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Tooth plates of the chimaeroids, holocephalian fishes, are unique dental hard tissues. Unlike the teeth of other animals, the tooth plates are located on the roof of the mouth and in the lower jaw. Their tooth plates consist, to a large extent, of lightly mineralized tissue (osteodentin) and hypermineralized tissue (pleromin). Notably, the mineral phase of pleromin is whitlockite, while that of other animals is apatite. Dietary habits of chimaeroids and wearing features of their tooth plates suggest an extreme hardness of pleromin, but this has never been investigated. We examined the microhardness of the tooth plate of  Chimaera phantasma  and found that pleromin in the biting region was extremely hard, comparable with the hardness of mature tooth enamel, whereas the hardness of immature pleromin was lower than that of bovine dentin. The hardness of osteodentin, on the other hand, was equivalent to that of bovine dentin and almost the same throughout the tooth plate. Immature pleromin was sparsely packed with oval crystals of whitlockite and, as pleromin matures, the space between crystals was filled with small intercrystalline materials. The maturing process of pleromin could partly contribute to its remarkable hardness and have some implications for designing novel biomaterials.

A unique mineralization mode of hypermineralized pleromin in the tooth plate of Chimaera phantasma contributes to its microhardness