New thoughts on how sharks evolved

This 410-million-year-old fossil with a bony skull uncovered in Mongolia may force a rethink of how sharks evolved.

Minjinia turgenensis, a new species, is an ancient cousin of both sharks and animals with bony skeletons, the researchers say – and that suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.

“It was a very unexpected discovery,” says Martin Brazeau from Imperial College London, lead author of a paper in the journal Nature Ecology & Evolution.

“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.”

To date, most early fossils of fish have been uncovered in Europe, Australia and the US, but the research team from the UK, Mongolia, Sweden and France decided to try its luck in western Mongolia where rocks of the right age had not been studied.

Among its discoveries was the partial skull of M. turgenensis, including the brain case.

Shark skeletons are made of cartilage, which is half the density of bone, and are known to evolve before bony ones. However, it was thought that sharks split from other animals on the evolutionary tree before this happened – keeping their cartilaginous skeletons while other fish, and eventually us, went on to evolve bone.

The new find suggests the ancestors of sharks first evolved bone and then lost it again, rather than keeping their initial cartilaginous state for more than 400 million years, the researchers say.

M. turgenensis belongs to a broad group of fish called placoderms, out of which sharks and all other jawed vertebrates – animals with backbones and mobile jaws – evolved.

Humans and bony vertebrates also have skeletons made of cartilage when developing as foetuses, but this is replaced by endochondral bone – the hard bone that makes up our skeleton after birth. Previously, no placoderm had been found with endochondral bone, but the skull fragments of M. turgenensis were “wall-to-wall endochondral”.

Brazeau and colleagues are being cautious not to over-interpret from a single sample, while they sift through the large amount of other material collected from Mongolia.

Finding further evidence to support an early evolution of endochondral bone could point to a more interesting history for the evolution of sharks.

“If sharks had bony skeletons and lost [them], it could be an evolutionary adaptation,” he says.

“Sharks don’t have swim bladders, which evolved later in bony fish, but a lighter skeleton would have helped them be more mobile in the water and swim at different depths. 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.”