Paleontologists in Mongolia have found the fossilized remains of Minjinia turgenensis, a new genus and species of placoderm fish that lived 410 million years ago (Early Devonian epoch). They’ve examined a partial braincase and skull roof of Minjinia turgenensis and found extensive endochondral bone, the hard bone that makes up our skeleton after birth. This discovery suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.
Sharks have skeletons made of cartilage, which is around half the density of bone.
Cartilaginous skeletons are known to evolve before bony ones, but 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.
Minjinia 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.
Previously, no placoderm had been found with endochondral bone, but the skull fragments of the ancient fish species were wall-to-wall endochondral.
This could suggest 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.
“It was a very unexpected discovery,” said lead author Dr. Martin Brazeau, a researcher in the Department of Life Sciences at Imperial College London and the Department of Earth Sciences at Natural History Museum, London.
“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.”
“If sharks had bony skeletons and lost it, it could be an evolutionary adaptation,” he added.
“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.”
The study was published online today in the journal Nature Ecology & Evolution.
M.D. Brazeau et al. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nat Ecol Evol, published online September 7, 2020; doi: 10.1038/s41559-020-01290-2