Fractal Dimension of Dental Crowns: An Innovative Method for Age Estimation in Toxodonts

Toxodonts constitute a group of extinct native South American ungulates that have been subject of extensive paleobiological research. Among the traits analyzed, enamel crest complexity (ECC; through fractal dimension) and occlusal surface tooth area (OTA) have recently been studied. However, they have not yet been applied to evaluate differences between ontogenetic stages, (e.g., juveniles, subadults, and adults). Here, we analyzed these traits in the p2-m3 of three species of Toxodontidae, Adinotherium ovinum (n = 8), Nesodon imbricatus (n = 11), and Toxodon platensis (n = 19), along ontogeny by standardized major axis regressions using OTA of m1 as an age proxy, and compared adult and young individuals (i.e., m3 not fully erupted) by Kruskal-Wallis test. Our results showed that ECC decreases with age in all teeth, and p2-m1 seems to share a common slope for Adinotherium and Nesodon. Kruskal-Wallis test showed significant differences between age and species in p3 and molars, with young individuals having higher ECC values than adults. There is a decreasing trend in ECC values from A. ovinum (highest), through N. imbricatus (intermediate), to T. platensis (lowest). These differences are more pronounced from m1 to m3. We conclude that both ECC and OTA can serve as effective tools for differentiating adult toxodonts from those that have not yet reached adulthood. The lower ECC in adult molars may be due to crown simplification during ontogeny, as teeth wear over time by abrasion from food particles during chewing. Conversely, OTA increases in adults as a mixed effect of wear and the larger size of their teeth compared to younger individuals.