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Exceptional Fossils Offer Peek into Prehistoric Australian Rainforest

McGraths Miocene fossil dig
Researchers excavating at the McGraths Flat fossil site. | Salty Dingo 2020

An analysis of exceptionally well-preserved fossils from a newly discovered site in New South Wales, Australia provides unprecedented insights into the rainforest ecosystems that dominated Australia during the Miocene, before the continent dried and the rainforests contracted.

The findings, presented in the January 7 issue of Science Advances, provide evidence that temperate rainforests persisted west of the Great Dividing Range — an expanse of mountain ranges, plateaus, and hills in eastern Australia — into the mid-Miocene, about 15 million years ago.

The study also helps illuminate the ecological interactions (including predator-prey relationships) that played out in these long-vanished forests, which have remained mysterious due to a lack of preserved invertebrate fossils. Some fossilized fish at the site, for example, contain the remains of insect larvae in their stomachs. In one case, the researchers unearthed a partially digested dragonfly wing.

"Past work on Australia's Miocene environments has been largely based on sites that preserve hard body parts, like bones and teeth," said Matthew McCurry, curator of paleontology at the Australian Museum, a lecturer at the University of New South Wales, and the first author of the study. "Because of that bias, we haven't had many chances to look at fossil insects, spiders or other more delicate organisms. Therefore, it has been difficult to envision what these ecosystems looked like. The discovery of McGraths Flat closes an important knowledge gap and will help us to better understand what life was like in the Miocene."

A Fortuitous Discovery

Examining a fossil with a hand lens. | Salty Dingo 2020

During the Miocene epoch, which occurred between about 23 and 5 million years ago, reduced precipitation caused rainforests around the world to shrink, leading to increasingly arid landscapes. Australia experienced a particularly pronounced shift from rainforests that dominated the landscape to the shrublands, grasslands, and deserts seen on the continent today, with modern Australian rainforests remaining in pockets near the east coast. However, a lack of well-preserved fossils has prevented scientists from understanding the nature of Australia's abundant rainforest ecosystems before this transition unfolded.

Then McCurry and colleagues learned about McGraths Flat, a sedimentary deposit bursting with well-preserved fossils that sits on private property in the Central Tablelands of New South Wales.

"The site was initially discovered by a local farmer [Nigel McGrath], who found fossilized leaves on one of his fields," said McCurry. "He alerted us to it, and when we visited we were pleased to discover that the site yields a much wider range of fossils, including the remains of insects, spiders, and fishes."

The researchers conducted seven short trips to recover fossil specimens from the site, which they later analyzed using various microscopy techniques. The team also used a tool based on relationships between leaf form and environmental conditions to predict the site's paleoclimate at the time the fossils were deposited.

A Peek into the Past

The researchers uncovered a stunning array of fossilized insects and arachnids from the site, including assassin bugs, a longhorn beetle, two termite wings, ants, and 13 spiders complete with delicately preserved setae (hair-like structures on their limbs). Many of the fossilized arthropods they found at McGraths Flat were previously undescribed, representing the first Miocene records of their families in Australia.

"The fossils that we find at the site are constantly surprising us," said Michael Frese, an associate professor at the University of Canberra and an author of the study. "We were particularly surprised by the level of fidelity in the preservation of some fossils. For example, it's rare to find fossils that preserve extremely fine details such as melanosomes, which allowed us to reconstruct color patters in fish and feathers."

McCurry and Frese noted that the fossils at McGraths Flat are preserved in a finely bedded geothite — a red iron oxide-hydroxide that wasn't previously believed to offer a source of well-preserved fossils.

Mygalomorph spider from McGraths Flat. | Michael Frese

"We think that the goethite has formed as iron rich groundwaters flowed into an oxbow lake and precipitated around animals and plants that fell into the water," said Frese. "Exactly how this process occurred is the subject of ongoing research. We are conducting geochemical analyses to trace the source of the iron and to determine how it facilitated the exceptional preservation of fossils at McGraths Flat. We hope that our observations lead to the discovery of similar deposits around the world."

The level of preservation at the site enabled the researchers to uncover numerous examples of species interactions frozen in time within the fossil treasure trove. The finds included a glochidium (a type of parasitic larva) of a freshwater mussel attached to a fossilized fish fin — the first example of its kind in the fossil record. The team also discovered pollen preserved on the head of one insect, revealing that it was likely an active pollinator millions of years ago.

In future research, McCurry and colleagues plan to describe and name all of the new species they have been discovering in the sedimentary deposit and to study how the site formed.