The researchers said the discovery found in preserved tissue on Australian placoderm fossils offers an insight into the early evolution of all vertebrates, the Australian Broadcasting Corp. reported Friday.
"Nothing like this has ever been found in the world," said the study's lead author Associate Professor Kate Trinajstic, a palaeontologist with Curtin University in Bentley. "We've actually found the muscles and we've found them in such quantity and preservation that for the first time we can actually map all of the muscles on a fish."
Placoderms, the earliest-known jawed vertebrates, swam Devonian seas 70 million years.
Researchers had assumed these fish had primitive structures such as sharks, but the discovery of the tissue on the fossils "tells us that primitive doesn't mean simple," Trinajstic said.
"These fish had a unique and complex musculature which is unknown in modern fishes," Trinajstic told ABC. "Along with having a complex muscle system, they also have a differentiated vertebra column, that's quite an advanced feature."
Another difference? The presence of abdominal muscles.
"Sharks and bony fish have quite simple muscles and they don't have the abdominal muscles, but this really ancient fish has abs," Trinajstic said.
She said researchers used the European Synchrotron Radiation Facility to scan the fossils, ABC said.
Trinajstic said decomposition or decay was prevented because of rapid burial once the fish died.
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