Enamel histology in extinct mammals: Standardization of a methodological framework applied to toxodont notoungulates (Mammalia)

Enamel histology may provide valuable insights into dental growth and life history traits. However, this method presents significant challenges for extinct species, particularly as the periodicity of the increments observed cannot be tested experimentally. Here, we propose a standardized methodological framework to formalize what has been done in the literature and help researchers working with fossil taxa. We applied this framework to a fossil dataset composed of 19 molars belonging to four species of Toxodontia (Notoungulata, Mammalia) - an extinct sub-order of South American notoungulates known for their specialized dental characteristics (crown height increase and change in eruption pattern) - with different crown height and age: the brachyodont species Pleurostylodon modicus from the Casamayor sub-unit of the Sarmiento Formation (Eocene; Argentina), and the three hypsodont species Eurygenium pacegnum from Salla (Late Oligocene; Bolivia), Adinotherium ovinum and Nesodon imbricatus from the Santa Cruz Formation (Middle Miocene; Argentina). Our protocol starts with the identification of Retzius lines as increments reaching the tooth surface and forming perikyma grooves. Retzius lines were often observed in restricted areas of the cervical region and/or outer enamel, as previously reported in the literature and confirmed in Toxodontia. Increments between successive Retzius lines were identified as laminations, corresponding to daily features. Lastly, although rare, sub-daily features in the form of cross-striations or fine laminations were also observed. The daily secretion rate (DSR) ranged from 6 to 22 μm/day, while the enamel extension rate (EER) ranged from 13 to 376 μm/day and the enamel formation front angle (EFFa) varies from 3 to 30°. There were however great disparities, especially depending on species and on crown region. In particular, the EER decreased drastically (and the EFFa increased) between the occlusal and the cervical regions. This decrease was less marked from the middle to the cervical region in more recent toxodonts (Adinotherium and Nesodon), suggesting a cruising speed in tooth growth probably associated with hypsodonty and a longer crown formation time. To investigate interspecific differences, we thus focused on the middle region of the crown. We noted lower values of DSR and EER as well as higher values of EFFa in Pleurostylodon, the smallest, oldest and only brachyodont species of the dataset, compared with the other taxa. However, as our sample is limited taxonomically, we could not clearly distinguish between the relative roles of hypsodonty, body mass, and phylogeny on the histological parameters. This work paves the way for a robust study of a larger dataset of Notoungulata to explore their tooth growth and life history along their repeated evolution of dental specializations.