Fossil of arthrodire heart.

An ancient fossil from one of our planet's earliest vertebrate organisms was found concealing an exciting surprise.

Inside a 380 million-year-old fossilized armored fish, paleontologists identified a mineralized heart, one that was exceptionally well-preserved in three dimensions.

This is an incredible find. Soft tissues are rare in the fossil record, tending to decompose before fossilization can take place. Rarer still are three-dimensional soft tissues.

And it gets better. Scans of the fossil allowed scientists to study its anatomy in 3D without needing to break the precious object apart. Thanks to its amazing state of preservation, details such as an atrium, ventricle and an outflow tract can be clearly identified in the fossil.

The ancient fish's heart was an S-shaped organ made up of two chambers, with the smaller chamber sitting on top of the larger one. This was much more advanced than paleontologists thought it would be, and could provide critical information about the evolution of the head and neck region, and how they changed to accommodate jaws, the researchers say.

"As a paleontologist who has studied fossils for more than 20 years, I was truly amazed to find a 3D and beautifully preserved heart in a 380-million-year-old ancestor," says palaeontologist Kate Trinajstic of Curtin University in Australia.

"Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a larger leap between jawless and jawed vertebrates. These fish literally have their hearts in their mouths and under their gills – just like sharks today."

This is the fossil in which an almost perfect, three-dimensional arthrodire heart was found.

The fossil hails from a site known as the Gogo Formation in the northernmost corner of Western Australia. During the Devonian, between 419.2 million years ago and 358.9 million years ago, this region was a vast reef thriving with life. Now, it's a fossil bed classified as a Lagerstätte – so exceptional that sometimes even soft tissues have been preserved.

The fossil was left by an animal from an extinct class of armored fishes called arthrodires. These creatures flourished for about 50 million years during the Devonian before disappearing during a major global extinction event towards the end of the period.

Consisting of a crude chunk of limestone rock adorned with a scattering of strangely biological features, the specimen once would have been a challenge to analyse without risking its destruction.

Fortunately, we no longer have to break fossils open to see what's inside.

"What's really exceptional about the Gogo fishes is that their soft tissues are preserved in three dimensions," says paleontologist Per Ahlberg of Uppsala University in Sweden.

"Most cases of soft-tissue preservation are found in flattened fossils, where the soft anatomy is little more than a stain on the rock. We are also very fortunate in that modern scanning techniques allow us to study these fragile soft tissues without destroying them. A couple of decades ago, the project would have been impossible."

With help from scientists at the Australian Nuclear Science and Technology Organisation and the European Synchrotron Radiation Facility in France, the team used neutron beams and synchrotron X-ray imaging to map the different mineral densities inside the fossil. These densities revealed not just the preserved bones of the arthrodire, but other less robust features – a spectacular heart, as well as a stomach, intestine and liver.

The scan revealed incredible details about the heart, including ventricles and atria.

The presence of the other organs allowed the team to study the anatomical layout of the fish.

"For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us," Trinajstic says.

"However, there was one critical difference – the liver was large and enabled the fish to remain buoyant, just like sharks today. Some of today's bony fish such as lungfish and birchers have lungs that evolved from swim bladders but it was significant that we found no evidence of lungs in any of the extinct armored fishes we examined, which suggests that they evolved independently in the bony fishes at a later date."

Previously excavated fossil specimens from the Gogo Formation allowed paleontologists to reconstruct and understand the musculature of the Gogo arthrodires. In addition, arthrodire embryos have been discovered in the formation.

The new specimen suggests that there might be even more treasures waiting to be uncovered in the Australian outback.

"These new discoveries of soft organs in these ancient fishes are truly the stuff of palaeontologists' dreams, for without doubt these fossils are the best preserved in the world for this age," says paleontologist John Long of Flinders University in Australia.

"They show the value of the Gogo fossils for understanding the big steps in our distant evolution. Gogo has given us world firsts, from the origins of sex to the oldest vertebrate heart, and is now one of the most significant fossil sites in the world. It's time the site was seriously considered for world heritage status."

The research has been published in Science.