shark bones and their relatives don’t have skeletons made of bones, like most other vertebrates. Their skeletons are made of cartilage—the same flexible connective tissue that lends shape to your nose and ears and provides structural support throughout the body. This is one of the key things that distinguishes sharks and their cousins (called class Chondrichthyes) from other fish, which belong to class Osteichthyes.
A shark’s skeleton is comprised of various kinds of cartilage—some areas are thicker or more dense than others. Those parts of the shark that need more strength, such as the spinal column, are made of calcified cartilage—a form of cartilage that has been bonded with calcium salts and becomes much tougher than non-calcified cartilage.
In addition to being more durable, calcified cartilage allows a shark to conserve energy by moving less. Unlike bone, which is rigid, a shark’s cartilage is flexible and buoyant, so it can move more quickly and easily in the water.
Another benefit of the flexible cartilage is that it makes sharks’ jaws more extendable. This helps them to gulp down large chunks of food and also lets them use their powerful jaws more efficiently when attacking prey.
A shark’s cartilage also enables it to heal faster than other animals. Bones can take a long time to heal, but cartilage can quickly repair itself.