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Heart of stone

New fossil record reveals an unforeseen pathway in the evolution of the cardiac muscle

A fish fossil of Rhacolepis buccalis collected at the Araripe Basin, with internal organs in an excellent state of preservation

Murilo de Carvalho / lnbio CT A fish fossil of Rhacolepis buccalis collected at the Araripe Basin, with internal organs in an excellent state of preservationMurilo de Carvalho / lnbio CT

Paleontology now has a heart. Brazilian researchers have found an organ preserved in the fossil of a fish that lived 115 million years ago in what is today the Northeast region of Brazil. For the first time in the paleontological record, a fossilized heart is being described  and, because of its excellent state of preservation, the petrified organ belonging to the fish Rhacolepis buccalis  has revealed to us a hitherto unknown stage in the evolution of the heart.

Three-dimensional images of the entire animal (measuring 13 cm in length) and its internal organs were produced from high-resolution CT scans.  To the researchers’ surprise, they found that the heart had five valves (one controlling the flow of blood throughout the rest of the body), as distinct from the single-valve fish of today.  “This shows that organisms don’t always become more complex as they evolve,” explains physician José Xavier Neto, a researcher at the National Biosciences Laboratory (LNBio) and coordinator for the group that discovered the fossilized heart. “In some cases, they become less so.”

The R. buccalis belongs to the Actinopterygii class of bony fishes, meaning those that are ray-finned.  Its heart appears to be at the evolutionary mid-point among the current species in this group, like the zebrafish (in Brazil, the paulistinha), which has a single cardiac valve, and other species (like the dozens of fish of the genus Polypterus) which have undergone little evolutionary change over the past 390 million years. “We don’t know the context in which this simplification occurred,” says Neto, who published the findings in the April 2016 edition of the journal eLife, “but it usually happens after what we know as a surge of complexity.” Also unknown is whether valve-loss was just a random change or amounted to a true evolutionary advantage for the species.

The 3D Fossil
Finding and describing a fossilized heart only became possible thanks to synchrotron-light technology, which has been making important contributions in the field of paleontology over the past few years. “Soft tissue, like the heart, is very hard to maintain itself in a state of preservation,” says paleontologist Mírian Pacheco of the Federal University of São Carlos (UFSCar) in Sorocaba, São Paulo. Pacheco also uses synchrotron light to study animal fossils (hard to find intact and mostly of invertebrates that lived approximately 540 million years ago) of the Ediacaran geological period.  Fossilized brains, ovaries, muscles, intestinal contents, umbilical cords and swim bladders have all been found, but, according to the researchers, never a heart.

AnthropoceneNeto joined the select group of researchers who described the fossilized invertebrates when he became convinced that he should focus his search on the R. buccalis a few years ago. While vacationing with his family in the south of their home state of Ceará, Neto spoke with geologists Francisco Idalécio Freitas, executive coordinator of Geopark Araripe, and José Artur de Andrade of the National Mineral Production Department, who advised him to focus his research on the R. buccalis. The species was very common to the Araripe Basin in the interior region of the states of Ceará, Pernambuco and Piauí, an area known for its well-preserved fossils of the Cretaceous period. When found, fossils of R. buccalis normally have the advantage of a three-dimensional form, thus increasing the likelihood that internal organs are preserved. In Campinas, biologists Laura Maldanis and Murilo de Carvalho prepared the samples and analyzed the material. Because the second-generation synchrotron at the National Synchrotron-Light Laboratory (adjacent to LNBio at the National Center for Energy and Materials Research, or CNPEM) did not have the capacity to produce images of samples (like those of the 15 x 5 cm fossilized fishes), the material was analyzed at the European Synchrotron Radiation Facility in Grenoble, France, using fourth-generation equipment capable of higher-energy beam generation.  “We weren’t able to see the heart cavities in the first CT scans,” Neto recalls, “but the resolution was so good that that we could see—in the fishes’ intestinal tracts—the shrimp that they had consumed.” Finally, two samples showed not only the heart, but also details of its inner region.

Such high degree of precision was possible because the synchrotron’s CT scan has a resolution almost 100 times greater than that of ordinary medical CT scanning devices. While conventional devices can distinguish points that are 500 micrometers apart, synchrotron light reduces this distance to six micrometers (each micrometer measures one one-thousandth of a millimeter). The device produces a series of “layers” of x-rays of the sample, creating a precise image of its internal “relief.” Software is then used to join the layers into a 3D image.  “The resulting image is almost as precise as seeing a dissected heart,” says Neto.  Yet another advantage of the synchrotron is that its light poses no danger of damaging the samples.

According to Pacheco, who is not a member of the research group, “this material can be analyzed again in terms of other characteristics.  The team’s achievements have placed Brazil at an international level of competition in the field of paleontology.” There is hope, if the present timetable is maintained, that Sirius—a new source of synchrotron light similar to the one in France—will be operational in Campinas so fossils and other materials can finally be analyzed in Brazil.

Molecular evolution of regulatory regions of HOX genes associated with the morphology of fish fins, with special emphasis on Chondrichthyes (nº 2012/05152-0); Grant Mechanism: Post-doctoral research grant; Principal Investigator: Marcelo Rodrigues de Carvalho – USP; Grantee: Murilo de Carvalho; Investment: R$255,270.00.

Scientific Article
MALDANIS, L. et al. Heart fossilization is possible and informs the evolution of cardiac outflow tract in vertebrates. eLife.  V. 5, No. e14698. April 19, 2016