Scientists have discovered, for the first time, a fossilised heart in a 119 million-year-old fish from Brazil called Rhacolepis. This has been possible thanks to the powerful X-rays of the ESRF, the European Synchrotron Radiation Facility, Grenoble, France. This breakthrough can provide clues about cardiac evolution.
⇐cover image: A representation of the fish Rhacolepis buccalis. Credits: D. Silva and F. Tadeu
The evolution of the chambered heart is a key point in the history of vertebrates. However, there have never been clues to elucidate this million-year-old process, since no cardiac structure of vertebrates was ever described in the fossil record. This is partly because the heart is made from soft muscle tissue, which normally decays fast after death and is very hardly preserved, unlike hard tissues such as bones. The discovery of a 3D fossilised heart totally preserved in a fossil is a major breakthrough.
An international team of researchers from Brazil, Sweden and the ESRF tested around 60 fossils from the fish Rhacolepis buccalis, which lived during the Cretaceous in a region of the planet that today is Brazil. In two of the samples analysed, the team discovered fossilised hearts. The preservation of this organ allowed the observation of the heart architecture in this extinct species, notably the conus arteriosus, a conical extension of the ventricle that helps regulate blood outflow via valves. While most living ray-finned fishes have one valve row in the conus (except in the basal-most clade, Elopomorpha, which bears two valve rows), the heart of Rhacolepis comprises five of them. Multiple valves are usually characteristic of basal, non-teleost, lineages and can be observed in living bowfin and garpike, as well as sharks. This intermediate state gives for the first time some insight about this reduction of the valve number during the evolution of the heart in ray-finned fish, the largest group of vertebrates alive today with nearly 30,000 species. Although this is an important key to the understanding of heart evolution in fishes, more observations like this are necessary to get a full comprehension of this evolutionary story.
The use of the ESRF was vital to the discovery of this fossilised heart. “The combination of large beam, high energy and high coherence made it the perfect tool for us”, explains Vincent Fernández, scientist at the ESRF. They used X-ray microtomography, which images the fossil in thin sections; these images can then be processed to render the heart slice by slice and digitally restore the features of the organ.
The corresponding author of the paper, José Xavier-Neto hopes that “this work will stimulate researchers all over the world to place their best-preserved fossils under synchrotron light to find further cardiac fossils, so we can rapidly fill up the gaps in their fascinating evolutionary history».
In an insight article published with the scientific paper, John A. Long, palaeontologist at Flinders University in Australia, defines the new finding as the “a bit of a Holy Grail for palaeontology”. And he adds that: “Such discoveries can contribute a wealth of new anatomical information that is essential for understanding evolutionary patterns”.
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Heart fossilization is possible and informs the evolution of cardiac outflow tract in vertebrates
Elucidating cardiac evolution has been frustrated by lack of fossils. One celebrated enigma in cardiac evolution involves the transition from a cardiac outflow tract dominated by a multi-valved conus arteriosus in basal actinopterygians, to an outflow tract commanded by the non-valved, elastic, bulbus arteriosus in higher actinopterygians. We demonstrate that cardiac preservation is possible in the extinct fish Rhacolepis buccalis from the Brazilian Cretaceous. Using X-ray synchrotron microtomography, we show that Rhacolepis fossils display hearts with a conus arteriosus containing at least five valve rows. This represents a transitional morphology between the primitive, multivalvar, conal condition and the derived, monovalvar, bulbar state of the outflow tract in modern actinopterygians. Our data rescue a long-lost cardiac phenotype (119-113 Ma) and suggest that outflow tract simplification in actinopterygians is compatible with a gradual, rather than a drastic saltation event. Overall, our results demonstrate the feasibility of studying cardiac evolution in fossils.
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