Why Are Sharks Not Bony?

it has been thought, for years, that sharks have cartilaginous skeletons because the group evolved before bony support structures evolved in other vertebrates. One of the reasons for this belief is that fossils of the extinct group of Placoderm fish were covered in bone scales and had ossified, jaws but appeared to have solely cartilaginous internal skeletons (like sharks and rays). Placoderms lived before bony fish evolved. Studies from Imperial College, London, have now shown, however, that a 410 m year old fossil Placoderm from Mongolia (Minjinia turgenensis), apparently had a bony cranium. Perhaps sharks elected to dispense with bony skeletons, possibly due to weight savings that facilitated speed. A bony skeleton also takes a lot of calcium phosphate.