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Carcharhiniformes
Heterodontiformes
Hexanchiformes
Lamniformes
Orectolobiformes
Pristiophoriformes
Squaliformes
Squatiniformes
† Symmoriida
† Cladoselachiformes
† Xenacanthida (Xenacantiformes)
† Iniopterygia
† Eugeneodontida
† Hybodontiformes The bull shark, Carcharhinus leucas, is the best known of several species that swim in both seawater and freshwater, as well as in deltas.[2] The skeleton of a shark is very different from that of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made from cartilage but they are still considered bones. They function in the same way as human bones do. Like its relatives, rays and skates, the shark's jaw is not attached to the cranium. The jaw's surface, like its vertebrae and gill arches, is a skeletal element that needs extra support due to its heavier exposure to physical stress and its need for extra strength. It has therefore a layer of unique and tiny hexagonal plates called "tesserae", crystal blocks of calcium salts arranged as a mosaic.[3] This gives these areas much of the same strength found in real and much heavier bony tissue. Generally there is only one layer of tesserae in sharks, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have been found to be covered with two to three layers or more, depending on the body size. The jaws of a large white shark may even have up to five layers.
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