Fossilized fish heart sheds light on vertebrate evolution –

The oldest known heart has just been discovered in a 380 million year old fish fossil discovered in Australia. The big surprise is that although this organ comes from a very distant ancestor, it is not so different from that of a human being.

In Australia, scientists have unearthed a beautifully preserved three-dimensional fossilized heart and other internal organs of ancient armored fish, a discovery that provides insight into the evolution of the body of vertebrates, including the To be human.

>> This is what a Late Devonian placoderm might have looked like:

Researchers have described the heart, the organ that pumps blood through the body’s circulatory system, in fish called placoderms, an extinct species that were among the very first vertebrate gnathostomes, or possessing a jaw. The Placodermi lived from the Silurian to the Devonian, between -440 and -358 million years ago.

The specimens discovered lived in a tropical reef around 380 million years ago, during the Devonian period. These fossils are 250 million years older than any known fish heart.

The fossilized liver, stomach, and intestine of these placoderms helped provide a more complete view of the internal anatomy at a pivotal time in the history of vertebrates—backbone animals including fish, amphibians, reptiles, birds and mammals.

>> Reconstruction of the anatomy of the gogo placodermethe Macnamaraspis caprios:

Placoderm fish anatomy discovered in the Gogo Formation in the Kimberley region of Western Australia. Under the gills, the heart. In pale salmon, the stomach. In dark red, the liver. You can also make out an embryo in the pink pouch − these fish gave birth to their young and did not lay eggs. [Trinajstic et al. 2022 – Science]

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The fossils were found in a place called the Gogo Formation in the Kimberley region of Western Australia near the town of Fitzroy Crossing. They are remarkable because soft tissues, unlike hard tissues like bones and teeth, are rarely preserved as fossils, in a solid three-dimensional structure, but rather flattened and therefore carrying very little information.

“The site is arguably one of the most important fossil sites in the world for understanding the early evolution of backbone animals, including the origins of the human body,” said vertebrate paleontologist Kate Trinajstic of the Curtin University and the Western Australian Museum, lead author of the study published in the journal Science.

“Our oldest jawed ancestors”

Placoderms, known for their bony armor on their heads and necks, represented “our earliest jawed ancestors”, according to Kate Trinajstic.

The newly described fossils belong to two species, named Crouching fishes et Ritchie’s cut, both about 25 centimeters long and with asymmetrical shark-like caudal fins, tooth-bearing jaws, and sharp blade-like edges. Their heads were broad with a rounded muzzle.

Placoderms had a more complex heart than one would expect for a primitive fish. It was S-shaped, similar to that of a shark, and consisted of two chambers: the smaller one on top and the larger one below. It was located at the front of the shoulder girdle in a position similar to today’s sharks and bony fish.

Its structure differs from that of later vertebrates. Amphibians and reptiles have a three-chambered heart, while that of mammals and birds has four. The research team suggests this made the animal’s heart more efficient and was the critical step that transformed it from a slow-moving fish to a fast predator.

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>> The position of the heart (in red) in the Gogo placoderm fossil, seen using the ESRF synchrotron:

>> The inside of the placoderm fossil, with the heart (in red):

Placoderm fish organs in detail.  In red, the heart. [Trinajstic et al. 2022 - Science]Placoderm fish organs in detail. In red, the heart. [Trinajstic et al. 2022 – Science]

In terrestrial vertebrates, which evolved from fish during the Devonian, the heart moved further back along the body – or downward from the perspective of a standing man or woman. If a person’s heart were in the same place as these placoderms, it would be at the base of the throat, in the center of the collarbone.

Visible liver and stomach

The liver of the placoderm was large and allowed the fish to be able to float, as is the case with sharks. The liver shows how placoderms evolved away from the organ layout of jawless fish. In this type of fish, called lampreys, the liver is crushed against the heart and envelops it from behind. Placoderms exhibited heart-liver separation like modern jawed vertebrates.

The stomach of placoderms, which is shaped like a flattened sac and is somewhat rectangular, has a distinctive, thick, honeycombed wall texture, apparently representing glandular tissue. The intestine has spiral valves to facilitate the absorption of food. There is no evidence that these animals possessed lungs.

>> The liver and stomach of the placoderm:

Gastric and hepatic wall of a placoderm fish, as well as the dorsal median. [Trinajstic et al. 2022 - Science]Gastric and hepatic wall of a placoderm fish, as well as the dorsal median. [Trinajstic et al. 2022 – Science]

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The importance of soft anatomy

The most important step in vertebrate evolution was “the transition from the ancestral jawless condition, reflected in modern lampreys and hagfishes [ndlr. des animaux aquatiques anguilliformes]to a vertebrate body with jaws,” according to paleontologist and study co-author Per Ahlberg of Uppsala University in Sweden.

“Today, the overwhelming majority of vertebrates belong to the group with jaws: sharks, rays, bony fishes and all terrestrial vertebrates, including humans. This transition has not only affected the evolution of the jaws, but also all kinds of changes in the soft anatomy – for example the evolution of a stomach, and the heart moving forward in the throat region,” he said.

“But, while fossils give us a reasonably complete picture of skeletal evolution, the equally important soft organs usually don’t fossilize at all, meaning we have to guess at the details of their transformation over the course of time. of evolution,” added Per Ahlberg.

These observations obtained using modern imaging techniques, such as neutron beams, would have been impossible just a few decades ago.

Sujet radio: Bastien Confino

Web article: Stéphanie Jaquet and Reuters