Researchers have discovered a 380-million-year-old heart and other organs in the fossil of an ancient jawed fish collected in Western Australia's Kimberley region.
Experts say it is the oldest three-dimensionally preserved fossil heart ever found in a jawed vertebrate, shedding new light on the evolution of humans.
The research, published in the Science journal on Friday, finds the position of the organs in the body of arthrodires - an extinct class of ancient armoured fish - is similar to modern shark anatomy.
Lead researcher and Curtin University paleontologist Kate Trinajstic said the discovery of the heart was remarkable given its age and level of preservation.
The find came as a surprise to a global team, including researchers from Australia, France and Sweden.
"We knew there was something in there. We didn't know the extent of it," Professor Trinajstic told AAP.
"It completely blew our minds that there was a heart there. We had no preconceived ideas that we could have got that preserved."
Researchers used neutron beams and synchrotron X-rays to scan the specimens, which were embedded in limestone.
They constructed 3D images of the soft tissues based on the different densities of minerals deposited by the bacteria and surrounding rock.
Scans revealed the fish was strikingly similar to a present-day shark, with a two-chambered, S-shaped heart located under its gills.
"It was as if you'd just put a modern shark in a CT scanner," Prof Trinajstic said.
"The surprise was that step (a two-chambered heart) had actually been made in such an early fish. Here we are, right at the beginning of the jawed vertebrates, and that step's already there."
She said the finding suggested there was a bigger leap between jawless and jawed vertebrates than was previously understood.
The fossils were collected in the Gogo formation in inland Kimberley - once a coastal reef with exceptional preservation qualities.
"There's no oxygen on the bottom so there's no scavengers or anything like that, which allows time for the fish to fossilise without being disturbed or munched on," Prof Trinajstic said.
Co-author and Flinders University professor John Long said the Gogo formation had proved to be one of the most significant fossil sites in the world.
"These new discoveries of soft organs in these ancient fishes are truly the stuff of paleontologists' dreams, for without doubt these fossils are the best preserved in the world for this age," Prof Long said.
"It's time the site was seriously considered for world heritage status."
