Cutting through the water with a grace and agility that makes them the apex predators of their ecosystems, the seemingly effortless way sharks move is made possible by one curious trait: They don’t have bones.
Instead, their skeletons are made of cartilage. Researchers have long believed that this boneless body schemata predated bony skeletons of other fish; indeed, sharks, they thought, were a blueprint for bones.
But a new fossil finding from western Mongolia challenges that understanding. Bony skeletons, it turns out, date back in fish evolution much further than expected. The finding may fundamentally alter what makes a shark a shark.
In a new study, researchers describe skull and braincase parts from an ancient fish fossil, Minjinia turgenensis, named for the region of Mongolia where it was found.
The fish was a placoderm, a class of prehistoric armored fish, and the fossilized bone parts are 410 million years old. The finding places the evolution of bones further back in time than previous estimates, and muddies the story of how sharks evolved as opposed to other fish. It challenges what we understand about one of the shark’s most remarkable traits — its cartilaginous frame.
Placoderms have been studied in the past for their bone and jaw development — traits that later continued not only in fish, but in other animals, too, including humans.
Sharks sure have jaws, but they don’t have bones. Rather, sharks’ skeletons are made of cartilage, a material which is half as dense as bone, helping these stealthy swimmers conserve energy.
Given sharks’ distinct boneless makeup, researchers thought sharks split off from other fish before they went on to develop bony skeletons.
But the new fossil, an ancient relative of both sharks and bony fish, suggests that, at some point in time, sharks may have been bonier than they are today.
The findings were published Monday in the journal Nature Ecology & Evolution.