The oldest chambered hearts have been discovered in fossils from Western Australia.
The two-chambered organs, which date back about 380 million years, are preserved within remarkable three-dimensional fossils of ancient, armored fish called placoderms, which were the first vertebrates to develop jaws more than 400 million years ago. These jawed fish represent an evolutionary leap toward the body plan present in most animals with a backbone today—including humans. The fossils reveal that it didn’t take long for evolution to land on this basic body plan: At this point in evolutionary history, the S-shaped heart in the placoderms was already well separated from the other organs, lodged near the newly evolved jaw. The heart’s separation from the abdominal organs is still seen today.
“A great deal of complexity occurred right at the beginning of our evolutionary history as jawed vertebrates,” says Kate Trinajstic, a paleontologist at Curtin University in Australia, who led the new research.
The fish fossils also contain livers and intestines, as well as stomachs that are so beautifully preserved that the folds of their lining are still visible. The organs are the oldest preserved in three dimensions in any jawed vertebrate. This kind of 3-D preservation is very rare, says study co-author Per Ahlberg, a paleontologist at Uppsala University in Sweden. The placoderms in the study, which grew to about 10 inches (25 centimeters) in length, once trawled an ancient reef made of sponges and stromatolites, sedimentary rocks deposited by microorganisms. When the fish died, some of their bodies washed off the reef into water layers containing very little oxygen and lots of hydrogen sulfide. The bacteria in these layers formed a biofilm around the bodies, which attracted minerals, encasing the fish in a protective crust that prevented them from becoming flattened like most fossils. “It’s kind of bizarre,” Ahlberg says. “You look at these guys, and they were probably semi-mineralized by the time they hit the seafloor.”
Trinajstic and her team had previously discovered mineralized muscle tissue in these same placoderm fossils. They did so using noninvasive techniques that image the fossils with beams of x-rays and neutrons. The researchers knew the fossils likely held much more. For the new study, published on September 15 in the journal Science, they analyzed the samples at the European Synchrotron Radiation Facility in Grenoble, France, and the Australian Nuclear Science and Technology Organisation. The imaging techniques at these facilities can show fine detail in structures that are only subtly different from the surrounding rock, says Lara Maldanis, a postdoctoral paleobiologist and physicist at the University of Grenoble Alpes, who was not involved in the new study.
For 380-million-year-old organs, the structures didn’t look so alien from modern anatomy. The intestines were spiraled like a cinnamon bun, and the stomach had a muscular layer and a layer of glands, indicating that the fish used digestive juices. The two-lobed liver was large and probably helped keep the fish buoyant, much like shark livers do today, Ahlberg says. The heart had two chambers, similar to the hearts of jawless vertebrates such as lampreys, but the chambers were stacked so that the atrium was toward the animal’s back and the ventricle was toward its chest. That’s a shift from the arrangement seen in older jawless vertebrates, where the chambers are oriented side by side, says Maldanis, who was part of a Brazilian team that reported the first-ever fossilized vertebrate heart in 2016.
Finding the heart is especially thrilling, says José Xavier Nero, chief health scientist of the Brazilian state of Ceará, who was the senior author of the 2016 study but was not involved in the new work. He and his team found their fossilized heart in a fish from Brazil dating back 113 million years.
“It is very exciting indeed,” Xavier Nero says. “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.”
Also exciting, Trinajstic says, is what apparently wasn’t there: lungs. Though it seems counterintuitive, fish were the first animals to evolve lungs. Some, like the appropriately named lungfish, still use them to breathe air. Others underwent an evolutionary overhaul and repurposed their lungs as swim bladders, air-filled pockets that provide buoyancy underwater. There was no sign of lungs in the placoderm fossils, Trinajstic says, suggesting that the development of these organs occurred in bony fish after they diverged from placoderms. (Cartilaginous fish, such as sharks, did not develop lungs and instead use their liver for buoyancy, as placoderms apparently did.)
The Australian rocks that preserved the placoderms also hold fossils of many other kinds of fishes, Ahlberg says, so future research will likely focus on comparing 3-D anatomy across species. And the placoderms may still hold more surprises. “I’d like to find a brain,” Trinajstic says.