Sharks do not have bones; their skeletons are composed entirely of cartilage. This places them in a specific group of fish known as elasmobranchs, a classification they share with rays and skates. This fundamental difference in skeletal material separates them from bony fishes, which belong to the class Osteichthyes.
The Shark’s Cartilaginous Skeleton
Cartilage is a strong, flexible connective tissue, the same material that forms a human’s nose and ears. It is significantly different from bone, being less dense and more pliable. This composition provides sharks with a lightweight framework. While the entire skeleton is cartilaginous, it is not uniform in its consistency throughout the shark’s body.
Certain parts of the skeleton that require greater strength, such as the spine and jaws, undergo reinforcement. This process, known as calcification, involves depositing calcium salts into the cartilage matrix. This makes these specific areas harder and more rigid, giving them characteristics similar to bone but without the added weight. Even with this reinforcement, these structures are considered calcified cartilage, not true bone.
Advantages of a Cartilage Skeleton
The cartilaginous nature of a shark’s skeleton offers functional benefits. A primary advantage is its low density. Cartilage is about half as dense as bone, which provides sharks with natural buoyancy. Because sharks lack a gas-filled swim bladder to control their position in the water column, the lighter skeleton helps them conserve energy and stay afloat.
The flexibility of cartilage is another significant benefit, contributing to a shark’s agility as a predator. This pliability allows them to bend their bodies for sharp turns and is integral to their side-to-side swimming motion, enabling them to move efficiently while hunting. The structure even allows some species to open their mouths wider, as the upper jaw is not fused to the skull.
Shark Fossils and Teeth
The composition of a shark’s skeleton directly impacts what is left behind in the fossil record. Because cartilage is soft and decomposes relatively quickly, complete fossilized shark skeletons are exceptionally rare. Consequently, most shark fossils discovered by paleontologists are not skeletal remains, but their teeth.
Shark teeth are made of dentin, a hard, calcified tissue that is covered in enamel, making them much more durable than cartilage. This composition allows them to fossilize readily, providing a rich and detailed record of ancient shark species. On rare occasions, the more heavily calcified parts of a shark’s skeleton, such as vertebrae or pieces of the jaw, can also survive the fossilization process.