Researchers have unearthed a 380-million-year-old heart from a prehistoric fish—the oldest three-dimensionally preserved heart from a vertebrate ever found. They published their findings in the journal Science on Thursday.
It’s “a mind-boggling, jaw-dropping discovery,” John Long, a paleontologist at Flinders University in Australia and co-author of the paper, tells BBC News’ Pallab Ghosh. “We have never known anything about the soft organs of animals this old, until now.”
Soft tissues are rarely preserved as fossils. Usually, they decay or are eaten, leaving only bones, shells or teeth behind. But these specimens had preserved stomachs, intestines and livers alongside the heart, per the study.
The now-extinct fish were placoderms, or armored fish that are “our earliest jawed ancestors,” Kate Trinajstic, lead author and paleontologist at Curtin University in Australia tells Reuters’ Will Dunham. They existed from about 416 million to 359 million years ago.
Limestone in the Gogo Formation of Western Australia preserved these fossilized specimens, and in the past, researchers have found fish muscles and embryos preserved there, per a statement.
Scientists say these finds could help us better understand our own evolution. The team scanned the fossils using neutron beams and synchrotron X-rays and discovered that the placoderms had surprising similarities to modern-day sharks—and to humans.
“For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us,” Trinajstic says in the statement. “However, there was one critical difference—the liver was large and enabled the fish to remain buoyant, just like sharks today.”
Some present-day fish, like lungfish and bichirs, have lungs that evolved from swim bladders that help keep them afloat, per the statement. But the team didn’t find evidence of lungs in the placoderms, suggesting that bony fish evolved lungs later and independently, write Long and Trinajstic in The Conversation.
The placoderms had two-chambered, S-shaped hearts that were located under the gills. Trinajstic tells Michael Ramsay of the Australian Associated Press she was surprised to see a two-chambered heart in such an early fish.
“Here we are, right at the beginning of the jawed vertebrates, and that step's already there,” she tells the publication. Having two chambers likely made the heart more efficient, allowing these fish to become quick predators, per the BBC.
Placoderm expert Zerina Johanson of London’s Natural History Museum, who wasn’t involved in the study, tells the BBC that the researchers have made an "extremely important discovery.”
“There are many things going on in these placoderms that we see evolving to ourselves today such as the neck, the shape and arrangement of the heart and its position in the body,” she tells the publication.
“It is very exciting indeed,” José Xavier Neto, who was part of a team that discovered the first-ever fossilized vertebrate heart in 2016, tells Scientific American’s Stephanie Pappas. “Only six years after our initial discovery, it’s great to see that other groups are also being able to report on fossil hearts. The field is really new, and we really need high-quality data.”