Australian fish fossils reach the heart of vertebrate evolution

Scientists in Australia have unearthed beautifully preserved fossilized hearts and other internal organs of ancient armored fish in a discovery that provides insight into the evolution of the bodies of vertebrates – including humans.

Researchers Thursday described the heart, the organ that pumps blood through the body’s circulatory system, in fish called placoderms that inhabited tropical coral reefs about 380 million years ago during the Devonian period. The fossils were 250 million years older than any previously known fish core.

The fossilized liver, stomach, and intestines from these skin plaques helped give a fuller view of the internal anatomy at a pivotal time in the history of vertebrates – animals with a backbone including fish, amphibians, reptiles, birds, and mammals.

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 things like bones and teeth, are rarely preserved as ridges, and even more often than not they are preserved in three strong dimensions, rather than flattened.

Vertebrate paleontologist Kate Triangstick of Curtin University and Western Australian Museum, and lead author of the study published in Science said. Trinagistic said Placoderms, known for the bony shields on the head and neck, represent our “early jawed ancestors.”

The newly described fossil consists of two species, called Compagopiscis croucheri and Incisoscutum ritchiei, both about 10 inches (25 cm) long with asymmetrical shark-like tail fins, jaw-bearing teeth and blade-like cutting edges, and broad, blunt-nosed heads. .

Placoderms had an S-shaped heart similar to that of a shark. It consisted of two chambers, one smaller at the top and one larger below, and was located at the front of the shoulder girdle in a similar position to sharks and bony fish today. 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 period, the heart moved backward along the body—or downward from the perspective of upright humans. If a person’s heart was in the same place as this skin, it would be at the base of the throat between the collarbones.

The liver of the plaques was large and enabled the fish to stay afloat, as in the case of sharks. The liver showed how placoderms evolved away from the organ arrangement of jawless fish. In jawless fish called lamprey, the liver is pressed against the heart and encases it from behind. Placoderms showed a separation of the heart from the liver like modern jawed vertebrates.

The stomach of the blastoderm, in the form of an oblate and somewhat oblong sac, has a characteristic parietal tissue, thick and beehive-shaped, apparently representing glandular tissue. The intestines have solenoid valves to help absorb food. There was no evidence of lungs.

Paleontologist and study co-author Per Ahlberg of Uppsala University in Sweden said the single biggest step in vertebrate evolution was the transition from an ancestral jawless state, which is reflected in modern lampreys and hagfish, to a jawed vertebrate body plan.

“Today, the vast majority of vertebrates belong to the group of jaws: sharks, rays, bony fish, and all land vertebrates including humans. This transformation included not only the evolution of jaws, but also all kinds of changes in the soft anatomy — for example the evolution of the stomach,” Ahlberg said. And the heart progresses to the throat area.

“But while fossils give us a reasonable complete picture of skeletal evolution, the usually equally important soft organs do not fossilize at all, which means we are left guessing about the details of their evolutionary transformation,” Ahlberg added.