Researchers have found new fish fossils containing the oldest preserved muscles ever seen, and this fish had abs.
Previously, it was thought the specialised stomach muscles were unique to land animals, however these new fossils show they evolved in a species of armour-plated fish.
Per Ahlberg, of Uppsala University in Sweden, and colleagues, found the fossilised fish in the Gogo formation of western Australia. Here, an ancient reef has been preserved very well.
The remains in the Gogo are preserved in three-dimensions, unlike normal soft tissue fossils which are flat.
The fossils found by Ahlberg belong to early jawed fish called placoderms, with their heads and parts of their bodies covered with tough armour.
"Some were quite scary, big-time predators," says Ahlberg - the Dunkleosteus, for example, grew to 10 metres long.
Ahlberg was able to piece together some of the fish's musculature using three placoderm fossils. He says: "In part, what we see matches what people expect.
"But there are also some very unexpected things."
Modern fish have muscles running around the length of their bodies, however the placoderms had strips of transverse muscles on their bellies, as well.
Ahlberg says: "These are quite specialised muscles. They're present in all land vertebrates, and they hold the belly in."
Modern fish do not require them as their bodies are supported by the water.
Co-author of the research, Kate Trinajstic of Curtin University in Perth, Australia, says they were stunned to discover the abs of the fossil fish.
There is no definite reason for these having abs, however Ahlberg says it may be because of the tough armour on the front halves of their body.
As it swam, the placoderm waved its tail from side to side meaning the back half of it would also have swung. This would have caused powerful shearing forces against the armour plates, so the abdominal muscles may have prevented that.
Philip Anderson, from the University of Massachusetts Amherst, agrees, though he was not involved in the study. He says: "If the whole back of that body starts moving back and forth, you'll start bending the plates."
The specialised muscles could have helped keep everything steady.
The placoderm also had two pairs of muscles spanning from its shoulder to the top of its head, pulling the head up and down with the help of a joint linking the head and shoulders.
These muscles may have made the placoderms greater predators, says Ahlberg. "When the fish opens its mouth, it's also raising the skull," he says.
"That opens the mouth very vigorously, so it becomes quite an effective biting mechanism."
Anderson adds, "It could have aided in bite performance and maybe how fast it opened its jaws."
Anderson says the new fossils raised new questions despite answering old ones. Palaeontologists will now have to figure why these fish had abs.