What material makes up a shark's skeleton?

A shark's skeleton is made mainly of cartilage, not bone. Sharks are cartilaginous fish in the class Chondrichthyes, a group that also includes rays and skates.

Cartilage is the flexible, lighter connective tissue found in places such as the human nose and joints. In sharks, it forms the internal skeleton, including the jaws and supporting structures. Shark cartilage is not completely soft: much of it is strengthened by tiny mineralized structures called tesserae, which provide support while preserving flexibility.

This skeleton helps sharks remain relatively light and maneuverable in the water. It is also different from the skeleton of bony fish, whose internal framework is largely mineralized bone. Another common mistake is assuming that shark teeth are cartilage because the skeleton is; teeth are separate structures made largely of dentine and enamel. Sharks continuously replace their teeth, with new rows moving forward as older teeth are lost. Their cartilaginous skeleton is a defining evolutionary feature, but it does not mean every part of the animal is cartilage.