World's oldest heart preserved in armored fish 380 million years old

World’s oldest heart preserved in armored fish 380 million years old

Researchers have discovered a 380 million year old heart – the oldest ever found – alongside a separate fossilized stomach, intestine and liver in an ancient jawed fish, shedding new light on the evolution of our own body. Credit: Alice Clement/Curtin University

A team of Australian scientists has discovered the world’s oldest heart, part of the fossilized remains of an armored fish that died around 380 million years ago. The fish also had a fossilized stomach, liver and intestine. All of the organs were laid out much like similar organs in modern shark anatomy, according to a recent paper published in the journal Science.

As noted earlier, most fossils are bones, shells, teeth, and other forms of “hard” tissue, but occasionally fossils are discovered that preserve soft tissue such as skin, muscle, the organs – or even the occasional eyeball. This can tell scientists a lot about aspects of the biology, ecology and evolution of these ancient organisms that skeletons alone cannot convey.

For example, earlier this year, researchers created a highly detailed 3D model of a 365 million year old ammonite fossil from the Jurassic period by combining advanced imaging techniques, revealing internal muscles that n had never been observed before. Among other findings, the researchers observed paired muscles extending from the ammonite’s body, which they speculate the animal used to retract further into its shell to avoid predators.

And last month, British researchers described their experiments monitoring the carcasses of dead sea bass for 70 days to better understand how (and why) the soft tissues of internal organs can be selectively preserved in the fossil record. One of the best ways soft tissue can turn into rock is to replace it with a mineral called calcium phosphate (sometimes called apatite). Specifically, the muscles, stomachs, and intestines tend to “phosphate” much more frequently than other organs like the kidneys and gonads. The authors concluded that the phosphorus content of specific organ tissues contributes to this unusual selection bias for which soft tissues are preserved in the fossil record.

Enlarge / The arthrodire placoderm fossil from the Gogo Formation in Australia where the 380 million year old mineralized core was discovered.

Yasmine Phillips/Curtin University

The fossilized specimens examined in this latest article were collected from the Gogo Formation in Western Australia, which was once a reef and is rich in exceptionally well-preserved Devonian fossils, such as the class of prehistoric armored fish known as placoderms. This preservation includes soft tissues, including nerves. In 2005, paleontologists even excavated a new species of placoderm, dubbed mother fish (“mother fish”), with an embryo still attached by an umbilical cord – evidence that at least some species of armored fish gave birth to well-developed live offspring.

According to the authors of this latest paper, placoderms were among the first jawed vertebrates, whose evolution involved significant changes in skeletal structure and soft anatomy. Because soft tissue preservation is so rare in the fossil record, samples collected at the Gogo Formation (and now housed in the public collections of the Western Australian Museum and the Museum of Victoria) may hold clues to how this transition occurred, in particular, how the head and neck region changed to accommodate the jaws.

Reconstruction of an arthrodiral placoderm from the Devonian.
Enlarge / Reconstruction of an arthrodiral placoderm from the Devonian.

Trinajstic et al., 2022

“What’s really exceptional about Gogo fish is that their soft tissues are three-dimensionally preserved,” said co-author Per Ahlberg from Uppsala University. “Most cases of soft tissue preservation are found in flattened fossils, where the soft anatomy is poorly developed. It is more than a stain on rock. We are also very lucky in that modern scanning techniques allow us to study these fragile soft tissues without destroying them. A few decades ago, the project would have been impossible.”

Paleontologists collected the samples by splitting calcareous concretions in the field, then gluing the broken pieces together for transport. The researchers were able to scan the intact samples using neutron beams and synchrotron radiation. Then they constructed 3D images of the soft tissue preserved inside based on the different densities of minerals deposited by the bacteria and the surrounding rock matrix.

Artist's rendering of the now-extinct battleship fish to which the 380-million-year-old heart once belonged.
Enlarge / Artist’s rendering of the now-extinct battleship fish to which the 380-million-year-old heart once belonged.

Curtin University

The result: the first 3D model of a complex flat S-shaped heart with two distinct cavities. The team also imaged a thick-walled stomach with intact intestines and a liver, separated from the heart; they also noted the absence of lungs. The fossilized liver was quite large and likely helped the fish stay buoyant, the authors said. This is the first time scientists have been able to see the arrangement of organs inside a primitive jawed fish.

“As a paleontologist who has studied fossils for over 20 years, I was truly amazed to find a beautifully preserved, 3D heart in a 380 million year old ancestor,” said co-author Kate Trinajstic, a vertebrate paleontologist at Curtin University. . “Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a bigger jump between jawless and jawed vertebrates. These fish literally have their hearts in their mouths and under their gills, just like sharks today.

DOI: Science, 2022. 10.1126/science.abf3289 (About DOIs).

List image by Yasmine Phillips/Curtin University

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