In the limestone ranges of Western Australia’s Kimberley region, close to the town of Fitzroy Crossing, you will discover among the world’s best-preserved ancient reef complexes.
Here lie the remnants of myriad prehistoric marine animals, including placoderms, a prehistoric class of fish that represents a few of our earliest jawed ancestors.
Placoderms were the rulers of the ancient seas, rivers and lakes. These were probably the most abundant and diverse fishes of the Devonian Period (419359 million years back)but become extinct by the end in a mass extinction event.
Studying placoderms is essential because they provide insight in to the origins of the jawed vertebrate body plan (vertebrates are animals with backbones). For example, placoderms have revealed once the first jaws, teeth, paired skull bones and paired limbs evolved. They’ve also taught us concerning the origins of internal fertilization and live birth in vertebrate evolution.
Now, in a paper published in Science, we detail our findings of the oldest three-dimensionally preserved heart from the vertebratein this case a jawed vertebrate. This placoderm heart is approximately 380 million yrs . old, and 250 million years over the age of the prior oldest vertebrate heart.
How did we take action?
Fish fossils from near Fitzroy Crossing were first reported from Gogo Station in the 1940s. Nonetheless it wasn’t before 1960s that beautiful 3D preservations were revealed, utilizing a technique that removes rock from bones with weak acetic acid.
However, this system became a double-edged sword. As the fine information on the bony skeleton were uncovered, soft tissues in the fossils dissolved away. It wasn’t until 2000 that the initial bits of fossilized muscle were identified in placoderms.
With the advent of an X-ray method called “synchrotron microtomography”first applied to the Gogo fossils in 2010more muscles were revealed from the Gogo placoderms, including neck and ab muscles.
Our work used this same technology showing, for the very first time, the current presence of a liver, intestines and stomach in a Devonian fish. A few of the specimens even showed remnants of these last meal: a crustacean.
We found the soft organs fossilized within an order of placoderms called arthrodires. We were holding the most typical and diverse of most known placoderms, seen as a a distinctive joint between their head and trunk armor.
The center of the placoderm
Probably the most exciting find for all of us was the center. We found our first placoderm heart using synchrotron imagining.
Then while tinkering with a technology called neutron imaging, we discovered another heart inside a different specimen.
Life will need to have been nerve-racking in the Devonian seas, because placoderms literally had their hearts within their mouths!
At this time in vertebrate evolution, the neck was so short that the center was located behind the throat and beneath the gills.
Fishes which are a lot more primitive than arthrodires, like the jawless lamprey, have their heart near their liver. And the chambers of the center (called the atrium and ventricle) sit hand and hand.
However, arthrodire placoderms had the center in a far more forward (anterior) position, behind the throat. And the atrium sat along with the ventriclesimilar to sharks and bony fishes today.
Today, 99% of most living vertebrates have jaws. Arthrodires supply the first anatomical evidence to aid the hypothesis that, in jawed vertebrates, the repositioning of the center to a far more forward position was from the evolution of jaws and a neck.
But that isn’t all. This movement of the center would likewise have made room for lungs to build up.
So did placoderms have lungs?
Probably the most challenging evolutionary questions today is whether lungs were within the initial jawed vertebrates. Although fish have gills, the current presence of lungs in a few fish might help with buoyancy, that is had a need to sink and rise in the water.
Today, lungs are just within primitive bony fishes such as for example lungfish and African reedfishes.
More complex bony fish (like the teleosts) stay afloat utilizing a swim bladder, whereas sharks have neither lungs nor a swim bladder, and instead work with a large fatty liver to greatly help with buoyancy.
But think about ancient placoderms? Previous studies (that have been somewhat controversial) suggested lungs were within a primitive placoderm called Bothriolepis.
Our analysis of the arthrodires from Gogo reveals the structures regarded as lungs in Bothriolepis are actually a liver with two lobes, so lungs are actually thought to have already been missing from placoderms.
Our discovery therefore shows an individual origin for lungs in bony fishes (osteichthyans). The movement of the center to a forward position from jawless fishes (Cyclostomata) could have allowed room for lungs to build up in later lineages.
The lack of lungs in placoderms suggests these fish relied on the liver for buoyancy, like modern sharks do.
An essential site
The preservation of organs is really a race against time. In some instances, an animal’s decomposition will aid soft tissue preservation, but an excessive amount of decomposition and the soft tissues decay away. For excellent preservation the total amount must be perfectly.
In the fossilized heart we found the atrium and ventricles are shown clearly, as the conus arteriosusa portion of the center that directs blood from the ventricle to the arteriesis much less well preserved.
Having the ability to make these discoveries before they’re lost forever is essential if we have been to totally understand the first evolution of vertebrates, like the origins of our body plan.
So beyond our immediate findings, our work has reinforced the importance of the Gogo site in the Kimberley among the world’s most significant sites to carry out this work.
Citation: Oldest vertebrate fossil heart ever found tells a 380 million-year-old story of evolution (2022, September 17) retrieved 17 September 2022 from https://phys.org/news/2022-09-oldest-vertebrate-fossil-heart-million-year-old.html
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