The microstructure of reptilian tooth enamel

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A survey of the enamel microstructure in 43 reptilian taxa reveals significant structural variety and complexity. Notably, all taxa, except the agamid lizard Uromastyx, possess enamel without prisms, distinguishing reptilian enamel from that of mammals. To address this, a new terminology was developed, akin to that used for mammalian enamel, which is hierarchical and devoid of amelogenetic or phylogenetic implications. This terminology encompasses five complexity levels: crystallite, module, enamel type, schmelzmuster, and dentition levels. Detailed analysis shows that phylogenetic constraints are minimal, with only advanced ornithopod dinosaurs and Ichthyosauria exhibiting unique enamel synapomorphies. Biomechanical constraints on enamel structure are also challenging to identify. The primary determinant of microstructure appears to be enamel surface morphology, characterized by ridges and wrinkles that likely serve adaptive functions. Reptilian amelogenesis is responsible for this morphology, as the enamel-dentin junction is smooth. The concept of amelogenesis as enamel surface morphogenesis is exemplified by durophagous reptiles, where unrelated groups converge on a bulbous tooth shape yet display distinct enamel microstructures. Other ecomorphotypes, like ziphodont carnivores, also show convergent evolution in enamel microstructure. These findings prompt critical questions regarding reptilian amelogenesis, highlighting the