Modern sharks are not "living fossil" replicas of their prehistoric ancestors but have evolved significantly over millions of years to develop their hallmark design, a consensus-busting study said Wednesday.

The newly discovered fossil skull of a shark-like ancestor of all jawed vertebrates adds to evidence that a bony skeleton, not a cartilaginous one, was the prototype.

The ancestor, which lived in the Palaeozoic era some 325 million years ago, turns out to have had characteristics of fish with cartilaginous skeletons, like sharks, and of bony ones like salmon and tuna, a reasearch team wrote in the journal Nature.

Scientists have long assumed that modern animals with bony skeletons evolved from a shark-like creature with a frame made of cartilage, thus acquiring bone over time.

Modern-day sharks and rays were thought to most closely represent the original jawed ancestor, having stayed basically unchanged.

But the latest study adds to a new school of thought that sharks shed their bony frames and replaced them with cartilage to become specialised, deep-sea hunters.

"Scientists, and the public, have long thought that today's sharks are 'living fossils' -- a group of animals that appeared a long time ago and hardly changed," study lead author Alan Pradel of the American Museum of Natural History, told AFP.

"Scientists based this on the fact that their skeletons are cartilaginous like those of jawless fish (a sister group comprised of lampreys and hagfish), which are assumed to be more primitive.

"Thus, scientists assumed that modern-day sharks represent the ancestral version of the skeleton of jawed vertebrates."

But the new fossil challenges this view.

Its skull is arranged very differently to those of sharks, much more like those of bony fishes, Pradel's team found.

"The common ancestor of jawed vertebrates more closely resembled today's bony fish than cartilaginous ones," said Pradel of the study findings.

"That turns the traditional scientific thinking on its head. Today's sharks are not living fossils, and they are very different to their ancestors."

The finding means that scientists can probably learn more from bony fishes like salmon and tuna about the early evolution of jawed vertebrates than from sharks, as was long believed.

[Image via Agence France-Presse]