In the popular culture of the fossil record, fish usually don’t make an imprint. Instead, our imaginations look up to the grandeur of dinosaurs. Yet it is the fish – down below and duller, perhaps – that fascinate vertebrate palaeontologists.
In the story of early vertebrate evolution, it is the fish that are the main storytellers. And many of them are from northern Western Australia, where the placoderms once roamed a massive barrier reef off the coast of Gondwana that is now a fossil-infused sedimentary inland formation.
Placoderms are a class of prehistoric fish and our earliest ancestors known to have jaws. About 400 million years ago, during the Devonian geologic period – which is known as the Age of the Fishes – placoderms swam happily in anything bigger than a pond. But our earliest jawed ancestors didn’t make it through the great mass extinction event of the Late Devonian. They ruled the oceans of ancient Earth, and now they’re ruling the fossil record.
Jaws: more gripping than Jurassic Park
Placoderms have been called the ‘dinosaurs of the seas’ and, because the head and thorax is covered with bony plates, they’re also known as ‘the armoured fish’. But their best feature is their jaws. This is because, prior to placoderms, our vertebrate ancestors were like lampreys, with a pliable mouth that moseyed around sucking up food. So while we’re captivated by the historic tragedy of the dinosaurs, and the evolution of opposable thumbs, jaws are the bigger deal.
Vertebrate palaeontologists and long-time collaborators Professor Kate Trinajstic and Professor John Long know placoderms, and the arthrodira in particular, probably better than anyone. Trinajstic explains that the arthrodire group of placoderms also had a neck joint that let the head rise and the jaws drop – which allowed feeding – and they ranged from moochers of the ocean floor to six-metre apex predators.
Hammer in hand, Trinajstic began exploring the Devonian reefs in Australia’s northwest about 25 years ago. After being discovered in the 1940s, investigations at the Gogo Formation have been ongoing since the late 1960s, with great discoveries made by Long and others striving to further understand vertebrate evolution. According to Trinajstic, unlike the portrayals of palaeontologists feathering away at rocks with little brushes, the only way to capture a placoderm is to thwack open the limestone concretion, or ‘nodules’ that contain marine fossils. And they’re quite common in the Gogo.
“After perishing, placoderms living on the reef would have floated out and sunk into deeper water where oxygen is absent, which gives more chance of preservation,” she explains.
“And the nodules are fairly easy to find. We pick them up, hit them with a hammer so that they break along the fossil line, which is the weakest line in the rock.”
But if these fossil nodules are scattered randomly over the landscape, like shells on a beach, there for the picking, or thwacking, why isn’t the Gogo swarming with bounty hunters?
Trinajstic says that while the Gogo is one of the world’s best-preserved ancient reef complexes, and the limestone concretions are common, the location is kept fairly quiet. “And most of the time we only find fossil ‘ghosts’, which are just a smear.”
But every so often they find more than a smear. Over the past 10 years Trinajstic has contributed several important research discoveries from the Gogo, and this year she struck gold – a nodule containing a placoderm heart that thumped 380 million years ago. The discovery has caused palpitations in the scientific body of vertebrate palaeontology, because the heart is somewhat older than the previous vertebrate heart discovered… by 250 million years.
Old fossils are new again
But it wasn’t a case of cracking open the nodule to reveal a tiny stone heart and yelling Eureka! Long found the nodule on a field trip years ago, but Trinajstic discovered the preserved heart only recently, from scans obtained by a combination of modern imaging techniques known as synchrotron radiation (SR) microtomography and neutron imaging.
It’s rare for palaeontologists to find fossils of animal tissue like organs and muscles. Soft tissue usually decays before it can be fossilised, but under rare perfect conditions, fossilised soft tissue can occur. The second challenge is that prior to SR microtomography, palaeontologists needed to apply a weak acid solution to the rock surrounding a fossil. And, along with the rock, any fossilised soft tissue – muscle, for example, that may have clung to the bones, also dissolved away. An earlier method to study the fossil’s soft anatomy involved the sectioning of the fossil, which of course destroyed the original version.
But now, scanning technologies, coupled with enormous computing power, enables palaeontologists to reveal and study fossil non-destructively, and will likely uncover more soft tissue fossils for the record.
The heart of the matter… is actually the liver
Trinajstic joined Curtin as a research fellow in 2009, and since then has been using scanning techniques on Gogo fossils discovered during the past 22 years, and revealing previously unknown musculature of the placoderms. In 2011 she received an esteemed fellowship from the Australian Research Council for the project ‘Fleshing out the fossil record’, to further investigate the development of the skeleton and specialised musculature in early vertebrates. The results have been astounding, a specimen of the genus Compagopiscis revealing when the first teeth evolved and studies on an Incisoscutum specimen revealing the evolutionary origins of internal fertilisation and live birth!
Then, in October, Trinajstic and her colleagues published a seminal paper in Science, describing how, at the Australian Nuclear Science and Technology Organisation (ANSTO) and at the European Synchrotron Radiation Facility in France, experts scanned Trinajstic’s fossil specimens – still embedded in their nodules – and created 3D images of the specimens within, and the significance of the discovering the 380-million-year-old soft tissue cluster of heart, liver, stomach and intestine.
While an old fish heart has much to tell us, it’s really the arrangement of the organs found with the heart – the liver in particular – that tells the better story. For starters, the heart is sitting under the gills, so the fish literally had its heart in its mouth, Trinajstic explains.
“Previously in jawless vertebrates, the heart sat closer to the liver, and the progression of the heart towards the head links with the evolution of our neck region, and the space it vacated made room for lungs to develop later in the bony fishes.”
In most modern fish species, the stomach sits behind the liver, and buoyancy comes from a large gas-filled organ known as a swim bladder. However, the stomach – which in one specimen even contained a bit of the fish’s final meal (some sort of crustacean) sits above the liver.
“The liver is the most interesting part. It’s large and would have helped the fish maintain its place in the water column, like most shark species today.”
Trinajstic says that all up, they have about 30 specimens of Compagopiscis and Incisoscutum, but only three have livers preserved and only one individual specimen has a full complement of organs.
“But it’s allowing us to piece all the evidence together. We can see these features were advanced in such early vertebrates, and understand more about how the head and neck region began to change to accommodate jaws.”
Global respect for the Gogo
As significant as what has been found is what hasn’t … lungs.
One of the tough evolutionary questions for palaeontologists is whether the earliest jawed vertebrates had lungs. Trinajstic and her colleagues may have just snapped that bone of contention.
“Most modern fish have swim bladders, and a few species have lungs whereas sharks did not. The question has been, which is the primitive? Did lungs evolve in placoderms and sharks lost them, or are they an advanced feature which evolved in the bony fish?”
“We’ve seen no evidence of lungs in the arthrodire fossils, which suggests that lungs evolved later in the bony fishes.”
Adding these latest findings makes the Gogo arthrodires the most fully understood of all jawed stem vertebrates. And, together, Trinajstic and Long (who is based at Flinders University) are promoting the Gogo Formation’s credentials for the World Heritage List, as one of the world’s most significant fossil sites.
Long has pointed out that “Gogo has given us world-firsts, from the origins of sex to the oldest vertebrate heart …They show the value of the Gogo fossils for understanding the big steps in our distant evolution.”
And while Trinajstic continues searching for more placoderm specimens for the puzzle, what she truly wants to find in the Gogo is a decent conodont.
Conodonts were a class of jawless marine vertebrates, possibly like lampreys, that rummaged around the ocean floor between the Cambrian and Triassic periods. Strangely, the Gogo hasn’t given up a complete conodont.
“We’ve found the mouth elements but never found the body,” she says.
But, as Long points out, the Gogo continues to yield secrets. And there’s still plenty of time.