Ontogeny of Enamel Thickness in the Teeth of Alligator and its Significance for Crocodyliform Dental Evolution

Abstract

Enamel is a key dental tissue responsible for mediating forces that are incurred through biting and feeding. Thus, disparity in enamel thickness among individual teeth as well as among species should provide important information on diet and feeding behavior. Crocodylians, including Alligator mississippiensis, evolved the highest bite forces among vertebrates, and thus their teeth likely also have adaptions to accommodate these extreme behaviors and loads. However, little is known about the ontogenetic and phylogenetic patterns of enamel thicknesses in the teeth of alligators, crocodyliforms and other sauropsids. Positional and ontogenetic changes in enamel thickness in alligators may reflect feeding ecology, as thicker enamel may be needed to resist higher bite forces. Moreover, craniodental diversity among extinct crocodyliforms suggests that enamel thickness may offer key ecomorphological insights into their evolution. Teeth and alveoli from three mandibular positions were sampled from an ontogenetic series of alligators and microCT scanned. Slice data and 3D models were used to measure the enamel thickness, enamel and dentin volume, and surface area of the enamel‐dentin junction. These data were complemented by a sample of molariform teeth from fossil crocodyliforms including the durophagous, Late Cretaceous alligatoroid Brachychampsa, its early Cenozoic relative Allognathosuchus, the herbivorous, Late Cretaceous Iharkutosuchus and several early Jurassic protosuchian taxa in order to assess variation and potential for ecological signal in the clade. We found that the most anterior teeth maintain the thinnest enamel compared to middle and posterior (molariform) positions throughout alligator ontogeny. Enamel thickness in all teeth scaled with isometry throughout alligator ontogeny. Comparisons of the molars of fossil crocodyliforms with those from alligators reveal a diversity of enamel thickness and complexity, suggesting phylogenetic as well as ecological signals may be impacting tooth structure, function, and evolution, warranting further investigation into the system.Support or Funding InformationThis work was funded by the Missouri Research Board, Life Sciences Undergraduate Research Opportunities Program, and the Biology Honors Program.