It is impossible to make full evolutionary sense of the vertebrates on the basis of those species that are alive today. is is because the structural dierences between the major classes of vertebrate are profound, and there are no contemporary intermediates. Elucidating the history of eukaryotic evolution has thus depended heavily on the remarkable fossil record. However, because soft tissues rarely lasted long enough to be fossilized, this evidence comes almost entirely from their fossilized bones and teeth, tissues that were originally hardened by the presence of hydroxyapatite. Teeth also contain dentine and enamel, the latter being ~96% mineral and the hardest substance made by chordates.