Mysterious ghost sharks' origins revealed

Ghost sharks are so rare, until now it's not been clear what they really are.

It's only been weeks since the species was caught on camera for the first time, and now scientists think they've proven beyond doubt what they really are. [link to Nat Geo story on first ever footage]

It turns out its nickname was bang-on - they're sharks.

The confusion stems from the creature's strange appearance, leading some to suggest ghost sharks, also known as chimaera, were more closely related to rays.

"They don't look much like sharks and rays," says Michael Coates, professor of organismal biology and anatomy at the University of Chicago.

"Their skull is especially difficult to understand… because it looks more like ourselves."

Their eyeballs are so huge, they distort the shape of the shark's brain. They have dots around their heads, believed to house cells sensitive to electricity, and when swimming they flap their fins like wings.

The males even have a retractable sex organ on top of their heads.

"It had been difficult to see how closely linked chimaeroids are to [sharks and rays] because they look so different."

The answer came from a 280 million-year-old fossil of extinct shark Dwykaselachus. At first glance its skull appears much like that of other sharks, but scanning of its interior shows it's remarkably similar to those of ghost sharks - including the massive eye sockets.

"These results indicate that chimaeroids are rooted within the order of... sharks," the scientists concluded.

Ghost sharks split off the evolutionary tree 400 million years ago - before dinosaurs roamed the Earth - but have barely evolved since.

"Understanding where they fit in the tree is important because they represent a valuable, a unique archive of information on biodiversity," says Dr Coates.

December's historic filming of a chimaera was also the first time one had been spotted in the northern hemisphere.