By | September 21, 2022
World's oldest heart preserved in 380 million-year-old armored fish

Scientists 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 bodies. Credit: Alice Clement/Curtin University

A team of Australian researchers has discovered the world’s oldest heart, 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 the organs were arranged much like similar organs in modern shark anatomy, according to a fresh newspaper published in the journal Science.

As we have previously reported, most fossils are bones, shells, teeth, and other forms of “hard” tissue, but occasionally fossils are discovered that preserve soft tissues such as skin, muscles, organs—or even the occasional eyeball. This can tell researchers much about aspects of the biology, ecology and evolution of such ancient organisms that skeletons alone cannot convey.

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

And last monthBritish researchers described his experiments monitoring carcasses of dead sea bass as they decayed over the course of 70 days to gain insights into how (and why) soft tissues of internal organs can be selectively preserved in the fossil record. One of the best ways that soft tissue can turn into stone is when it is replaced by a mineral called calcium phosphate (sometimes called apatite). Specifically, muscles, stomachs and intestines tend to “phosphatize” much more often than other organs such as kidneys and gonads. The authors concluded that the phosphorus content of specific organ tissue contributes to this unusual selection bias for which soft tissues are preserved in the fossil record.

Arthrodire placoderm fossil from the Gogo Formation in Australia where the 380 million year old mineralized heart was discovered.
Magnify / Arthrodire placoderm fossil from the Gogo Formation in Australia where the 380 million year old mineralized heart was discovered.

Yasmine Phillips/Curtin University

The fossilized specimens examined in this latest paper 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 armored prehistoric fish known as placoderms. The preservation includes soft tissues, including nerves. In 2005, paleontologists even unearthed a new species of placoderm, called Materpiscis (“mother fish”), with an embryo still attached by an umbilical cord—evidence that at least some species of armadillo gave birth to well-developed live offspring.

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

Reconstruction of a Devonian arthrodire placoderm.
Magnify / Reconstruction of a Devonian arthrodire placoderm.

Trinajstic et al., 2022

“What’s really exceptional about the Gogo fish is that their soft tissues are preserved in three dimensions,” said co-author Per Ahlberg at Uppsala University. “Most cases of soft tissue preservation are found in flattened fossils, where the soft anatomy is little more than a speck on the rock. We are also very fortunate because modern scanning techniques allow us to study these delicate soft tissues without destroying them. For a a couple of decades ago the project would have been impossible.”

Paleontologists collected the samples by splitting limestone concretions in the field and then taping 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 tissues preserved within based on the different densities of minerals deposited by bacteria and the surrounding matrix of rock.

Artist's representation of the now-extinct armadillo to which the 380-million-year-old heart belonged.
Magnify / Artist’s representation of the now-extinct armadillo to which the 380-million-year-old heart belonged.

Curtin University

The result: the first 3D model of a complex, flat s-shaped heart with two distinct chambers. 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 afloat, according to the authors. It is the first time scientists have been able to see the arrangement of the organs inside a primitive jawed fish.

“As a paleontologist who has been studying fossils for more than 20 years, I was truly amazed to find a 3D and beautifully preserved heart in a 380-million-year-old ancestor,” said co-author Kate Trinajstic, a vertebrate palaeontologist at Curtin University. “Evolution is often seen as a series of small steps, but these ancient fossils suggest that there was a bigger leap 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 DOI).

List image by Yasmine Phillips/Curtin University

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