The world’s oldest heart found in a placoderm

Aided by advanced X-ray technology, it has been possible to study the inner organs of placoderm fossils in this lump of limestone.

“This is the period in which the major groups of vertebrates appear and begin to develop in different directions, so it is enormously interesting to understand the anatomy of the vertebrates that lived then,” says Per Ahlberg, professor at the Department of Organismal Biology and one of the researchers behind the study which was recently published in the journal Science.

The most important step

He and his research group have long been interested is the process by which jawed vertebrates evolved, resulting not only in the appearance of jaws but also in changes in the internal organs. The first vertebrates appeared over 500 million years ago, at which point they had no jaws. They are known as jawless fish, like the hagfish and lampreys of today. Per Ahlberg calls the appearance of jaws the most important step in the evolution of vertebrates, as nearly all of the vertebrates of today have jaws, including ourselves.

Aided by advanced synchrotron tomography and neutron tomography, he and his colleagues have now been able to study the well-preserved discoveries from north-western Australia. The images they produced showed that the internal organs of the placoderms were positioned in the same way as today’s jawed vertebrates.

“There’s a real stomach, which we see for the first time. We can see the walls and glandular structure of that stomach. We can see the liver’s structure and that the heart is positioned where it should be in a jawed fish – that is, at the base of the throat, well separated from the liver. So far, these confirm the traits we see in all living jawed vertebrates,” notes Ahlberg.

Age of Fishes

The discoveries originate from the Devonian geological period, which is also often called the Age of Fishes because fishes underwent enormous development and diversification during this time. Examples include a division appearing between bony fish with a bone-based skeleton and cartilaginous fish with a cartilaginous skeleton, a category consisting today of sharks and rays. Unlike bony fish, cartilaginous fish lack lungs or a swim bladder. They must swim constantly in order to avoid sinking and they also have a very large liver filled with oil. The same appears to have applied to the placoderms.

“We cannot find any traces of lungs or swim bladders. Moreover, we have found that the liver is quite large and may, at a guess, have contained oil for a little extra buoyancy,” explains Ahlberg.

The fact that the placoderms’ bodies have been preserved so incredibly well, even three-dimensionally, is due to the unusual phenomenon whereby fossilisation likely started at the same moment the fish died. A potential explanation for this is that the deep water was free of oxygen right up to the sunlit zone.

“It appears that there were green sulfur bacteria in the water – they want the combination of sunlight but not oxygen – and that they in some strange way affected the water’s chemical composition and the chemistry inside dead fish that sunk through the water column and began to mineralise as soon as they fell towards the sea floor,” adds Ahlberg.