Sharks defy the common belief that all vertebrates possess a skeleton made of bone. The entire skeleton of a shark, including its jaws, is composed of cartilage. Sharks are classified as cartilaginous fish, or Chondrichthyes, meaning their structural framework is built from the same flexible connective tissue found in the human nose and ears. This difference from bony fish (Osteichthyes) impacts everything from a shark’s buoyancy to the mechanics of its powerful bite.
The Cartilaginous Skeleton
Shark cartilage is durable, flexible, and only about half as dense as true bone. Unlike bone, which is highly vascularized, shark cartilage is non-vascular and lacks blood vessels. This prevents the continuous remodeling process characteristic of bone in other vertebrates.
To achieve necessary rigidity, the skeleton is strengthened through mineralization or calcification. In high-stress areas like the jaws and vertebrae, calcium salts are deposited into the cartilage matrix. This creates calcified cartilage, which is similar in strength to bone but retains the lightweight nature of cartilage.
The jaw cartilage surface is reinforced with tiny, interlocking hexagonal plates called tesserae, which are crystal blocks of calcium salts. Larger species, like the great white shark, may have multiple layers—up to five in some specimens—to handle extreme biting forces. These calcified plates provide structural support without converting the tissue into true bone.
How the Jaw Operates
The shark’s jaw mechanics are unique because the upper jaw is not fused to the cranium, a setup known as a hyostylic suspension. The jaws are loosely connected to the skull by ligaments and muscles, allowing for high mobility enabled by the flexible cartilaginous structure.
This flexibility permits most modern sharks to protract, or thrust, their entire jaw apparatus forward when attacking prey. The contraction of specialized muscles, such as the preorbitalis muscle, forces the upper jaw to project, resulting in a wider gape and a more effective grip. This action is coordinated with the downward movement of the lower jaw, maximizing the bite surface area.
The teeth are not rooted in the jaw cartilage but are embedded in soft connective tissue. They are arranged in multiple rows that function like a conveyor belt, continuously developing and moving forward to replace damaged teeth. This replacement system, which can replace a lost tooth in as little as 24 hours in some species, ensures the shark always has a sharp, functional set of teeth.
The Evolutionary Advantage of Cartilage
The cartilaginous skeleton is a successful evolutionary strategy for sharks, which have thrived for over 400 million years. The primary benefit is weight reduction, as cartilage is significantly less dense than bone. This lightweight structure helps sharks achieve near-neutral buoyancy because they lack a gas-filled swim bladder, the organ most bony fish use to regulate depth.
By minimizing skeletal mass, sharks reduce the muscular energy required to avoid sinking, making them efficient swimmers. The flexibility of the cartilage also contributes to their agility, allowing for quick bursts of speed and tight turns when pursuing prey. This combination of lightweight material and reinforced strength has allowed the Chondrichthyes class to remain apex predators.