Ancient armoured fish had complex musculature — including abdominal muscles — the discovery of uniquely preserved tissue on Australian placoderm fossils has revealed.
The discovery provides an insight into the early evolution of all vertebrates says the study's lead author Associate Professor Kate Trinajstic, a palaeontologist with Curtin University.
'Nothing like this has ever been found in the world.' —Person quoted
"Nothing like this has ever been found in the world," says Trinajstic.
"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, ruled Devonian seas for 70 million years.
Researchers long assumed these fish had primitive structures like sharks, but the discovery of tissue on the 380-million-year-old fossils, reported in the journal Science, proves otherwise.
"It tells us primitive doesn't mean simple. These fish had a unique and complex musculature which is unknown in modern fishes," says Trinajstic.
"Along with having a complex muscle system, they also have a differentiated vertebra column, that's quite an advanced feature."
One thing that sets placoderms apart from sharks is the presence of a notable joint between the skull and shoulder girdle.
"These guys had an incredible bony neck joint and they also had armour so they could only move their head up and down because they've got a hinge joint on the armour," says Trinajstic.
"They're the first animal with a neck, and the first animal with its shoulder separated from the back of the head.
Another defining feature is 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," says Trinajstic.
"The next time we see abdominal muscles similar to this fish, is in tetrapods, animals that walk on to land."
The researchers used the European Synchrotron Radiation Facility to scan their fossils, so as not to damage the three dimensional preservation.
Trinajstic says the three dimensional preservation of these fossils, including their soft tissues such as muscles and nerves, is incredibly unique, because such tissues almost never fossilize.
The three species of placoderms -- Eastmanosteus, Compagopiscis and Incisoscutum -- were discovered in the the Gogo formation, near Fitzroy Crossing in the Kimberley region of Western Australia.
The site is preserved like a snapshot of a Devonian ecosystem.
"This isn't a weird anomaly, we've got a whole suite of these fish preserved," she says.
The unusually high degree of preservation was caused by rapid burial once the fish died, preventing decomposition or decay.
"It's called the medusa effect where something's literally turned to stone," Trinajstic explains.
Bacteria is also thought to have helped in the process by encasing fossils in a microbial slime which held the remains together and also trapped minerals such as phosphorous, which aided in the muscle tissue being preserved.
Previous work by Trinajstic and colleagues showed these fish had teeth much like our own.