Rewinding developmental trajectories shows how bears break a developmental rule

Mammals have evolved a broad variety of dental morphologies. Nevertheless, the development of the mammalian dentition is considered highly conserved. Molar size proportions exemplify this as a system where small changes in shared developmental mechanisms yield a defined range of morphological outcomes. The Inhibitory Cascade (IC) model states that as molars develop in a sequence, the first developing anterior molars inhibit the development of subsequent posterior ones. The IC model thus predicts a trend of linear tooth size change along the molar row, as has been observed in a wide range of mammalian taxa with otherwise differing dental morphologies. Perhaps the starkest exceptions to the IC rule are bears, in which the second molar is the largest and the third one is disproportionally small. Here we sought to calculate when and how during development, the bear dentition moves away from the IC prediction. We examined molar proportions in eight bear species, and estimated tooth sizes during development. The results indicate that development of bear molars already deviates from IC expectation during patterning. However, during the earlier cap stage, size proportions of bear molars still seem to adhere to the IC model predictions. Overall, these analyses suggest that irrespective of the final outcome, the process of initial splitting of the molar-forming region into individual teeth is conserved and follows the IC rule.