Abdominal muscles are not something usually seen in marine life, but researchers found a fossil fish with "rippling" abs in Australia.

The 380-million-year old armour-plated fishes, called placoderms, boasted the earliest know muscles ever found in a vertebrate.

Curtin University's Associate Professor Kate Trinajstic said soft preserved soft tissue in fossils is extremely rare, usually there is only bone left, according to a news release.

Past studies had used the tissue from bone scarring to recreate an idea of what the fish' muscles looked like, the new research proved the past findings were incorrect.

"We were stunned to find that our ancient fossil fishes had abs! Abdominal muscles were thought to be an invention of animals that walked onto the land but this study revealed that these muscles appeared much earlier in our evolutionary history," Trinajstic said.

The study used specialist synchrotron scanning to put together a new recreation of the fish' muscles. The scan showed the fish had tendons arranged in a "helix-like" pattern.

These tendons "connected the tail skin to the muscles and helped propel the fish through the water like a modern shark," according to the study.

Professor Per Ahlberg from Uppsala University in Sweden, another author of the study, said the team had been looking to go further than just mapping out the musculature of this one ancient fish.

"We have managed to produce something close to a dissection guide for placoderms, nothing like this has ever been possible for such early vertebrates," he said.

Professor Philippe Janvier of the Natural History Museum in Paris, who was not on the research team, believes the new finding is a "milestone" in the study of early vertebrates. The findings give scientists a much clearer picture of the anatomy the "root of the jawed vertebrates" possessed.

Professor John Long from Flinders University who found some of the specimens said this means a lot to modern paleontology. According to him the next step is to find how soft tissue evolved from fish to human.

To see photos of the fossil scan click here.