December 2022 – January 2023

The Nation Reviewed

Heart of old

By Jock Serong
Illustration by Jeff Fisher
How an Australian discovered the oldest heart on the fossil record in the Kimberleys

Professor Kate Trinajstic sounds relaxed when she talks about the complex work of a palaeontologist. It’s an adventure for her, a fascination and a source of wonder – despite also being a fiendishly technical hunt for elusive quarries. For Trinajstic, a discovery she recently revealed in the journal Science is the culmination of 25 years in the field and the lab, following a mid-life career shift from cardiac nursing – a curious coincidence, it turns out. The search for traces of internal organs in some of Earth’s most ancient inhabitants led Trinajstic and her colleagues to discover the oldest heart in the fossil record.

Since the early 2000s, Trinajstic, a professor at Perth’s Curtin University, has been doing fieldwork in the fossil-rich area of the Kimberley known as the Gogo Formation. “We go every three to five years. My colleague [Flinders University Professor] John Long has been going since the ’80s. We four-wheel-drive from Fitzroy Crossing, but that’s as much as I’m willing to tell you about the location, because there’s a black market in these fossils.”

There would be few working environments on the Australian continent more challenging than the Kimberley: the heat, the remoteness, the terrain, and the spiked and fanged battalions of flora and fauna. Trinajstic maintains, in her cheerful way, that the heat isn’t an issue. “We work in the dry season and it’s usually 35 to 40 degrees but I don’t mind – I don’t seem to function below about 30.” The palaeontologists work in black soil, in what are now valleys between the low ranges of the region. Trinajstic and her colleagues are examining the sea floor of a Devonian reef.

The Devonian period – the Age of the Fishes – spanned 60 million years. It was dominated by aquatic animals; on land, only arthropods (insects and spiders) had developed. There were no flowering plants and no mammals, birds, dinosaurs or reptiles. Gondwana was Earth’s major landform. Although there were early forests – crowds of giant ferns – most of the action was happening in the seas, and there the dominant fishes were placoderms – large, armoured predators with an anatomy close to that of modern fishes.

The fishes are about 25 to 30 centimetres long. The placoderm species did throw up some 9-metre monsters, but those are found in the Cleveland Shale in Ohio. The biggest individual uncovered in the Kimberley is about 1.5 metres long. In the Gogo Formation, placoderms died on the reefs, floated out from the shallows and dropped into oxygen-depleted mud, where the decaying carcasses were coated in a bacterial film that attracted calcium carbonate to form a protective rock layer. In this way, soft tissues became mineralised, forming fossils, whereas more commonly fossils were formed from harder tissues such as bone.

The fossils appear in limestone nodules. “They look like hot-dog rolls and they’re not embedded in bedrock but are loose in the ground,” Trinajstic says. “Cracking nodules” to extract the fossils is a surprisingly rough business. “If you watch it on telly, people are working away with toothbrushes. My favourite tool is the sledgehammer.”

The groundbreaking discovery wasn’t only of a heart, or a single fossil. “It was seven fossils,” Trinajstic says. “We found two that had hearts, another four had other organs like stomachs and livers, and one had intestines. John Long cracked one concretion in the field – he literally hit it with a pick, and we could see the intestines within.” Across the seven fossils the team had a full, composite placoderm. “You rarely have a full specimen to work on. And the further back you go in time, the less likely you are to find full fossils. But here we were 380 million years back and dealing with multiple beautifully preserved specimens.”

The discovery of the heart was a stepwise process, an accumulation of decades of scientific work. Back in 1978, Dr Alan Bartram, a scientist in the Natural History Museum in London, first found soft tissue in early bony fish. In 2000, Trinajstic was working with a colleague, Dr Carina Marshall, who found patches of muscle in a placoderm. “We realised then that the acids we were using on the samples were destroying muscle.” Trinajstic and her colleagues had a breakthrough invitation in 2008 to work with a team led by Professor Per Ahlberg in Uppsala, Sweden, and to use the European Synchrotron Radiation Facility in France to make these previously destroyed muscle fibres visible.

A further trip to the synchrotron in 2010 examined whether the placoderms had lungs. “I looked slice by slice at the scans, getting more and more excited,” Trinajstic says. “The scans were taking 10 to 20 hours and we had the machine running around the clock, working in shifts 24/7 on scanning and data analysis.” By the time Trinajstic flew back to Sweden to work with Ahlberg, “we were able to identify organs”.

Trinajstic and her team identified a heart in a specimen they’d scanned in France, but although the blood vessels around it were clearly visible, the organ itself was very poorly preserved. Then, two years ago, Trinajstic tried neutron scanning at the Australian Nuclear Science and Technology Organisation in Sydney (“a lot closer than France, though the wineries aren’t as good”). Her experience with the synchrotron data meant that Trinajstic had a much clearer idea of what she was looking for – and she found it. “I said to the people there, ‘That’s the heart.’ It was incredibly exciting.”

The placoderm’s heart Trinajstic uncovered is about 2 centimetres long, “about from the top of your thumb to the first joint” and its top chamber is smaller than the bottom one, just as it is in ours. In evolutionary terms, its discovery also helps explain the forward location of our hearts in our bodies, and the evolutionary drive toward the development of necks – the ability to mobilise our heads separately from our trunks.

While medical scientists have been largely interested in what the discovery will reveal about the evolution of the heart, Trinajstic says that what they’re learning about other organs is significant too. Livers had a role in buoyancy control, for example. Her work has also confirmed placoderms didn’t have lungs. “Were lungs ancient and lost in sharks, or are they modern and only appeared in bony fishes? Now we know – it’s the latter.” What that tells us is that cartilaginous fishes – principally sharks – separated from the evolutionary chain before lungs were developed.

What does it mean to find the oldest heart in the fossil record? Aside from being a source of wonder in its own right, what can it explain to us as our own hearts measure our tiny mortal allocation, 380 million years later?

“It shows us that at the very start of the evolutionary journey to us, there was already a great deal of complexity,” says Trinajstic. “When we think of evolution, we tend to think of primitive animals being simple. But this is a 380-million-year-old fish with a heart very much like our heart, and a shark’s liver.

“When I started, no one thought you could get soft tissue in these fish. Here we are, right at the beginning of the jawed vertebrates’ evolution and we’ve already got the jawed vertebrate bauplan [“body plan”]. We know too that they had internal fertilisation and gave birth to live young. Now we’ve got the heart, the stomach, the liver. And I’m going to keep looking… I’d love to find a brain – it’s a possibility.”

This quixotic search harks back to The Wizard of Oz: wanderers in search of a heart, a brain and what might be paraphrased as guts. But what do such ethereal threads mean to a scientist who splits nodules with a sledgehammer?

“I don’t get any unscientific feelings about this being the world’s oldest heart. I don’t think about love, or the soul, or romance when I see it,” Trinajstic says, then thinks for a moment. “Although I once felt a fleeting sadness seeing two little embryos fossilised inside a mother fish that had died. They never got the chance to be born.”

Jock Serong

Jock Serong is the founding editor of Great Ocean Quarterly. He writes feature articles in the surfing media, and his novels include On the Java Ridge, Preservation and The Burning Island.

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