Caribbean Reef Shark

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Upper and lower teeth of the bluntnose, six-gill shark (Hexanchus griseus), showing contrasting tooth shapes
Courtesy of D Ross Robertson, Shorefishes of the Tropical Eastern Pacific Online , CC-BY-NC-SA

About Sharks (Infraclass Selachii): Teeth

Sharks have a reputation for dangerous, killing teeth. While most sharks are not dangerous to humans, they do have a lot of teeth. A working row of teeth in the front is followed by other replacement rows behind. The teeth are formed in skin that gets pushed forward, like a conveyor belt, bringing a new tooth or row of teeth. As many as 30,000 teeth may be replaced in a shark's lifetime. New teeth are not only less worn, but tend to be larger, keeping pace with the shark's growth. Shark teeth come in a variety of shapes, adapted to a shark’s diet. Often, all that remains of sharks in the fossil record is teeth, since they do not have calcified skeletons. Fortunately, the tooth shape helps paleontologists identify fossil sharks. The oldest fossil shark teeth are from the Devonian (about 400 million years ago).

When a zebra shark (Stegostoma fasciatum) stops swimming, it sinks to the bottom
Courtesy of Morningdew, via Wikimedia Commons, CC-BY-SA

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About Sharks (Infraclass Selachii): Skeleton

Sharks have a skeleton made of light-weight cartilage, rather than calcified bone. When they swim, by beating their tail fin back and forth, the flexible skeleton allows their body to bend. Because they lack internal sacks (gas bladders) for buoyancy, they must swim constantly to stay afloat. Sharks use their stiff, pectoral fins to provide lift, like airplane wings. It turns out that sharks also have a fluid-based (hydrostatic) skeleton. Muscles attach directly to their skin, and the skin acts like the outside of a balloon, withstanding high pressures as muscles contract. The pressure in a swimming shark can rise as high as twice that in a full car tire. Crisscrossing layers of the protein collagen make shark skin tough and rigid to withstand the pressure. Sharks have been equated with swimming balloons.