Australian fish fossils get to the heart of vertebrate evolution

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Australian scientists have unearthed fossilized hearts and other internal organs from ancient armored fish, a discovery that sheds light on the evolution of the bodies of vertebrates, including humans.

Researchers on Thursday described the heart of fish called placoderms that inhabited a tropical reef about 380 million years ago, during the Devonian period. The fossils were 250 million years older than any other fish heart known so far.

The fossilized liver, stomach, and intestine of these placoderms helped provide a more complete view of internal anatomy at a crucial time in the history of vertebrates, animals with backbones such as fish, amphibians, reptiles, birds, and mammals.

The fossils were found at a place called the Gogo Formation, in the Kimberley region of Western Australia. They are remarkable because soft tissues, unlike hard tissues such as bones and teeth, are rarely preserved as fossils, let alone three-dimensional full-bodied, as in this case, rather than flattened.

Placoderms, known for their bony armor on their heads and necks, represented “our earliest jawed ancestors,” said paleontologist Kate Trinajstic of Curtin University and the Western Australian Museum, lead author of the study published in the journal Science.

The newly described fossils correspond to two species, called Compagopiscis croucheri and Incisoscutum ritchiei, both about 25 centimeters long, with asymmetric tail fins similar to those of sharks, jaws with teeth and cutting edges similar to blades, and broad heads. and roma.

Placoderms had an S-shaped heart similar to that of a shark. It was made up of two chambers, a smaller one at the top and a larger one at the bottom, and located at the front of the shoulder girdle in a position similar to that of sharks and bony fish today.

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Its structure differs from that of later vertebrates. Amphibians and reptiles have a three-chambered heart, while mammals and birds have a four-chambered heart.

In terrestrial vertebrates, which evolved from fish during the Devonian, the heart has moved farther back along the body, or downward from the perspective of upright humans. If a person’s heart were in the same place as these placoderms, it would be at the base of the throat, between the collarbones.

The liver of the placoderms was large and allowed the fish to maintain buoyancy, as in sharks. The liver showed how placoderms had evolved away from the organ arrangement of jawless fish.

In jawless fish called lampreys, the liver is squashed against the heart and wraps around it from behind. Placoderms showed a separation between the heart and liver like modern jawed vertebrates.

The flattened, somewhat rectangular pouch-shaped stomach of placoderms has a distinctive thick, alveolate wall texture, apparently representing glandular tissue. The intestine has coiled valves to facilitate the absorption of food. There is no evidence of lungs.

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