The oldest vertebrate fossil heart ever found tells a 380-million-year-old evolutionary story

In the limestone ranges of the Kimberley region of Western Australia, near the town of Fitzroy Crossing, you’ll find one of the world’s best-preserved ancient reef complexes.

Here lie the remains of numerous prehistoric sea creatures, including placoderms, a prehistoric class of fish that represents some of our earliest jawed ancestors.

Placoderms were the rulers of the ancient seas, rivers and lakes. They were the most common and diverse fish of the Devonian (419-359 million years ago) – but died out at the end in a mass extinction event.

Studying placoderms is important because they provide insight into the origin of the vertebrate body plan (vertebrates are animals with backbones). Placoderms, for example, have revealed when the first jaws, teethpaired skull bones and paired limbs evolved. They also taught us about the origin of internal fertilization and live birth in vertebrate evolution.

Now, in a newspaper published in Science, we describe our findings of the oldest three-dimensionally preserved heart of a vertebrate – in this case, a vertebrate with jaws. 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 forties. But it wasn’t until the 1960s that pretty 3D preservations were revealed, using a technique that removes stone 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 in the fossils dissolved. It was not until 2000 that the first pieces of fossilized muscle were identified in placoderms.

With the advent of an X-ray method called “synchrotron microtomography” – first used on the Gogo fossils in 2010 – more muscles were revealed from the Gogo placoderms, including neck and abdominal muscles.

Our work used the same technology to demonstrate the presence of a liver, stomach and intestines in a Devonian fish for the first time. Some specimens even showed remnants of their last meal: a crustacean.

We found the soft organs fossilized in an order of placoderms called arthropods. These were the most common and diverse of all known placoderms, characterized by a unique connection between their head and torso armour.

The heart of the placoderm

The most exciting find for us was the heart. We found our first placoderm heart using synchrotron imagination.

When I experimented with a technology called neutron imagingwe discovered a second heart in another specimen.

Life in the Devonian seas must have been nerve-wracking as placoderms literally had their hearts in their mouths!

At this point in vertebrate evolution, the neck was so short that the heart was in the back of the throat and below the gills.

Fish that are even more primitive than arthropods, such as the jawless lamprey, have their hearts close to their livers. And the chambers of the heart (called the atrium and ventricle) sit next to each other.

On the other hand, arthrodire placoderms had the heart in a more anterior (anterior) position, at the back of the throat. And the atrium sat 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, the repositioning of the heart to a more anterior position was related to the evolution of the jaws and a neck.

But that is not everything. This movement of the heart would also have made room for the development of the lungs.

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 aid in the buoyancy needed to sink and rise in the water.

Today, lungs are only present in primitive bony fish such as lungfish and African reedfish.

More advanced bony fishes (such as the teleost) stay afloat using a swim bladder, while sharks do not have lungs or swim bladder and instead use a large fatty liver to aid in buoyancy.

But what about old placoderms? Previous studies (which were: somewhat controversial) suggested that lungs were present in a primitive placoderm called Bothriolepis.

Our analysis of Gogo’s arthropods reveals that the structures thought to be lungs in Bothriolepis are in fact a two-lobed liver, so lungs are now thought to be missing in placoderms.

Our discovery thus shows a single origin for lungs in bony fishes (osteichthyans). The movement of the heart to a forward position of jawless fishes (Cyclostomata) would have given room for the lungs to develop in later generations.

The absence of lungs in placoderms suggests that these fish depended on their liver for buoyancy, as modern sharks do.

A crucial place

Organ preservation is a race against time. In some cases, an animal’s decomposition will promote soft tissue preservation, but too much decomposition and the soft tissues will decay. For excellent preservation, the balance must be just right.

In the fossilized heart, we found that the atrium and ventricles are clearly depicted, while the conus arteriosus – a part of the heart that directs blood from the ventricle to the arteries – is not very well preserved.

Being able to make these discoveries before they are lost forever is crucial if we are to fully understand the early evolution of vertebrates, including the origins of the human body plan.

So in addition to our direct findings, our work has reinforced the importance of the Gogo site in the Kimberley as one of the world’s premier sites for carrying out this work.

This article was republished from The conversation under a Creative Commons license. Read the original article.