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It has been long assumed that sharks are one of the more primitive forms of fish.

But new analysis of a 415 million-year-old fossilised fish has shed doubt on the idea that sharks are the primitive forerunners of modern jawed vertebrates.

Modern-day sharks have internal skeletons made of cartilage, and don't have any bony armour on the outside of their faces, unlike other fish living today.

A fossilised 415 million-year-old fish called Janusiscus (shown above) provides critical evidence for a well-developed external skeleton (shown in blue) in the shared ancestor of bony fishes and cartilaginous fishes such as sharks. This means that sharks are not as primitive as assumed, experts say

Now researchers from the University of Oxford and Imperial College London have suggested that modern sharks shed their bony skeletons early in their evolutionary history.

When examining the fossil, they found it suggested that the last common ancestor of all jawed vertebrates, was not very shark-like.

They used X-rays and CT scans to look inside the skull and found that the structure surrounding the brain was reminiscent of cartilaginous fish (chondrichthyans) such as sharks and rays.

The fish fossil's 'two faces' led to it being named Janusiscus after the double-faced Roman god Janus. 

Modern-day sharks have internal skeletons made of cartilage, but don't have any bony armour on the outside of their faces, like other living fish. A blue shark accommpanied by a pilot fish is pictured

Because of its external features, experts had wrongly thought it belonged to the bony fish family, osteichthyans, a group which includes familiar fish such as cod and tuna as well as all land-dwelling creatures with backbones.

Dr Matt Friedman of Oxford University's Department of Earth Sciences said: 'This 415 million year-old fossil gives us an intriguing glimpse of the 'Age of Fishes', when modern groups of vertebrates were really beginning to take off in an evolutionary sense.

'It tells us that the ancestral jawed vertebrate probably doesn't fit into our existing categories.'

The scans revealed Janusiscus lacks a division across the bottom of its brain case, which places it an early stage of evolution - but it also has a bony external skull.

It shares features with two distinct evolutionary branches of early fish: those with skeletons of cartilage, like sharks, and the bony fish that went on to become land vertebrates and, ultimately, humans.

Chondrichthyans, such as sharks, have often been viewed as primitive, and treated as proxies for what the 'ancestral' jawed vertebrate would have looked like, largely due to their lack of bony skeleton, according to the study, published in Nature.

Dr Friedman explained: 'This mix of features, some reminiscent of bony fishes and others cartilaginous fishes, suggests that humans may have just as many features that you might call "primitive" as sharks.

'Losing your bony skeleton sounds like a pretty extreme adaptation but with remarkable discoveries from China, Janusiscus strongly suggests that that the ancient ancestors of modern sharks and their kin started out just as 'bony' as our own ancestors.' 

The fossil was previously thought to belong to the bony fish, osteichthyans, a group which includes familiar fish such as cod and tuna (illustrated witha  stock image) as well as all land-dwelling creatures with backbones, but is now known to be an ancestor of the sharks too

The fossil was found in Siberia in 1972 and is currently held in the Institute of Geology at the Tallinn University of Technology, Estonia.

Dr Martin Brazeau from Imperial's Department of Life Sciences spotted its potential in providing new insights into the period of evolution.

He said: 'I knew straight away that this was a fossil that merited further investigation.

'Although Janusiscus does have a bony skull - which led to it being first classified as a bony fish - it lacked many other features you'd expect in that group.

'It also lacked a feature seen in both early bony fishes and sharks, which is a division across the brain case. This clearly places it at an early stage of evolution, before the two branches split.

'It shows that fish from this time, which are ancestors of both sharks and bony fish, had bones too, and sharks must have lost them at a later point.

'Janusiscus has helped us to look at sharks differently and will ensure they are no longer dismissed as being "frozen" at a primitive stage of evolution.'