Mineralized periodontia in extinct relatives of mammals shed light on the evolutionary history of mineral homeostasis in periodontal tissue maintenance.

AIM: Dental ankylosis is a rare pathological condition in mammals, however, it is prevalent in their extinct relatives, the stem mammals. This study seeks to compare the mineralized state of the periodontal attachment apparatus between stem and crown mammals and discuss its implications for the evolution of non-mineralized periodontal attachment in crown mammals, including humans. MATERIALS AND METHODS: Thin sections of a fossil mammal and three stem mammals were compared to reconstruct periodontal tissue development across distantly related lineages. RESULTS: Comparisons revealed that the extinct relatives of mammals possessed the same periodontal tissues as those in mammals, albeit in different arrangements. The ankylotic condition in stem mammals was achieved through extensive alveolar bone deposition, which eventually contacted the root cementum, thus forming a calcified periodontal ligament. CONCLUSIONS: Dental ankylosis was part of the normal development of the stem mammal periodontium for millions of years prior to the evolution of a permanent gomphosis in mammals. Mammals may have evolved a permanent gomphosis by delaying the processes that produced dental ankylosis in stem mammals. Pathological ankylosis may represent a reversion to the ancestral condition, which now only forms via advanced ageing and pathology.