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Fossils of a prehistoric rainforest hide in Australia’s rusted rocks

The area, McGraths Flat, is not Australia’s only
Miocene deposit, but these new fossils are a paleontological boon because of
their exquisite preservation. Over the past three years, paleontologists have
excavated flowers, insects and even a bird’s wispy feather.

The scientists’ discoveries, published Friday in the
journal Science Advances, help reconstruct Australia’s Miocene rainforest in
extensive detail, and the site “opens a whole new area of exploration for
Australian paleontology,” said Scott Hocknull, a paleontologist at Queensland
Museum who was not involved in the research.

Fifteen million years ago, a river carved through the
jungle, leaving an oxbow lake (known as a billabong in Australia) in its wake
at McGraths Flat. Nearly devoid of oxygen, this stagnant pool kept scavengers
at bay, allowing plant material and animal carcasses to accumulate. As
iron-rich runoff from nearby basalt mountains seeped into the billabong, the
pool’s low pH caused the iron to precipitate and encase the organic material.
As a result, the fossils at McGraths Flat are preserved in a dense, iron-rich
rock known as goethite.

This method of fossilisation is uncommon, Hocknull
said. Because quality fossils are rarely found in igneous rocks,
paleontologists often overlook them. However, the fossils from McGraths Flat
illustrate that goethite, which is common in Australia, can yield remarkable
fossils.

“There’s no shortage of goethite,” Hocknull said.
“We’re essentially a rusting country.”

Because of their iron-tinted origins, many of the
fossils from McGraths Flat glimmer with a metallic sheen. In addition to
pristine plants, the goethite is crawling with fossilised insects. As they
split apart the brick-colored slabs of stone, researchers have discovered a
miniature menagerie of giant cicadas, dragonflies and parasitic wasps. And many
are remarkably preserved — some ancient flies sport the detailed imprints of
their compound eyes.

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The site has also yielded more than a dozen archaic
arachnids. While insects have sturdy exoskeletons, Michael Frese, a virus
expert and paleontologist at the University of Canberra and a co-author of the
study, likens spiders to “squishy bags of liquid.” As a result, Australia’s
fossil record of spiders was nearly nonexistent before McGraths Flat.

The fossils are so well preserved that the
paleontologists were able to observe relationships between species — something
that is often difficult to parse from fossil sites, according to Matthew
McCurry, curator of paleontology at the Australian Museum and the study’s lead
author. For example, the team observed parasites fastened to a fish’s tail and
a nematode that had infiltrated a longhorn beetle.

Frese utilised an electron microscope and
microphotography techniques to examine the rainforest’s inhabitants. While
imaging a fossilised sawfly, Frese discovered a clump of pollen on the beelike
insect’s head.

“We can tell which flower was visited by this
particular sawfly before it fell into the water and met its untimely end,”
Frese said. “That would not be possible if the quality of preservation was not
as high.”

The pollen also revealed that the rainforest was
surrounded by drier environments, making it likely that McGraths Flat
represents a remnant patch of a once larger forest. According to McCurry, this
makes sense considering the climatic trends of the Miocene.

When these insects scurried around the iron-tainted
billabong, Australia was drifting northward, away from Antarctica. As it
traveled, its climate drastically dried out, causing the rainforests to retract
and leading to widespread extinctions.

The researchers believe McGraths Flat offers an
intimate glimpse of how this dramatic climate transition affected particular
species within the rainforest ecosystem. For instance, some insects found at
McGraths Flat endured drier conditions while others are now found only in northern
Australia’s remnant pockets of rainforest.

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“Studying these fossil ecosystems, we can see which
species were better able to adapt to those changes,” McCurry said. “We can
potentially predict which are most at risk in terms of future changes.”

Frese said that McGraths Flat was particularly useful
for reconstructing ancient ecosystems because of the breadth of species it
preserved.

“Our site is different because it’s all small fossils,
but in the end, I think it will tell us more about what has happened in the
ecosystem,” he said. “You do not need to find a 1-ton terror bird to tell this
story.”

©2022 The New York Times Company

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