In the limestone ranges of Western Australia’s Kimberley region, near the town of Fitzroy Crossing, you’ll find one of the world’s best preserved ancient reef complexes.
Here lie the remains of countless prehistoric marine animals, including the echinoderm, a class of prehistoric fish that represents some of our earliest jawed ancestors.
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 eventually became extinct in a mass extinction event.
The study of plaque skin is important because it provides insight into the origins of the body plan of jawed vertebrates (vertebrates are animals with a backbone). For example, skin cells revealed when the first jaws, teethAnd the paired skull bones And the double sides evolved. They also taught us about origins Internal fertilization and live birth in vertebrate evolution.
Now, in paper published In Science, we detail our findings for the oldest preserved 3-D heart of a vertebrate – in this case a jawed vertebrate. This skin core is about 380 million years old, 250 million years older than the previous one vertebrate heart.
How did we do that?
Fish fossils were first reported near the Fitzroy Crossing jojo station in the forties. But that didn’t happen until the sixties beautiful 3D preservatives were detected using the technique of removing rocks from bones with weak acetic acid.
However, this technique proved to be a double-edged sword. While the finer details of the skeleton were revealed, the soft tissues of the fossils have faded away. It was not until 2000 that the first pieces of fossilized muscle in Placoderms were identified.
With the advent of an X-ray method called “synchrotron micro-imaging” – first used in gogo fossils in 2010 – more muscles have been revealed from gogo placoderms, including Neck and abdominal muscles.
Our work used this same technique to show for the first time the presence of a liver, stomach, and intestines in a Devon fish. Some specimens even showed the remains of their last meal: crustaceans.
We found the soft organs fossilized in an arrangement of skin called Arthropods. These were the most common and varied of all known chromatic leathers, and featured a unique joint between the head shield and torso.
mucous membrane heart
The most exciting discovery for us was the heart. We found our first heart out of the skin using synchrotron visualization.
Then while experimenting with a technique called Neutron imagingwe detected a second heart within a different sample.
Life must have been nerve-wracking in the Devonian seas, because their hearts literally held theirs!
At this point in vertebrate evolution, the neck was so short that the heart was located at the back of the throat and under the gills.
Fish that are more primitive than arthropods, such as Jawless lampreyTheir heart is close to their liver. The heart’s chambers (called the atrium and ventricle) sit side by side.
On the other hand, dermatophytes had the heart in an anterior (anterior) position, at the back of the throat. And the atrium sat above the ventricle – similar to sharks and bony fish today.
Today, 99% of living vertebrates have jaws. Arthrodires provides the first anatomical evidence to support the hypothesis that repositioning of the heart to a more advanced position in jawed vertebrates is related to the development of the jaws and neck.
but that is not all. This movement of the heart would also have given way to the evolution of the lungs.
Does placodermat have lungs?
One of the most challenging evolutionary questions today is whether lungs were present in early jawed vertebrates. Although fish have gills, the presence of lungs in some fish can aid in buoyancy, which is necessary for sinking and rising in the water.
Today, lungs are found only in primitive bony fish such as lungfish and African reed fish.
More advanced bony fish (such as teleosts) Stay afloat with a swim bladder, while sharks have neither lungs nor a swim bladder, instead they use a large fatty liver to aid in buoyancy.
But what about old skin? Previous studies (which were somewhat controversial) suggested that the lungs were located in the primitive spiral ectoderm called Potreolips.
Our analysis of Gogo arthropods reveals structures believed to be lungs Potreolips It is actually a liver with two lobes, so it is now believed that the lungs are missing from the plaque skin.
Our discovery therefore shows a single origin of lungs in bony fishes (osteichthyans). The movement of the heart to the anterior position of the jawless fish (Cyclostomata) would have allowed room for the lungs to develop in later subspecies.
The lack of lungs in the placoderm indicates that these fish relied on their livers for buoyancy, as modern sharks do.
Preserving members is a race against time. In some cases, animal decomposition helps preserve soft tissues, but excessive decomposition and soft tissue wear off. To maintain excellent balance, the balance must be absolutely correct.
In the fossilized heart, we found that the atria and ventricles were clearly visible, while the arterial cone – the part of the heart that directs blood from the ventricle to the arteries – was not well preserved.
Being able to make these discoveries before they are lost forever is critical if we are to fully understand the early evolution of vertebrates, including the origins of the human body plan.
Beyond our immediate findings, our work has reinforced the importance of the Gogo site in Kimberley as one of the most important sites in the world to carry out this work.