Back in the year 2000, news of a fossilised dinosaur heart stirred up surprise and excitement among experts and fossil fans. Unfortunately, the find was revealed to be a wishful mistake years later, and many of us sighed in disappointment and resigned ourselves to waiting for the real thing.

Well, there’s good news. A heart has finally been discovered – but not with the remains of a dinosaur. The very first fossilised heart found in any prehistoric animal belongs to a fish, a creature that lived many millions of years ago in an area that is now Brazil (scroll down for video). 

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Researchers used specialised scanning techniques to put together 3D models of the fish, complete with colour-coded organs. (Scroll down for video!) Image: 2016, Maldanis et al

In general, decomposition is fast and fossilisation is slow. All the squishy parts – skin, muscles, organs and the like – are usually decayed away by the time the tougher bones and teeth are converted to mineral. But under certain conditions, decomposition can be held off long enough for fossilisation to capture soft parts as well.

These exceptions are rare, but they never fail to make headlines. Preserved dinosaur skin and muscle, ancient fish nerves and blood vessels, and even a fossilized brain, are all proof that nature occasionally breaks the rules.

Enter Lara Maldanis, PhD student at the Brazilian Synchrotron Light Laboratory, and Murilo Carvalho of the Brazilian Biosciences National Laboratory. If brains and muscles could be fossilised, they reasoned, why not hearts? Maybe the answer was that scientists just weren’t looking hard enough. So, along with a team of colleagues, they decided to look harder.

Rhacolepisbuccalis Fish _2016_04_22
The Santana Formation in Brazil is famous for its beautifully preserved fish. Rhacolepis is particularly common, so it’s not surprising that this would be the species to surprise us with a fossil heart. Image: Wikimedia Commons.

The team turned to a fossil formation in Brazil, famous for its excellent preservation, and used a tongue-twisting technique called "X-ray synchrotron microtomography", which let them peek at the insides of fish fossils without damaging them.

Their search paid off. They found not one, but two fossil fish with mineralised hearts preserved inside, recorded in exquisite three-dimensional detail. Confirmed by high-tech scans and multiple experts, these are officially the first fossilised hearts ever discovered for a vertebrate animal.

These record-setting fish are called Rhacolepis, and they’re more than 113 million years old. That means they were swimming the seas of South America close to the same time that dinosaurian titans like Giganotosaurus and Argentinosaurus were stomping around on the land. Rhacolepis is an ancient member of the teleost group of fish, which comprises nearly all bony fish still alive today.

Scanning layer by layer through every millimetre of the fossils, the researchers were able to reconstruct 3D digital images of the hearts. These models display incredible details, including the different heart chambers, the pericardium membrane that surrounds those chambers and even the tiny valves that control blood flow.

And those details revealed a secret: this fish didn’t have the same kind of heart structure that most teleost fish have today. But it also didn’t have the structure we’d expect to see in the ancient ancestors of teleosts. The heart of Rhacolepis is in-between, a transitional state between the ancient version and the newer one. This little secret is a big deal for science: it’s our first-ever glimpse into the history of fish heart evolution.

The discovery has palaeontologists eager for more. "The find demonstrates the immense potential for more discoveries of this nature," says John Long of Flinders University. "Such discoveries can contribute a wealth of new anatomical information that is essential for understanding evolutionary patterns."

So what fishy parts will turn up next in fossilised form? Intestines? Eyeballs? How about some prehistoric fish liver? Eager researchers are on the case armed with state-of-the-art tools, so it might not be long before ancient organs are in the headlines again.


Top header image: 2016, Maldanis et al