The world’s oldest heart found in a placoderm

30 September 2022

LImestone with fossils

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

Most researchers can only guess how animals that lived hundreds of millions of years ago were built internally, given that internal organs disintegrate. However, researchers have now found extremely well-preserved hearts, stomachs, livers and intestines in 380-million-year-old placoderm fossils.

“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.

Model of eating fish
The placoderm Compagopiscis hunted in a tropical
reef around 380 million years ago. However, here
it is being eaten by the lobe-finned fish Onychodus.
Photo: Curtin University

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.

Per Ahlberg
Per Ahlberg, professor at the Department of
Organismal Biology. Photo: Janet Ahlberg

“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.

Placoderm facts

Placoderms appeared some 440 million years ago, and it was among them that the very first jaws are found. The armour found on their bodies led to them being called ‘armoured’ fish. The head and front half of the body was covered in bony plates. There were several hundred different species, of which the largest could grow to nine metres long. Fossils show that they underwent internal fertilisation and that at least some of them gave birth to living young. The placoderms were one of many groups of animals that disappeared permanently 359 million years ago during the mass extinction at the end of the Devonian geological period.

 

Last modified: 2021-02-14