In the study published in Science, the researchers prepared and analyzed the muscles in a small number of specimens from three different species.
"(The ancient fish) have already revealed soft tissues such as nerve and muscle cells, the oldest known vertebrate embryos, and even a preserved umbilical cord," Young said.
The latest study went further and mapped the musculature of the ancient fish for the first time, possible after researchers realized that soft tissues had been preserved in some of the specimens, though it was being destroyed in the earlier process of acid etching the skeletons.
Curtin University associate professor Kate Trinajstic, a chief investigator on the ANU-based research into early vertebrate evolution, said the team had been "stunned to find that our ancient fossil fishes had abs!"
"Abdominal muscles were thought to be an invention of animals that first walked onto the land but this discovery shows that these muscles appeared much earlier in our evolutionary history," she said.