A fossilized shark skull that’s more than 280 million years old could be a missing evolutionary link between sharks, and their strange deep-sea relatives known as ghost sharks. That’s not all: by virtually reconstructing the ancient shark’s brain, the researchers discovered that modern day ghost sharks are more distantly related to today’s sharks than we thought.
The skull belonged to an extinct type of shark called a symmoriiform that roamed the oceans around 330 million years ago. A group of researchers led by evolutionary biologist Michael Coates with the University of Chicago discovered that the skull contains a weird mix of traits from both ghost sharks, and primitive sharks. Their findings, published today in the journal Nature, suggests that the group of fish that gave rise to ghost sharks branched off from the group that evolved into sharks more than 359 million years ago, which is when a mass extinction killed off 75 percent of life on Earth. That’s much earlier than previous estimates of 334 million years ago.
Ghost sharks went viral at the end of December after the Monterey Bay Aquarium Research Institute released a video of an elusive, wayward species that typically swims around Australia and New Zealand. Researchers caught this one on camera with an underwater drone off the coast of California, more than two miles beneath the ocean’s surface. It was the first time this particular species has been captured on video, and the first time it’s been seen in the Northern Hemisphere.
Watch Coates explain his team’s discovery:
Today’s ghost sharks probably look a lot like the ghost sharks of 340 million years ago, Coates said in a video. They’re closely related to the family of fishes that include sharks, skates, and rays. But they have unusual skulls with massive eye sockets, and on their skin there is a patchwork of grooves filled with sensory cells. Instead of teeth, ghost sharks mash their food on hardened tooth plates, kind of like a platypus.
The discovery came from the serendipitous convergence of modern technology, and old bones. A strange, primitive symmoriiform skull had been sitting in the South African Museum since the 1980s, and its oddness had bothered Coates for years. His colleague took a CT scan of the fossil — using the same technology that lets doctors check living patients for concussions to see inside the ancient skull. From the CT scan, they were able to virtually reconstruct the shape of the shark’s brain. They discovered even though the outside of the skull looked like that of the primitive shark, a central part of the brainstem was elevated, like a ghost shark’s. And the eye sockets were massive — an adaptation that helps today’s ghost sharks see in the dim light of the deep ocean.
So, the ancient brain of a symmoriiform looked a lot like a ghost shark brain. That means that symmoriiforms and ghost sharks probably shared a common ancestor that branched off from the ancestors of today’s sharks and rays earlier than we thought. The fact that both the symmoriiform and ghost sharks have massive eyes is a clue that their common ancestor started the process of adapting to life in deeper waters, where the ghost sharks have stayed ever since.