Fossils of a Prehistoric Rainforest Hide in Australia’s Rusted Rocks

Tableland Central Australia, hundreds of miles northwest of Sydney, is today dominated by tall grasses and trees. But scientists recently discovered that some of the rusty rocks in the area hide traces of the lush rainforest that covered the area 15 million years ago during the Miocene epoch.

The area, McGraths Flat, is not the only Miocene deposit in Australia, but these new fossils are a boon of fossils due to their remarkable preservation. Over the past three years, paleontologists have excavated flowers, insects, and even a bird’s downy feathers.

The scientists’ findings, published Friday in Science Advances, are helping to reconstruct Australia’s Miocene rainforest in great detail, said Scott Hocknall, a paleontologist at the Queensland Museum, and the site “opens up a whole new avenue of Australian paleontological exploration”. participate in the research.

Fifteen million years ago, a river carved through the forest, leaving Rainbow Lake (known as Billabong in Australia) in its wake at McGraths Flat. This stagnant pool, almost devoid of oxygen, keeps the scavengers in place, allowing plant matter and animal carcasses to accumulate. As iron-rich runoff from the nearby Basalt Mountains seeped into Billabong, the low pH of the pool caused iron precipitation and encapsulation of organic matter. As a result, the fossils at McGraths Flat have been preserved in a dense, iron-rich mineral known as goethite.

Dr. Hocknall said this method of fossilization is uncommon. Because high-quality fossils are rarely found in iron deposits, paleontologists often overlook them. However, fossils from McGraths Flat show that goethite, which is common in Australia, can produce impressive fossils.

“There is no shortage of goethite,” Dr. Hocknall said. “We are basically a country that rusts.”

Due to its iron-dyed origins, many McGraths Flat craters shimmer with a metallic sheen. In addition to virgin plants, goethite crawls with fossilized insects. Researchers discovered a miniature group of giant cicadas, dragonflies and parasitic wasps when they separated brick-colored stone slabs. And many are remarkably well preserved – some ancient flies bear the detailed imprints of their compound eyes.

The site also spawned more than a dozen ancient spiders. While insects have strong exoskeletons, Michael Friese, a virologist and paleontologist at the University of Canberra and co-author of the study, likens spiders to “sacks of spongy fluid.” As a result, the fossil record of spiders in Australia was virtually nonexistent prior to McGraths Flat.

The fossils have been so well preserved that paleontologists have been able to observe 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 lead author of the study. For example, the team observed parasites anchored in the tail of a fish and nematodes that infiltrated the long-horned beetle.

Dr. Fries used electron microscopy and micro-imaging techniques to examine the rainforest’s inhabitants. While photographing a fossilized sawfly, Dr. Freese discovered a clump of pollen on the head of an insect resembling a bee.

“We can tell which flower this sawfly visited before it fell into the water and met its abrupt end,” said Dr. Freese. “This would not be possible if the preservation quality was not high.”

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

When these insects swarmed around iron-contaminated billabong, Australia was drifting north, away from Antarctica. As it traveled, its climate dried up dramatically, causing rainforests to retreat and leading to large-scale extinctions.

The researchers believe McGraths Flat offers a glimpse into how this dramatic climate shift is affecting specific species within the rainforest ecosystem. For example, some insects found on McGraths Flat tolerated drier conditions while others are now only found in the remaining rainforest pockets of 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 anticipate which ones are most at risk in terms of future changes.”

Dr. Freese said the McGraths Flat has been particularly useful for rebuilding ancient ecosystems because of the breadth of species that have preserved them.

“Our site is different because it has tiny fossils, but in the end, I think it will tell us more about what happened in the ecosystem,” Dr. Freese said. “You don’t need to find a one-ton bird of terror to tell this story.”

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