Sharks, rays, and chimaeras belong to the class of fish known as Chondrichthyes. Their entire skeleton, including the jaws, is made of cartilage and connective tissue, not the true bone found in most other vertebrates. The absence of a bony skeleton means their skeletal structure is fundamentally different from the bony fish and mammals that make up the vast majority of vertebrates.
The Cartilaginous Foundation
The primary structural material in a shark’s jaw is hyaline cartilage, a flexible and durable tissue that is significantly less dense than bone. This cartilage must be strong enough to withstand the immense biting forces generated by large predatory sharks. This strength is achieved not through ossification, but through a unique reinforcement mechanism known as tessellation.
The cartilage is armored with a mosaic of minute, hexagonal plates called tesserae, composed of calcium phosphate hydroxyapatite crystals. These tesserae form a shell over the softer, unmineralized cartilage core. The plates are linked by organic fibers, creating a flexible yet remarkably strong composite structure that resists high cycle loading. Most sharks have a single layer of tesserae, but the jaws of larger, more powerful species, such as bull and great white sharks, can have multiple overlapping layers, sometimes up to five, for maximum support.
The reinforcement involves a layer of thousands of minute, hexagonal plates called tesserae that cover the cartilage. These tesserae are composed of calcium phosphate hydroxyapatite crystals, forming a mineralized shell over the softer, unmineralized cartilage core. The plates are typically hundreds of micrometers wide and are joined by organic fibers, creating a strong, composite material that resists fatigue and high mechanical stress. While most sharks have a single layer of these plates, the jaws of large, powerful predators like the great white shark can have up to five layers of tesserae, providing exceptional strength without true ossification.
Flexibility and Protrusion
The cartilaginous nature of the jaws enables unique movement mechanics, distinct from the fixed, fused jaws of many bony vertebrates. Shark jaws are not rigidly attached to the skull (chondrocranium). Instead, they are suspended by flexible ligaments and powerful muscles. This loose attachment allows the upper jaw (palatoquadrate) and the lower jaw (mandibular arch) a significant range of motion.
The most dramatic result of this flexible suspension is the ability of a shark to protrude its entire jaw forward and slightly downward during a strike. Specific muscles contract to force the upper jaw outward, momentarily detaching it from the skull to extend the animal’s reach and secure a better grip on prey. This protrusion occurs simultaneously with a rapid opening of the mouth, allowing the shark to adjust the angle of its bite and create a wider gape. The ligaments and muscles control this movement, allowing for both the forceful extension needed for a strike and the secure retraction afterward.
The cartilaginous structure of the jaws is not rigidly fixed to the skull, or chondrocranium, as it is in many other vertebrates. Instead, the upper jaw, known as the palatoquadrate, and the lower jaw, or mandibular arch, are suspended by a complex arrangement of flexible ligaments and powerful muscles. This loose attachment is fundamental to the shark’s feeding strategy, allowing for significant mobility.
The most notable movement is the ability of the shark to protrude its jaws outward and slightly downward during a predatory strike. Muscles such as the preorbitalis contract to thrust the upper jaw forward, momentarily separating it from the skull to increase the animal’s reach and secure a better grip on its prey. This jaw protrusion occurs simultaneously with the rapid opening of the mouth, enabling the shark to adjust its bite angle and maximize the surface area of its teeth during impact. The entire mechanism is an intricate interplay of soft tissue and reinforced cartilage, providing a balance of force and flexibility.
The Tooth Replacement System
The teeth are a non-skeletal component continuously attached to the jaw cartilage by connective tissue, rather than being set in sockets like human teeth. This attachment allows for a highly efficient system of continuous replacement. Sharks are polyphyodont, meaning they regenerate their entire dentition repeatedly throughout their lives.
New teeth develop in a continuous sheet of tissue called the dental lamina, located on the inner side of the jaw. These developing teeth are organized in rows, functioning like a “conveyor belt” system. As functional outer teeth are lost or damaged, the next tooth in the row moves forward to replace it. This constant regeneration ensures the shark always has a fresh set of sharp teeth ready for use, with some species losing and replacing thousands of teeth over a lifetime.
An important functional aspect of the jaw is the dental apparatus, where the teeth are not set into sockets like those of mammals, but are attached to the jaw cartilage by connective tissue. This arrangement facilitates the animal’s continuous and rapid tooth replacement, a phenomenon known as polyphyodonty. Sharks are constantly regenerating their dentition throughout their lives to ensure they always have a sharp and effective cutting tool.
New teeth develop in a specialized tissue called the dental lamina, located on the inner side of the jaw, forming multiple rows of replacement teeth. This system functions like a “conveyor belt,” where new teeth develop behind the functional teeth. As the outer, used teeth are lost or damaged during feeding, the next tooth in the row migrates forward to take its place. This continuous cycle ensures that the shark’s dentition is always sharp, with some species shedding and replacing thousands of teeth over their lifetime.