Did these fossils have a change of heart? Extremely well-preserved fossils and detailed imaging technology has allowed a team of scientists from Brazil, Sweden, and France to peer inside of a 113 million year old fish and view the oldest vertebrate chambered heart ever discovered. This research is presented this week in the journal eLife lead by co-first authors Lara Maldanis and Murilo Carvalho at the Brazilian Biosciences National Laboratory in Campinas, Brazil.
Research into the evolution of organ systems that are mainly comprised of soft parts is often stymied by the fact they are not easily preserved in the fossil record—although recent close examination of specimens with advanced imaging and microscopy techniques shows these structures may be preserved more often than previously though. Last year, for example, researchers in China were able to uncover a 540 million year old example of a central nervous system in a fossilized arthropods.
Up until this point though, no diagnosable heart tissue was ever found in a vertebrate. Some may remember the dinosaur called Willo on display in the North Carolina Museum of Natural Sciences that, at first, was said to possess a fossilized heart in its chest cavity. This claim was later debunked in 2011—the heart is merely an iron rich concretion of environmental origin.
This is why the find of a fossilized vertebrate heart in two specimens of a Cretaceous fish from Brazil, Rhacolepis buccalis, is so remarkable. Studying cardiac evolution in vertebrates is nearly impossible. The heart, such a critically important organ to the existance of the full range of vertebrate life, typically can only be observed from its modern-day state. Now, the heart of Rhacolepis can be viewed in great detail thanks to the synchrotron, a futuristic-sounding X-ray scanner that has revolutionized what paleontologists can learn about previously hidden structures in fossils.
The two specimens of Rhacolepis in this study were found in nodules, meaning they were almost fully encased in rock. This makes the fossils hard to study up close, but also preserves them in full 3D glory. Thanks to the synchrotron, Maldanis and Carvalho were even able to count the number of valve rows present in its heart. Rhacolepis belongs to the class of fish that makes up 99% of fish species called actinopterygians, or “ray-finned fishes”. Examining the heart of Rhacolepis allows the cardiac morphology of ray-finned fishes to be traced over millions of years of evolutionary history.