gaps in all other Paleozoic amniotes. Analysis of the specialized dentition of the bolosaurid parareptiles Bolosaurus from North America and Belebey from Russia, utilizing a combination of histological and tomographic data, reveals unusual patterns of tooth development and replacement. The data confirm that bolosaurid teeth have thecodont implantation with deep roots, the oldest known such example among amniotes, and independently evolved among much younger archosauromorphs (including dinosaurs and crocodilians) and among synapsids (including mammals). High-resolution CT scans were able to detect the density boundary between the alveolar bone and the jawbone, as confirmed by histology, and revealed the location and size of developing replacement teeth in the pulp cavity of functional teeth. Evidence provided by the paratype dentary of Belebey chengi indicates that replacement teeth are present along the whole tooth row at slightly different stages of development, with the ontogenetically more developed teeth anteriorly, suggesting that tooth replacement was highly synchronized. CT data also show tooth replacement is directly related to the presence of lingual pits in the jaw, and that migration of tooth buds occurs initially close to these resorption pits to a position immediately below the functional tooth within its pulp cavity. The size and complex shape of the replacement teeth in the holotype of Bolosaurus grandis indicate that the replacement teeth can develop within the pulp cavity to an advanced stage while the previous generation remains functional for an extended time, reminiscent of the condition seen in other amniotes with occluding dentitions, including mammals.

Keywords: Beleby; Bolosaurus; CT; Dentary; Histology; Parareptile; Thecodont; Tooth attachment; Tooth replacement.

Grants and funding

This work was made possible by a Natural Sciences and Engineering Research Council Discovery Grant (No. 482696) to Robert R. Reisz. The costs of CT scanning IVPP specimens were covered by Jilin University. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.