Talk about a stone-cold heart. A team of researchers discovered the first-ever fossilized heart found inside a fish, which could shed more light on the species' evolution.

Although paleontologists are making huge strides in learning and unraveling the mysteries of fossils of eons ago, they still face some huge challenges thanks to massive growth of science and technology. 

For example, it's difficult to study fossils of soft tissue or organs like the heart or the liver. Unlike bones, these parts of the body decay very quickly especially with the rapid interaction with bacteria. What's left of the organism is what becomes part of the fossil.

With the discovery of the fossilized heart, however, Brazilian and European researchers have shown that it is already possible. Not only that, but with the fossil being very old, it may offer more information on the evolution of the ray-finned fish, the biggest species of marine vertebrates to date.

The Discovery

The fossilized heart was part of Rhacolepis buccalis found in the Santana Formation in Brazil. The fish comes from an already extinct family and is believed to have existed between 113 and 119 million years ago based on spores or pollen embedded on the rock layers.

Using a non-invasive imaging technique called the synchrotron X-ray tomography, the researchers were able to obtain different "slices" of the components of the organ, which are then reconfigured until they form the fish's heart.

Because they already had a clear view of the heart, the researchers used a tool to study it and saw something unusual: while the heart already resembled that of the more recent species due to the presence of valves in the major artery, it also had valve structures similar to the older ones. To be more specific, this fish actually possessed five valves, far too many than what can be found in new fish vertebrates.

What This Means In Fish Evolution

Previously, researchers have come across blood cells in dinosaur bones, but they actually appeared to be much younger than the fish. Some have also found soft-bodied worms, but they are invertebrates.

The fish's heart, which may have been fossilized because of "rapid burial under special chemical conditions," is therefore one of the oldest soft tissue fossils discovered in a vertebrate.

These types of fossils don't come often, so "the find demonstrates the immense potential for more discoveries of this nature, enabling more discussion of the comparative anatomy of soft organs in extinct organisms and how they have evolved through time," wrote John Long, strategic professor in paleontology of Flinders University.

The study is now featured on eLife Sciences.