Nanoscale crystal orientations in tooth enamel in monsters of the past and their modern counterparts*

Teeth and especially the outer enamel(oid) layer are adapted to the particular feeding habits and wear of the animals forming them. Where tooth shape is ambiguous, enamel microstructure is used to infer whether or not a fossilized tooth belonged to a predator. However, most of the existing literature focuses on the physical elongation of enamel nanocrystals, and not on their crystalline orientations, which have been shown to a) not always correlate with the elongation direction of the nanocrystals (https://doi.org/10.1038/s41467-019-12185-7) and b) correlate with enhanced hardness and stiffness (https://doi.org/10.1016/j.actbio.2020.07.037). We used PIC (polarization-dependent imaging contrast) mapping at the calcium L-edge (https://doi.org/10.1021/jacs.8b05547) to reveal the crystal orientations of enamel(oid) crystals in monsters of the past, including T. rex, megalodon and ancient and modern sharks and bears. Comparison of PIC from these different top predators indicates that the crystal orientation patterns are well preserved over evolutionary time, and that they correlate with function.