ABOVE: Reconstruction of a jawed armored fish (order Arthodira) From Trinajstic et al. 2022

Three hundred and eighty million years ago, the Gogo Lagerstätte was one of the first coral reefs. Now, this massive outcrop lies in the middle of the Australian desert. Its limestone crags border an expanse of black, silty soil, which contains immaculately preserved fossils from the Devonian period.

Since 1940, researchers have collected fossils of ancient fish hidden inside the rocks scattered throughout Gogo. A few decades ago, they made a surprising discovery: The oxygen-poor environment of the ancient oceans not only preserved the bones of early vertebrates, it also preserved their soft tissue and organs. And in a paper published in Science today (September 15), scientists in Australia report that they have, for the first time, visualized the soft organs inside multiple specimens of our early jawed ancestors. To their surprise, they found striking similarities between the internal body plan of these ancient vertebrates and modern-day animals, including humans.

Most frequently, what’s preserved in the fossil record is bone. “You only get occasional instances of preservation of soft tissue,” explains Michael Coates, a paleontologist at the University of Chicago who was not involved in the study. “This is the first time researchers are pulling together a lot of soft tissue information to think about the reorganization of the heart and other tissues to the front end of the body.”

The study focuses on jawed fish within the extinct Arthrodira order, specifically Bothriolepis compagopiscis and Bothriolepis incisoscutum. While they were alive, the fish lived and foraged in shallow water. Once they died, the tide swept them further out to sea to oxygen-deprived, bacteria-rich depths. There, bacteria coated them inside and out with a thick slime, which attracted calcium carbonate deposits over time, preserving them. “The dying bacteria replicates the shape of the organ,” explains study coauthor Kate Trinajstic, a vertebrate paleontologist at Curtin University.

Kate Trinajstic examining a fossil at Curtin University
Credit: Adelinah Razali

Those carbonate shells were no match for the weak acid that researchers used for decades to dissolve the rock and uncover the bones within fossils, a technique Trinajstic says inadvertently destroyed any soft tissue that had been preserved. But “in 2000, one of my colleagues noticed some little white fragments and looked at it under a microscope and realized it was muscle.” It took her team 13 years to find enough muscle to map the musculature of one fish, a feat that became possible thanks to the European Synchrotron facility, which allowed the researchers to image the samples without using acid, says Trinajstic.

The researchers analyzed the shape of the organs using propagation phase contrast synchrotron x-ray microtomography (PPC-SRmCT) and neutron tomography, techniques for displaying detailed cross-sections of the insides of solid objects.

Trinajstic recalls feeling shock when she and her colleagues first looked through the scans at another synchrotron facility, the Australian Nuclear Science and Technology Organisation, and saw a preserved heart of a 380-million-year-old fish. “There was this incredible sense of wonder when I saw it,” she says. “I never in my wildest dreams thought I’d see a heart so well preserved in something so old.”

In addition to the heart, the team also identified the liver and digestive tract of multiple specimens of these early jawed vertebrates, which allowed them to draw comparisons to modern-day and other extinct fishes.

I never in my wildest dreams thought I’d see a heart so well preserved in something so old.

—Kate Trinajstic, Curtin University

The heart of modern-day jawless fish like lampreys is thought to be very much like that of the jawless fishes that were around in the Devonian period and earlier. Jawless fishes have a heart with chambers that sit side by side, a relatively small liver, and a long, unsegmented digestive tract. The study found that jawed fish, which evolved from jawless ancestors, seem to have simultaneously developed several modern traits early on in their history. In addition to jaws, they also developed a unique, S-shaped heart where one chamber sits on top of the other, which was repositioned to the back of the throat just beneath the gills. In lampreys, hearts are positioned further back in the body, behind the gills. These ancient jawed fish also had a large, buoyant liver, a layered stomach, and a digestive tract with multiple chambers, all of which resemble those of modern fish.

Limestone fossil of fish.
The Gogo fish fossil where the 380-million-year-old preserved heart was found

“I think the expectation is that evolution is slow and stepwise,” says Trinajstic, but these findings show that some features that we see in modern vertebrates all came about at roughly the same time. “What we’re seeing is that . . . when there’s an innovation, it tends to be not one thing, it tends to be a whole cascade of things bursting together.”

Trinajstic says that in some specimens, they found remnants of the fish’s last meal inside of its digestive tract. Previously, the researchers had also found embryos, indicating that these fish gave birth to their young instead of laying eggs. But Trinajstic and her team did not find lungs in these arthrodires, providing evidence against the theory that lungs evolved early on in fish. Sharks, which also have an ancient lineage, don’t have lungs, while modern-day bony fish do. “One of the big questions was whether or not lungs were a primitive characteristic,” Trinajstic says. Previously, scientists weren’t sure whether or not sharks once had lungs but lost them sometime in evolutionary history. “This has actually shown that certainly one of our earliest jawed vertebrates, the placoderms, don’t have lungs,” she says, suggesting that sharks may have never had them in the first place.

Trinajstic says that the findings now allow researchers to distinguish between competing hypotheses about the phylogeny of early jawed fish, showing that the internal body plan of modern-day sharks is closer to that of Bothriolepis than other fish from the placoderm order.

Coates says he’s less surprised to see a large liver and segmented intestinal tract in jawed fish, but says that the shape and position of the heart in these early jawed vertebrates is “something we haven’t been able to look at before. . . . It’s remarkable stuff.”

Trinajstic says that going forward, she’d like to look at other placoderm specimens and hunt for other organs. “I’ve got some now with heads, and I can see that they’ve got their eyes there. The next step is, do I have a brain in there somewhere?”