Do Sharks Have Bones? Cartilage Skeletons Explained

What is a shark skeleton made of?

Cartilage is a tough, rubbery tissue made of protein fibers, mainly collagen, set in a gel-like material. In most vertebrates, much of the cartilage in an embryo is later replaced by bone. In sharks, it never is. Their skull, jaws, backbone, gill supports and fin supports all stay cartilage for life.

Pure cartilage would be too soft for a large, powerful animal, so sharks reinforce it. A thin outer layer of the skeleton is covered with a mosaic of tiny, hard, six-sided tiles called tesserae, made of calcium phosphate, the same mineral found in bone. This makes the skeleton strong while keeping it light and slightly flexible.

The parts that work hardest get extra reinforcement. Most sharks have one layer of tesserae, but the jaws of big predators such as the bull shark, tiger shark and great white shark have two, three or more layers, and the jaws of a large great white may have up to five. The vertebrae are also mineralized in rings, and these rings form the growth bands that scientists count to estimate a shark’s age.

Why cartilage works so well for sharks

Cartilage has several advantages for a fish that never comes ashore:

• It is light. Cartilage is only about half as dense as bone. Sharks have no gas-filled swim bladder, so they rely on a light skeleton and a large, oily liver to avoid sinking.
• It is flexible. A bendy skeleton helps sharks turn tightly and swim efficiently, and the springy jaws can open very wide.
• It is cheap to build. Growing and maintaining a cartilage skeleton takes less energy and mineral than a bony one.

Sharks also get support from outside. Their skin contains tough, criss-crossing collagen fibers that act like a corset, giving the swimming muscles something to pull against. The skin is covered with dermal denticles, tiny tooth-like scales that protect the body and reduce drag.

The weakness of cartilage shows only out of water. Sharks have no rib cage, so a large shark stranded on land can be injured by its own weight.

Which parts of a shark are hard?

Even without bone, a shark has several hard parts:

• Teeth, made of dentine with a hard, mineral-rich outer layer, constantly replaced throughout life, as explained in How many teeth does a shark have?
• Dermal denticles, the tiny tooth-like scales that cover the skin and make it feel like sandpaper when rubbed the wrong way.
• Calcified jaws and vertebrae, strengthened with mineral tiles and rings.
• Fin spines in some sharks, such as dogfish and horn sharks, which have a sharp spine in front of each dorsal fin.

The fins themselves are supported by cartilage near the body and by long, flexible protein fibers called ceratotrichia farther out.

Why shark fossils are mostly teeth

Because cartilage usually decays before it can be preserved, the fossil record of sharks consists mostly of teeth, along with denticles, spines and calcified vertebrae. A single shark sheds thousands of teeth in its life, so fossil teeth are common in rocks laid down in ancient seas.

Whole fossil sharks are rare treasures. They form only where bodies were buried quickly in fine mud with little oxygen. The Cleveland Shale of Ohio, laid down in the Late Devonian roughly 360–370 million years ago, has yielded famous complete specimens of Cladoselache, an early shark-like fish with even the outline of its body and fins preserved. Giant extinct sharks such as megalodon, by contrast, are known almost entirely from teeth and a few vertebrae, which is why their size is still debated. How far back the shark record goes is explained in How long have sharks existed?

Did sharks ever have bones?

For a long time, scientists thought cartilage skeletons were a primitive feature, older than bone. The picture is now more complicated. Many early fish had bony armor, and in 2020 researchers described an ancient armored fish called Minjinia from Mongolia, more than 400 million years old, with true bone inside its skull. This suggests that the ancestors of sharks may have had some internal bone and that sharks later lost it.

If so, the shark’s light cartilage skeleton is not a leftover from a simpler past but a successful design that has served sharks for hundreds of millions of years.

Rays, skates and chimaeras share the same skeleton

Sharks are not the only fish without bones. Rays, skates and sawfish have the same kind of cartilage skeleton, and they are close relatives of sharks; their flat bodies evolved from shark-like ancestors. The family tree is explained in Are stingrays related to sharks?

Chimaeras, also called ghost sharks or ratfish, are more distant cousins in the same class, Chondrichthyes, a name that means “cartilage fish”. A 2021 global assessment covered about 1,200 species of cartilaginous fish: roughly 540 sharks, 610 rays and 50 chimaeras.

The largest skeleton of cartilage belongs to the whale shark, the biggest fish in the world, which commonly reaches 8–12 m (26–39 ft). Its vertebrae lay down growth bands too: a radiocarbon study of whale shark vertebrae estimated one 10 m (33 ft) female at about 50 years old.