Do sharks have bones?

Sharks, rays and chimaeras belong to the cartilaginous fishes. Like other vertebrates, they begin development with cartilage, but sharks do not replace that framework with bone as many bony fishes and land vertebrates do. The result is not a body without a skeleton. It is a different skeletal material organized into a skull, jaws, fin supports and vertebral column.

Ordinary comparisons with the flexible cartilage in a human ear are useful only up to a point. Shark skeletal tissues vary by location and species, and the structures carrying the greatest loads can be heavily reinforced.

Much of a shark's cartilage is covered by a mosaic of mineralized plates called tesserae. Fibers connect the tiles while an outer fibrous layer wraps the skeletal element. This composite construction can stiffen the surface without converting the entire structure into bone.

Vertebral centra may be mineralized differently from the tessellated cartilage around other skeletal parts. X-ray studies have found bioapatite in shark centra that is chemically related to the mineral in bone, while still retaining a distinct organization. Calling the skeleton 'cartilage' therefore describes its tissue identity, not an absence of hard material.

Large sharks accelerate, turn and transfer force through the body while the vertebral column bends repeatedly. Mechanical tests on mineralized vertebral cartilage from several elasmobranch species found strength and stiffness comparable with mammalian trabecular bone. The useful comparison is not that cartilage is better or worse, but that sharks reinforce it for the particular loads of swimming.

Teeth are highly mineralized and replaced throughout life, so they dominate the shark fossil record. Calcified vertebral centra, jaws and patches of tessellated cartilage can also preserve under favorable conditions. The rest of the skeleton usually decays more readily than bone, which is why an extinct shark may be known from thousands of teeth but only a small number of partial skeletons.

It is tempting to describe cartilage as a primitive step on the way to bone. Modern cartilaginous fishes are not unfinished bony fishes; their lineage has been evolving for hundreds of millions of years. A shark skeleton is a specialized composite system with its own growth, repair and mechanical tradeoffs.