Researchers find the oldest heart in vertebrates using X-rays and neutrons

15-09-2022

An international team led by Curtin University (Australia) has discovered a 380-million-year-old heart of a vertebrate – the oldest ever found – alongside a separate fossilised stomach, intestine and liver in an ancient jawed fish, partly using the ESRF’s X-rays. The results shed new light on the early evolution of living jawed vertebrates, which includes the mammals and humans.

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The new research, published today in Science, found that the position of the organs in the body of placoderms - extinct armoured fishes that flourished through the Devonian period from 419.2 million years ago to 358.9 million years ago - is similar to modern shark anatomy, offering vital new evolutionary clues.

Lead researcher John Curtin Distinguished Professor Kate Trinajstic, from Curtin’s School of Molecular and Life Sciences and the Western Australian Museum, said the discovery was remarkable given that soft tissues of ancient species were rarely preserved and it was even rarer to find 3D preservation. “As a palaeontologist who has studied 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,” Trinajstic says.

“Evolution is often thought of as a series of small steps, but these ancient fossils suggest there was a larger leap between jawless and jawed vertebrates. These fish literally have their hearts in their mouths and under their gills - just like sharks today.”

This research presents – for the first time – the 3D model of a complex S-shaped heart in a placoderm that is made up of two chambers with the smaller chamber sitting on top. Trinajstic said these features were advanced in such early vertebrates, offering a unique window into how the head and neck region began to change to accommodate jaws, a critical stage in the evolution of our own bodies.

“For the first time, we can see all the organs together in a primitive jawed fish, and we were especially surprised to learn that they were not so different from us,” Trinajstic explains. “However, there was one critical difference – the liver was large and enabled the fish to remain buoyant, just like sharks today. Some of today’s bony fish such as lungfish and bichirs have lungs that evolved from swim bladders but it was significant that we found no evidence of lungs in any of the extinct armoured fishes we examined, which suggests that they evolved independently in the bony fishes at a later date.” The Gogo Formation, in the Kimberley region of Western Australia where the fossils were collected, was originally a large reef.

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This is what Gogo fish looked like. Credits: Bruce Currie, Paleozoo.

 

 With the help of scientists at the European Synchrotron in France and the Australian Nuclear Science and Technology Organisation in Sydney, researchers used synchrotron X-rays and neutron beams to scan the specimens, still embedded in the limestone concretions. Researcher and co-author Sophie Sanchez, from Uppsala University and visiting scientist at the ESRF, explains the role of the facility in the research: “Fossilised soft tissues are remnants of organic matter. It is extremely challenging to reveal fossilised organic matter with X-rays, as they barely absorb light particles. To counter this, we used a protocol we had newly developed at that time, at the ESRF. This protocol enabled the enhancement of the contrast between the fossilised organs and the revelation of their three-dimensional organisation”. Scientists then constructed 3D images of the soft tissues inside the concretions, based on the different densities of minerals deposited by the bacteria and the surrounding rock matrix.

This new discovery of mineralised organs, in addition to previous finds of muscles and embryos, makes the Gogo arthrodires the most fully understood of all jawed stem vertebrates and clarifies an evolutionary transition on the line to living jawed vertebrates, which includes the mammals and humans. Co-author Professor John Long, from Flinders University, says: “These new discoveries of soft organs in these ancient fishes are truly the stuff of palaeontologists’ dreams, for without doubt these fossils are the best preserved in the world for this age. They show the value of the Gogo fossils for understanding the big steps in our distant evolution. Gogo has given us world firsts, from the origins of sex to the oldest vertebrate heart, and is now one of the most significant fossil sites in the world. It’s time the site was seriously considered for world heritage status.”

Co-author Professor Per Ahlberg, from Uppsala University, adds: “What's really exceptional about the Gogo fishes is that their soft tissues are preserved in three dimensions. Most cases of soft-tissue preservation are found in flattened fossils, where the soft anatomy is little more than a stain on the rock. We are also very fortunate in that modern scanning techniques allow us to study these fragile soft tissues without destroying them. A couple of decades ago, the project would have been impossible.”

Reference:

‘Exceptional preservation of organs in Devonian placoderms from the Gogo lagerstätte’, Science, 15th September 2022. DOI 10.1126/science.abf3289

Top image: The Gogo fish fossil where the 380-million-year-old, 3D preserved heart was discovered by researchers. Credit: Curtin University