In the limestone ridges of Western Australia’s Kimberley region, near the town of Fitzroy Crossing, you’ll find one of the best-preserved ancient reef complexes in the world.
Here lie the remains of a myriad of prehistoric marine animals, including placoderms, a prehistoric class of fish that represent some of our earliest jawed ancestors.
The placoderms were the rulers of the ancient seas, rivers, and lakes. They were the most abundant and diverse fish of the Devonian Period (419–359 million years ago), but ultimately became extinct in a mass extinction event.
The study of placoderms is important as they provide insight into the origins of the body plan of jawed vertebrates (vertebrates are animals with backbones). For example, placoderms have revealed when the first mandibles, teethpaired skull bones paired limbs evolved. They have also taught us about origins of Internal fertilization and live birth in the evolution of vertebrates.
now on paper published in Scienceswe detail our findings of the oldest three-dimensionally preserved heart of a vertebrate, in this case a jawed vertebrate. This placoderm heart is about 380 million years old and 250 million years older than the previous oldest vertebrate heart.
How did we do it?
Fish fossils near Fitzroy Crossing were first reported from Gogo Station in the 1940s. But it was not until the 1960s that handsome 3D preservations were revealed, using a technique that removes rock from bones with weak acetic acid.
However, this technique turned out to be a double-edged sword. As the fine details of the bony skeleton were uncovered, the soft tissues of the fossils dissolved. It wasn’t until 2000 that the first pieces of fossilized muscle in placoderms were identified.
With the advent of an X-ray method called “synchrotron microtomography,” first used on Gogo fossils in 2010, more Gogo placoderm muscles were revealed, including neck and abdominal muscles.
Our work used this same technology to show, for the first time, the presence of a liver, stomach and intestines in a Devonian fish. Some of the specimens even showed remains of their last meal: a crustacean.
We find the fossilized soft organs in an order of placoderms called arthrodires. These were the most common and diverse of all known placoderms, characterized by a unique junction between the armor of the head and the trunk.
The heart of the placoderm
The most exciting find for us was the heart. We found our first placoderm heart using synchrotron imagination.
Later, while experimenting with a technology called neutron imageswe discovered a second heart inside a different specimen.
Life must have been stressful in the Devonian seas, because placoderms literally had their hearts in their mouths!
At this point in vertebrate evolution, the neck was so short that the heart was located in the back of the throat and below the gills.
Fish that are even more primitive than arthrodires, such as the jawless lamprey, they have their heart close to their liver. And the chambers of the heart (called the atrium and ventricle) sit next to each other.
On the other hand, arthrodire placoderms had their hearts in a more forward (anterior) position, at the back of the throat. And the atrium was on top of the ventricle, similar to sharks and bony fish today.
Today, 99% of all living vertebrates have jaws. Arthrodires provide the first anatomical evidence to support the hypothesis that, in jawed vertebrates, repositioning of the heart to a more forward position was related to the evolution of the jaws and neck.
But that is not all. This movement of the heart would also have made room for the lungs to develop.
So did placoderms have lungs?
One of the most challenging evolutionary questions today is whether lungs were present in the earliest jawed vertebrates. Although fish have gills, the presence of lungs in some fish can help with buoyancy, which is necessary for sinking and rising in the water.
Today, lungs are only present in primitive bony fishes such as lungfish and African junkfish.
But what about ancient placoderms? Previous studies (which were somewhat controversial) suggested that lungs were present in a primitive placoderm called Bothriolepis.
Our analysis of Gogo’s arthrodires reveals that structures thought to be lungs in Bothriolepis are in fact a two-lobed liver, thus lungs are now thought to be lacking in placoderms.
Our discovery therefore shows a unique origin for lungs in bony fishes (osteichthyans). The movement of the heart into a forward position in jawless fishes (Cyclostomata) would have left room for lungs to develop in later lineages.
The absence of lungs in placoderms suggests that these fish it relied on its liver to float, as modern sharks do.
a pivotal site
Organ preservation is a race against time. In some cases, the decomposition of an animal will help in the preservation of the soft tissues, but too much decomposition and soft tissues decay. For excellent preservation, the balance must be correct.
In the fossilized heart we find that the atria and ventricles are clearly shown, while the conus arteriosus, a section of the heart that directs blood from the ventricle to the arteries, is not as well preserved.
Being able to make these discoveries before they are lost forever is crucial if we are to fully understand early vertebrate evolution, including the origins of the human body plan.
Beyond our immediate findings, our work has reinforced the importance of the Kimberley Gogo site as one of the most important sites in the world to carry out this work.
Citation: Oldest Fossil Vertebrate Heart Ever Found Tells 380-Million-Year Evolution Story (Sep 17, 2022) Retrieved Sep 17, 2022 from https://phys.org/news/2022-09-oldest -vertebrate-fossil-heart -millions-of-years.html
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