A terrifying species of shark that dwells in the depths of the ocean has been discovered by researchers.

Ancestors of the Atlantic sixgill have been swimming the seas for 250 million years, since before the age of the dinosaurs, yet it has only just come to light.

Experts used DNA analysis to find that these creatures are a different species to their counterparts, which inhabit the Indian and Pacific Oceans.

Because of their new classification, Atlantic sixgill sharks will now have a better chance at long-term survival, scientists say.

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A terrifying species of shark that dwells in the depths of the ocean has been discovered by researchers. Ancestors of the Atlantic sixgill have been swimming the seas for 250 million years. This image shows an Atlantic sixgill pup found off the coast of Belize

Biologists based at the Florida Institute of Technology analysed the sharks' DNA to make their discovery.

They found significant genetic differences between individuals in what had been considered a single species - Hexanchus nakamurari, or the bigeyed sixgill shark.

These difference were significant enough to rename the Atlantic variety and create a separate species, Hexanchus vitulus.

Atlantic sixgill sharks measures up to six feet (1.8 metres) long, far smaller than their Indo-Pacific relatives which can grow up to 15 feet (4.6 metres) or longer.

They have unique, saw-like lower teeth and six gill slits, as their name suggests, as opposed to the typical five seen in sharks.

In a written statement, study lead Toby Daly-Engel 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.'

Sixgill sharks are among the oldest creatures on Earth and live at the bottom of the ocean, sometimes thousands of feet below the surface.

The fact that they reside at such depths has made them especially challenging to study.

Sixgill sharks were originally thought to comprise just two species: the bluntnose sixgill shark (Hexanchus griseus) and the bigeye sixgill (Hexanchus nakamurai).

A team led by Florida Institute of Technology used DNA analyses to confirm that the species is different to the bigeye sixgill shark (Hexanchus nakamurai), which looks very similar to the Atlantic sixgill shark. This image shows an adult Atlantic sixgill shark off the coast of Belize

This map shows capture locations of sixgill sharks in Belize, the northern Gulf of Mexico, and the Bahamas from 2007 to 2016. Circles represent the Atlantic sixgill shark (Hexanchis vitulus) and squares represent the bluntnose sixgill shark (Hexanchus griseus).

Uncertainty surrounding the classification of the bigeye sixgill shark has been around for some decades.

There's also been debate as to whether individuals from the Atlantic constituted a separate species.

Dr Daly-Engel first suspected that there was a third species of sixgill shark when she was tagging bluntnose sixgill sharks at the University of Hawaii, according to National Geographic.

When analysing the DNA samples, she found that those being sent to the lab didn't match with those of the species already analysed.

Sixgill sharks were originally thought to comprise just two species: the bluntnose sixgill shark (Hexanchus griseus) and the bigeye sixgill (Hexanchus nakamurai ). Pictured is a bluntnose sixgill shark seen while exploring Santa Rosa Reef, south of Guam on April 20, 2016

Now, a newly identified species of sixgill shark, the Atlantic sixgill shark (Hexanchus vitulus), settles the debate as to whether bigeye sixgill sharks (Hexanchus nakamurai, pictured) from the Atlantic constitute a separate species

Dr Daly-Engel teamed up with researchers from other institutions and organisations to analyse 1,310 DNA base pairs of two mitochondrial genes.

She hoped this would prove there were enough molecular differences between samples of bigeyed sixgill sharks to make this distinction official.

'Because we now know there are two unique species, we have a sense of the overall variation in populations of sixgills,' Dr Daly-Engel added.

'We understand that if we overfish one of them, they will not replenish from elsewhere in the world.'

Another benefit of the research is a better understanding of shark diversity.

'Particularly diversity in the deep ocean,' Daly-Engel said, 'which we don't know much about.'

The full findings of the study were published in the journal Marine Biodiversity.