What material forms most of a shark's skeleton?

Most of a shark’s skeleton is made of cartilage, not bone.

Sharks are cartilaginous fish, grouped with rays, skates, and chimaeras in the class Chondrichthyes. Their internal skeleton is built primarily from cartilage, a flexible and durable connective tissue that is less dense than bone. This lighter framework reduces the energy needed for swimming.

Cartilage does not mean sharks lack hard structures. Their jaws and vertebral elements can be strengthened by mineral deposits, and their skin is covered with tooth-like dermal denticles. Their teeth are also mineralized and are continuously replaced, even though the central skeleton remains cartilaginous.

The phrase “most of the skeleton” is scientifically safe because shark skeletons are not simply soft or flimsy. Their cartilage provides support while retaining flexibility. Unlike bony fish, sharks also lack a rib cage that would support the body out of water, so a stranded shark can be injured by its own weight. Fossil shark skeletons are therefore rare; teeth and dermal denticles preserve much more readily.