Multiple prismatic calcium phosphate layers in the jaws of present-day sharks (Chondrichthyes; Selachii)

Summary Jaws of large individuals, over 2 m in total length, of the shark speciesCarcharodon carcharias (great white shark) andIsurus oxyrinchus (mako shark) of the family Lamnidae, andGaleocerdo cuvieri (tiger shark) andCarcharhinus leucas (bull shark) of the family Carcharhinidae were found to have multiple, up to five, layers of prismatic calcium phosphate surrounding the cartilages. Smaller individuals of these species and other known species of living chondrichthyans have only one layer of prismatic calcium phosphate surrounding the cartilages, as also do most species of fossil chondrichthyans. Two exceptions are the fossil shark generaXenacanthus andTamiobatis. Where it is found in living forms, this multiple layered calcification does not appear to be phylogenetic, as it appears to be lacking in other lamnid and carcharhinid genera and species. Rather it appears to be functional, only appearing in larger individuals and species of these two groups, and hence may be necessary to strengthen the jaw cartilages of such individuals for biting.