The Atlantic sixgill is believed to be now of the oldest-surviving species, with its early ancestors around before dinosaurs roamed Earth.
Experts were able to find the new type of shark – related to other sixgill species that live in the Pacific and Indian Oceans – using genetic testing.
Very little is known about the shark, such as how many of them there are, because of how far down they live.
They are known to thrive at depths of up to 6,000 feet and are rarely seen by humans, the Florida Institute of Technology said.
“We understand that if we overfish one of them, they will not replenish from elsewhere in the world”
Toby Daly-Engel, assistant professor
The shark is described as 6ft long, with saw-like teeth and six or seven gill slits, which is how it earned its name.
Toby Daly-Engel, assistant professor of biological science at Florida Tech, who was behind the study, said: “We showed that the sixgills in the Atlantic are actually very different from the ones in the Indian and Pacific Oceans on a molecular level, to the point where it is obvious that they’re a different species even though they look very similar to the naked eye.
“Because we now know there are two unique species, we have a sense of the overall variation in populations of sixgills.
“We understand that if we overfish one of them, they will not replenish from elsewhere in the world.”
The groundbreaking discovery comes just weeks after a cryptozoology expert said ancient species of giant shark are yet to be discovered.
Jonathan Downes claimed giant sea monsters are lurking in the deep ocean, saying: “There is a lot of evidence that there are some very big sharks out there that we haven’t discovered yet.
“One thing I am almost certain to exist is something very closely related to the whale shark.
“It’s either a different colour of one we haven’t discovered yet, or it is actually a completely new species of giant shark.”