Science

‘A Rosetta Stone’: Australian fossil site is a vivid window into 15m-year-old rainforest | Fossils

TAustralian paleontologist Matthew McCurry was excavating Jurassic fossils when a farmer came across news of something he saw in his field — a petrified leaf in a piece of hard brown rock.

Fossil papers aren’t usually something home writes about, but the place was nearby, so McCurry and his colleague Michael Freese went to have a look.

What they found in this dirt field near the town of Gulgung in New South Wales five years ago has terrified paleontologists – at least those few who know the secret -.

The rocks are littered by the inhabitants of a rainforest that existed in that dry and arid spot now about 15 million years ago.

“There is a whole ecosystem that is preserved,” says McCurry, curator of paleontology at the Australian Museum and lecturer at the University of New South Wales.

When their hammers sliced ​​the iron-rich rocks, thousands of fossils were uncovered – from flowering plants to fruits, seeds, insects, spiders, pollen and fish. There will be dozens of new species.

The research team is looking for insect fossils at the site; On their first visit, McCurry and Frieze found small aquatic insects preserved in the rock. Photo: Salt Dingo / Courtesy of The Australian Museum
Fossilized leaf from a new fossil site at McGraths Flats, New South Wales, Australia
A petrified sheet found in the site’s iron-rich rocks. The huge collection of leaf fossils allowed the team to appreciate the area’s climate. Photo: Salt Dingo / Courtesy of The Australian Museum

McCurry and colleagues disclosed the site and their preliminary findings in the journal Science Advances on Saturday AST.

“Palaeontologists all over the world will be drooling when they see this paper,” said Professor John Long, a famous fossil hunter from Flinders University, who peeked at some of the fossils a year ago.

Such a collection of samples in one place has allowed Australian scientists to build an incredibly detailed picture of an unknown ecosystem from a period known as the mid-Miocene – a time before the continent dried up completely to become what it is today.

In addition to the vast number of different specimens at the site – known as the McGraths Flat – it is the clean preservation of the fossils that provides unprecedented depth of information.

Under the microscope, there are details of less than a micron in width (the spider’s thread is about three microns long).

The spider’s breathing apparatus and the contents of the fish’s stomachs can be seen. Cells that could show the original color of the feather were preserved. A saw bug was frozen in time with dozens of pollen grains attached to its head.

McGraths Flat . Fossil Door Spider
“You have whole organisms.. soft tissue.. cellular preservation. There is a spider that maintains its respiratory system beautifully. Photo: Michael Fries / Courtesy of The Australian Museum

Since that first visit, McCurry and his colleagues have discovered a treasure trove of fossils. When rocks fracture, they tend to split the fossilized remains in half like an instant autopsy, revealing internal organs and tissues.

The fish’s stomachs have been well preserved, and McCurry says they can see what that fish ate – about 15 million years ago – in the moments before its demise.

“We can see food in the stomach, like a dragonfly wing. But it’s usually insect larvae.

Long saw some of the fossils last year when he visited McCurry at the Australian Museum.

Fossils are often preserved as small fragments or fragments. Occasionally you may get a whole organism. But this is really exceptional protection,” he says.

“You have whole organisms… soft tissues… cellular preservation. There’s a spider with its respiratory system beautifully preserved. It’s Xanadu.”

“There is all the diversity with a huge variety of organisms from fungi to plants and fish, and also you have their interaction. There is evidence of behaviour. It has all the traits of world-class fossil deposits, of which we have very little in Australia.”

“It is part of the Rosetta Stone for the whole environment of the middle Miocene. We have no other window into that period that tells us what that part of Australia was like.”

lucky digs

New fossil sites are rare finds, and this site has been almost overlooked. McCurry admits he got past it at least once, oblivious to what was there.

On his first visit to Frieze, they found iron-rich rocks, extraordinarily difficult to break up, and of a type unknown to fossil preservation.

But immediately, the couple stumbled upon what they thought were water bugs. Using a microscope that Frieze had in his car, they were able to see preserved little midges. “That’s when we realized how special the fossils were,” McCurry says.

Associate Professor Michael Freese segments rocks to find fossils at McGraths Flats, NSW, Australia
Associate Professor Michael Freese, who has been examining the fossils for several years, says he is amazed at their detail. Photo: Salt Dingo / Courtesy of The Australian Museum

Finding fossilized pollen has allowed scientists to accurately date the site.

Little is known about the ecosystems of the mid-Miocene period.

McCurry says there are likely to be “dozens, if not hundreds” of new species of science that have already been collected. The researchers found potential new species preserved in sediments between 50 cm and 80 cm thick at a rate of more than one a day. There have been eight field excavations so far.

Electron microscope image of a pentagram pollen
Pollen Nothofagidites. This artificially colored scanning electron micrograph shows a fossil pollen grain (Nothofagadites cf. deminitus), indicating that the plants of Nothofagus (and middle rainforests) had a greater geographical range in the Miocene than today. Photo: Michael Fries / Courtesy of The Australian Museum

Although only two square meters were excavated at a time, about 2,000 samples were collected. Now follows the painstaking process of checking each one against known records of plants and animals.

Using analysis of a huge collection of plant leaves at the site, the team was even able to estimate the area’s climate. The warm months were around 26°C, while the cold months were as low as 7°C.

Almost 1 meter of rain could fall in a month in the rainy season – the region’s modern climate is hotter but drier, with an average rainfall of only 70 mm per month.

feather fossil

While many members of the team browse a wide variety of plants and animals, Dr. Jacqueline Nguyen, an expert on bird evolution at the Australian Museum, has mostly been focused on the only evidence found so far of birds that were in the rainforest. That is, a single fossilized feather the size of a fingerprint.

“Fossified feathers are incredibly rare,” she says. “Most of them are from the Cretaceous period, but from the Miocene we only have this. I am very excited.”

Fossil feathers are so detailed that Nguyen and her colleagues were able to see the parts of the cells that give feathers their colour. This feather – perhaps from the bird’s body rather than the wing – is likely to be dark or iridescent.

“Although it is just a single feather, it is a tantalizing hint of what is to come. Perhaps we will find a bird skeleton.”

A fossilized feather appears yellow on the brown rocks in which it is found
“Fossified feathers are extremely rare,” says Dr. Jacqueline Nguyen. In this image, about the size of a fingerprint, parts of the cells that gave the feather its color have been identified. Photo: Michael Fries / Courtesy of The Australian Museum

Freese, a virologist by training, has examined the fossils under microscopes for several years.

“I was amazed by the details,” he says. “I like the way the fossils present themselves. Usually you only see the surface, but here it always splits in half and you see the inside of a spider’s leg or the inside of the pollen.”

The secret of preserving the fossils is up for debate, but McCurry believes that this would have happened over hundreds of years rather than in a sudden event.

Iron-rich water, possibly from nearby prominent outcrops, can flow into a shallow area, periodically removing oxygen from the water, killing organisms or encapsulating plants and animals in sediments that turn into rocks in the field.

McCurry admits he is relieved to be able to tell the world about this discovery.

“This has been a marathon,” he says. “It’s a really important discovery and it’s going to keep us going for a long time.”

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