Fossils from a prehistoric rainforest lurk in the rusty rocks of Australia

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Australia’s central highlands, hundreds of kilometers northwest of Sydney, are dominated by grasses and spindly trees today. But scientists recently discovered that some of the region’s rusty rocks hide traces of the lush rainforests that covered the region 15 million years ago during the Miocene era.

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

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

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

This method of fossilization is rare, said Dr Hocknull. Because quality fossils are rarely found in igneous rocks, paleontologists often overlook them. However, the McGraths Flat fossils illustrate that goethite, common in Australia, can produce remarkable fossils.

“There is no shortage of goethite,” said Dr Hocknull. “We are essentially a rusty country. “

Due to their iron-tinted origins, many McGraths Flat fossils glow with a metallic luster. In addition to virgin plants, goethite is teeming with fossilized insects. Separating the brick-colored stone slabs, the researchers 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.

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

The fossils are so well preserved that paleontologists have been able to observe the relationships between species – something that is often difficult to analyze from fossil sites, according to Matthew McCurry, curator of paleontology at the Australian Museum and senior author of the ‘study. For example, the team observed parasites attached to the tail of a fish and a nematode that had infiltrated a longhorn beetle.

Dr. Frese used an electron microscope and photomicrograph techniques to examine the inhabitants of the rainforest. While photographing a fossilized sawfly, Dr Frese discovered a clump of pollen on the bee-like 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,” Dr Frese said. “It would not be possible if the quality of preservation was not so high.”

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

When these insects scurried around the iron-contaminated billabong, Australia was drifting north, away from Antarctica. During its journey, its climate dried up considerably, causing tropical forests to shrink and leading to widespread extinctions.

Researchers believe McGraths Flat offers an intimate glimpse into how this dramatic climate transition affected particular species within the rainforest ecosystem. For example, some insects found at McGraths Flat have endured drier conditions while others are now only found in the remaining pockets of rainforest in northern Australia.

“By studying these fossil ecosystems, we can see which species were best able to adapt to these changes,” said Dr. McCurry. “We can potentially predict which ones are most at risk in terms of future changes. “

Dr Frese said McGraths Flat was particularly useful for rebuilding ancient ecosystems because of the extent of the species it preserves.

“Our site is different because it is just little fossils, but in the end I think it will tell us more about what happened in the ecosystem,” said Dr Frese. . “You don’t have to find a one-ton terrorist bird to tell this story.”

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