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

Australia’s Central Tablelands, hundreds of miles northwest of Sydney, are dominated today by grasses and spindly trees. But scientists recently discovered that some of the area’s rusted rocks conceal traces of the lush rainforests that covered the area 15 million years ago during the Miocene Epoch.

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 mineral known as goethite.

This method of fossilization is uncommon, Dr. Hocknull said. Because quality fossils are rarely found in iron deposits, 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,” Dr. 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 fossilized insects. As they split apart the brick-colored slabs of stone, the 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.

The site has also yielded more than a dozen archaic arachnids. While insects have sturdy exoskeletons, Michael Frese, a virologist 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, the 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.

Dr. Frese utilized an electron microscope and microphotography techniques to examine the rainforest’s inhabitants. While imaging a…