Why Sharks Have No Bones Despite Their Size and Strength

The evolution of the shark is not what most people would expect. Instead of sharks evolving from a massive predator, the original creature was a small, leaf-shaped fish. This fish lacked eyes and fins. It also did not have bones. As the oceans began to fill with bony fish, one of those fish, the Acanthodians, began to take on the characteristic looks of today’s shark. These fish were also the first to develop cartilaginous skeletal structures, which all modern sharks are known for having. As a result, the composition of shark skeletons can be traced back nearly half a billion years. This has also helped sharks survive four mass extinctions, thanks to their ability to conserve energy and swim long distances with ease.

Shark Skeletons Are Not Made From Bone At All

Sharks are one of the oldest species of animals still alive today. While they may not be the massive megalodons that lived during the Early Miocene and Early Pliocene eras, as recent as 3.5 million years ago, sharks have managed to maintain their marine apex predator status, having very few predators to contend with. Apex predators are typically thought of as strong, formidable, and massive. Interestingly, sharks possess all these qualities despite having no bones in their bodies. Instead, shark skeletons are made from cartilage.

Sharks have cartilage instead of bone because cartilage is strong, yet more flexible and lighter than bone. This allows sharks to maneuver more easily in the water, enabling them to attack prey with agility and escape from predators. According to Joshua Moyer, PhD, Lecturer in the Department of Ecology & Evolutionary Biology at Yale University, cartilage also helps sharks withstand physical forces in the water.

“Cartilage is roughly half as dense as most bones, and its higher water content allows it to withstand physical forces differently,” Moyer explains to A-Z Animals. “That’s why many terrestrial vertebrates use cartilage as shock absorbers in joints.”

Moyer continues, “That means that if a fish, like an early relative of sharks, lacked air-filled organs like a swim bladder, it would need to exert energy to actively maintain its position in the water column. A less dense skeleton means less energy is required to move through the water and generate lift.”

However, the exceptional strength of shark jaws is not due to cartilage alone; additional reinforcement makes their jaws especially powerful.

Sharks Have Strong Jaws Despite Being Formed From Cartilage

Shark teeth and jaws are often found fossilized, but the rest of the shark’s skeleton rarely is. This is because the teeth and jaws, according to Moyer, are reinforced by “deposits of calcium salts” that give sharks their signature strong teeth and jaws.

“In sharks and their relatives, the cartilages of the jaws are reinforced by deposits of calcium salts between the cartilage and the thin layer of tissue that surrounds the cartilage, called the perichondrium,” Moyer explains to A-Z Animals. These calcium deposits do not penetrate or ossify the cartilage, which would turn it into bone as happens in humans, but instead cover the outside of the cartilage beneath the perichondrium.”

Moyer continues, “The result is a mosaic of globular and prismatic deposits of calcium structures called tesserae. In the jaws of larger sharks or those that feed on hard, crunchy prey, there may be several layers of tesserae in the corners of the mouth to reinforce the jaws.”

Without this protection, nothing would remain of sharks after they die, and little to no information would be known about prehistoric sharks and how they evolved into the species we know today.

Chances Of Finding A Shark Skeleton Fossil Are Slim To None

Sharks have been around for 400 million years. Given that they have been around longer than dinosaurs, it would make sense that there would be fossils of these creatures, both massive and smaller in stature, found globally. However, this is not the case. While some complete shark fossils have been discovered, they are few and far between, and according to Moyer, very specific conditions are required for this to occur.

“It is very rare to recover fossilized shark cartilage, although under very specific conditions, the tesserae of cartilaginous skeletal elements may be preserved, or skeletons may be rapidly buried in anoxic sediment that protects the cartilage from decomposition,” Moyer explains to A-Z Animals. “In those cases, you can get lucky and recover more complete skeletal remains. When this happens, it tends to involve smaller species of sharks and rays that live closer to the seafloor where rapid and complete burial may occur.”

As a result, shark teeth are the most commonly found fossils. This, as Moyer explains, has to do with the fact that “fully formed teeth have calcium salts in dentine and an enamel-like tissue called enameloid, not cartilage.”

Occasionally, the vertebral centra (the center of the vertebra) can also be found, as these elements are rich in calcium salts and do not break down over time like cartilage does.

A Shark Ancestor’s Skeleton Was Made Of Bone, Not Cartilage

Given that sharks today are primarily composed of cartilage, it raises the question of whether their ancestors ever had bones. As it turns out, they did. It is a result of evolution and the advantages that come with a skeleton being built of cartilage, which sharks and other cartilaginous fish developed into the species they are today.

According to Moyer, “Biologists say that chondrichthyans secondarily lost bone.” This has to do with evolution taking away a trait that a species once had.

“In evolutionary biology, a secondary loss refers to a trait that had been expressed in an evolutionary lineage but no longer is,” Moyer tells A-Z Animals. “We can infer that sharks’ ancestors did have a bony skeleton at one time because modern sharks still have regions of their skeletons where the same mineral present in bones, calcium hydroxyapatite, is deposited, albeit not in the same way that it is in bone.” 

Due to the need to be more buoyant, agile, and able to escape predators, cartilaginous fish evolved to the point where bone was no longer present in their bodies. It is uncertain how many species existed during each evolutionary era, as there is no fossil record of many of them. But if they were like the sharks of today, their aerodynamic bodies would have made them a formidable force in the ocean millions of years ago.