The oldest heart in the world preserved in 380 million years of armored fish

Researchers have discovered a 380 million-year-old heart – the oldest ever – along with a separate fossilized stomach, intestines and liver in an ancient-jawed fish, shedding new light on the evolution of our bodies. Credit: Alice Clement / Curtin University

A team of Australian scientists have discovered the world’s oldest heart, which is part of the fossilized remains of an armored fish that died about 380 million years ago. The fish also had a fossilized stomach, liver, and intestine. All organs are arranged much like similar organs in modern shark anatomy, according to a recent research paper published in Science.

As mentioned earlier, most fossils are bones, shells, teeth, and other forms of “hard” tissue, but sometimes fossils that preserve soft tissues such as skin, muscles, and organs – or even the occasional eyeball. This could tell scientists so much about aspects of biology, ecology, and evolution of such ancient organisms that skeletons alone could not 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 previously unobserved internal muscles. Among other findings, the researchers observed paired muscles extending from the ammonites’ body, which they thought the animal used to pull itself further into its shell to avoid predators.

And last month, British researchers described their experiences observing the carcasses of seabass as they rotted over a period of 70 days to gain insight into how (and why) the soft tissues of internal organs can be selectively preserved in the fossil record. One of the best ways soft tissues can turn into rocks is when they are replaced by a mineral called calcium phosphate (sometimes called apatite). Specifically, the muscles, stomach, and intestines tend to “phosphate” more frequently than other organs such as the kidneys and gonads. The authors conclude that the phosphorous content in tissues of certain organs contributes to this unusual selection bias for which soft tissues are preserved in the fossil record.

Zoom / Articular endoderm fossil from the Gogo Formation in Australia where the 380 million-year-old mineral core was discovered.

Jasmine Phillips / Curtin University

The fossilized specimens examined in this latest research were collected from the Gogo Formation in Western Australia, which was once a coral reef and is rich in 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 excavated a new type of placoderm that they named mother fish (“mother fish”), with an embryo still attached to an umbilical cord – evidence that at least some species of armored fish gave birth to well-developed living offspring.

According to the authors of this latest research paper, plaque skin was among the oldest jawed vertebrates, the evolution of which involved significant changes in skeletal structure and soft anatomy. Due to the scarcity of soft tissue preservation in the fossil record, specimens collected in the Gogo Formation (and now in the public collections of the Western Australian Museum and Victoria Museum) can hold clues about how this shift occurred—specifically how the head and neck region changed to accommodate the jaws. .

Devonian articular endoderm reconstruction.
Zoom / Devonian articular endoderm reconstruction.

Trinajstic et al., 2022

“The really exceptional thing about the Gogo fish is that its soft tissues are preserved in three dimensions,” said co-author Per Ahlberg of Uppsala University. More than just a stain on the rock. We are also very fortunate that modern scanning techniques allow us to study these fragile soft tissues without destroying them. Two decades ago, the project was impossible.”

Paleontologists collected the samples by splitting limestone concretes in the field, then gluing the broken pieces together for transportation. The researchers were able to scan healthy samples using neutron beams and synchrotron radiation. Next, they built 3D images of the soft tissue preserved within it based on different densities of minerals deposited by bacteria and the matrix surrounding the rocks.

Artistic representation of the now-extinct armored fish to which the 380 million year old heart belongs.
Zoom / Artistic representation of the now-extinct armored fish to which the 380 million year old heart belongs.

Curtin University

The result: the first 3D model of a complex, flat, S-shaped heart with two separate chambers. The team also photographed a thick-walled stomach with a healthy intestine and liver separated from the heart. They also note the absence of lungs. The fossilized liver was very large and likely helped the fish to stay thriving, according to the authors. It is the first time that scientists have been able to see the arrangement of organs inside a rudimentary fish.

“As a paleontologist who has studied fossils for more than 20 years, I was truly amazed to find the beautifully preserved 3D heart of a 380 million-year-old ancestor,” said co-author Kate Triangstick, 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 much bigger leap between jawless vertebrates and jaws. 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).

Menu photo by Jasmine Phillips / Curtin University

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