Fossilised organs from 380-million-year-old fish help scientists get to the heart of our own evolution

The world’s oldest fossilised heart has been found in an armoured fish dug up at the Gogo Formation in Western Australia’s Kimberley region. Alongside the 380-million-year-old heart were separate fossils of a stomach, intestine and liver.

Researchers involved in analysing the fossils believe that these ancient, jawed fish organs can tell us about how our own bodies evolved.

The research, published in the journal Science, looks at the fossilised organs of an “arthrodire” body. Arthrodires are an extinct class of armoured fish, or placoderm, that were common in the Devonian period of Earth’s geological history which lasted from around 419 million years to 359 million years ago.

Positioning of the organs in the Devonian fish’s body resemble the anatomy of modern sharks.

Though arthrodires spent all their time in the water, it is believed that such jawed fish were the first vertebrates (animals with an internal spine) to take steps onto land. Hence, they can inform how all modern land vertebrates from birds and lizards to humans and other mammals evolved.

Arthrodires came in a variety of shapes and sizes from diminutive specimens only 15 centimetres long, to the massive 6-metre-long superpredator Dunkleosteus, which could chew through bone like a hot knife through butter.

Lead researcher Professor Kate Trinajstic, from Curtin University and the Western Australian Museum, says the discovery of the fossil of soft tissues – which are rarely preserved – in such an old specimen is remarkable.

“As a palaeontologist 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 Trinajstic. “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.”

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For the first time, this research shows a 3D model of a complex S-shaped arthrodire heart made up of two chambers with the smaller chamber on top. The find also shows that these fish were yet to evolve lungs, suggesting that this developed in later placoderms.

As these fish began to evolve jaws, it seems their organs also changed to accommodate changes in their neck and head regions. This is a critical stage in the evolution of our own bodies.

 “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 adds. “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 birchirs have lungs that evolved from swim bladders but it was significant that we found no evidence of lungs in any of the extinct armoured fishes, which suggests that they evolved independently in the bony fishes at a later date.”

In the Devonian, the Kimberley region would have been Australia’s first Great Barrier Reef, says co-author and Flinders University palaeontologist Dr Alice Clement.

“During the Devonian Period this site was a warm, shallow, tropical reef teeming with life,” Clement explains.

“There is still evidence of atolls and fringing reef today. There are more than 50 species of ancient fish described from the site including a range of placoderms, as well as some small, primitive ray-finned fish and the most species of lungfish known from anywhere in the world. There were also invertebrates such as “sea scorpions” (eurypterids) and other primitive crustaceans called phyllocarids and concavicarids. Rather than being made of coral, the reef itself was built by stromatoporoid sponges and tabulate corals.”

The palaeontologists were aided by scientists from the Australian Nuclear Science and Technology Organisation in Sydney and the European Synchrotron Radiation Facility in France, to direct neutron beams and synchrotron x-rays to scan the specimens still embedded in the limestone concretions.

From the scans, the team was able to construct three-dimensional images of what was once soft tissue.

“Advances in scanning technology have revolutionised palaeontology in recent years,” Clement adds. “As scanning technology becomes more powerful and capable of imaging large and irregular specimens, more and more secrets from fossils are revealed from within. The synchrotron imaging reveals differing densities from within a sample, whereas the neutron beam images fossils differently depending on the chemical composition of the sample.”

Previous digs from the region have revealed muscles and embryos of the fish. With the most recent discovery of fossilised organs make the Gogo arthrodires most fully understood of all the jawed fish from the period.

“They show the value of the Gogo fossils for understanding the big steps in our distant evolution,” says co-author Professor John Long, also from Flinders. “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.”

“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,” adds co-author Professor Per Ahlberg, from Uppsala University in Sweden.

Clement says the team aims to answer some of the “big questions in evolution,” such as early brain evolution in fishes, the evolution of terrestriality (of the first land animals, the tetrapods) and the origins of the vertebrate body plan.