LIFE STAGE PREDICTION IN ALLIGATOR MISSISSIPPIENSIS AND CROCODYLUS ACUTUS USING ALVEOLAR GROWTH

Estimating life stages of fossil specimens is critical to many paleobiological questions. However, the osteohistological techniques used in aging studies are necessarily destructive, limiting their use in many circumstances. Here we propose a non-destructive technique based on crocodylian dental developmental patterns. Crocodylian teeth initially develop in a trough lacking discrete alveoli. During odontogenesis, sockets form iteratively by growth of alveolar bone (AB), which expands with each tooth cycle. Once mature, tooth replacement occurs at regular intervals, and AB thickness changes concurrently. We hypothesize that these two factors are reliable proxies for crocodylian life stage periodicity. We digitally reconstructed µCT data of the skulls and jaws of hatchling, yearling, juvenile, and adult American alligators (Alligator mississipiensis) and crocodiles (Crocodylus acutus), capturing two characteristic dental morphologies in extant crocodylians—overbite and interdigitation of the tooth rows. Completed alveoli were counted on left dentaries, and AB / non-AB were segmented separately in the jaw. AB thickness of the most procumbent caniniform and molariform teeth was measured in transverse view. Alveolar completion and AB thickness were then tested as predictors of life stage using Kruskal-Wallis tests and linear discriminant analysis (LDA).

The Kruskal-Wallis tests returned a significant relationship (p < 0.05) between the number of completed alveoli and life stages in both species, with alveolar completion occurring distally across ontogeny. Combining alveoli counts and AB thicknesses also returned significant (p < 0.05) results for each species. The LDA for the combined data resulted in an average correct classification of over 80% for each species, greater than alveolar count-only dataset. These preliminary results suggest that alveolar bone growth is informative to the periodicity of tooth replacement and that alveolar completion is a reliable indicator of life stage in both Alligator and Crocodylus—regardless of dental arrangement. These results indicate that alveolar bone traits hold potential as non-destructive crocodylian aging metrics that can provide crucial ecologically-informative data to address paleontological questions in thecodont archosaurs.