No bones about it

Ancient fish forces rethink of how sharks evolved to be expert swimmers

What makes a shark a shark? A new finding challenges one of scientists' key theories about these marine predators.

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.

Three-dimensional CT scan of the braincase of Minjinia turgenensis. Imperial College London/Natural History Museum

Evolution of bones — Sharks are known for their limber, boneless bodies. But at some point in the course of history, these expert killers of the marine world may have had bones, too.

Then, they lost them.

"Conventional wisdom says that a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us,” said lead study author Martin Brazeau in a statement.

If sharks indeed had bony skeletons, then lost them, it may have been in order to adapt to their environment, Brazeau explains. Since sharks don't have a swim bladder — a feature that came later for bony fish — lightweight cartilage would have helped them stay nimble in the water, the researchers theorize.

Learning more about shark skeleton evolution can also help us understand how they came to be the formidable predators we know and fear today.

“This may be what helped sharks to be one of the first global fish species, spreading out into oceans around the world 400 million years ago," Brazeau said.

Abstract: The vertebrate skeleton comprises two main systems: the exoskeleton (external achondral dermal bones) and the endoskeleton (internal chondral bones and cartilages, as well as some intramembranous bones). An ossified exoskeleton evolved at least 450 million years ago in jawless stem gnathostomes, but the endoskeleton in these taxa is not endochondrally ossified. Endochondral bone, in which the cartilaginous endoskeletal precursor is invaded by and eventually replaced by bone, is widely considered an osteichthyan apomorphy, and such a reliable identifying character that it gives the group its name. Extant chondrichthyans lack dermal bone and possess a mainly cartilaginous endoskeleton enveloped by a structurally diverse range of tessellate calcified cartilage. Outgroups of the gnathostome crown also lack endochondral ossification. Galeaspids surround their cartilaginous skeleton in globular calcified cartilage, while osteostracan and ‘placoderm’ endoskeletons were sheathed in perichondral bone. Consequently, the last common ancestor of jawed vertebrates was long thought to have been perichondrally ossified, but lacking endochondral ossification.
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