A newly discovered fossil deposit in the Central Tablelands northwest of Sydney provides unique insights into the region’s prehistoric ecosystem. While the landscape today is characterized by bush, grass and desert areas, it was overgrown with rainforests around 15 million years ago. This is suggested by numerous well-preserved fossils of plants, insects, spiders and fish. However, pollen finds already show the first indications that the dehydration was beginning at that time.
During the Miocene, around 23 to five million years ago, the landscape and ecosystems in Australia changed dramatically. If the continent was still fertile and species-rich at the beginning of the epoch, climate changes around 14 million years ago resulted in a massive extinction of numerous animals and plants. Australia became drier and the former rainforests gave way to bush, grass and desert landscapes that characterize large parts of Australia to this day. “The lack of well-preserved fossils has so far made it difficult to understand the structure of Australian ecosystems before they dried up,” writes a team led by Matthew McCurry from the Australian Museum in Sydney.
Rainforest at the transition to drought
It wasn’t until 2017 that a unique deposit of well-preserved Miocene fossils was discovered, the McGraths Flat, named after its discoverer. The site is located near the town of Gulgong in central New South Wales, around 250 kilometers northwest of Sydney. McCurry and his team have excavated fossils in McGraths Flat for three years and have now published the first comprehensive analysis. “Until now, it has been difficult to say what these ancient ecosystems looked like, but the degree of conservation of this new fossil site means that even small, fragile organisms like insects have become well-preserved fossils,” says McCurry.
The fossils were dated to an age of eleven to 16 million years – exactly the phase in which the climate change took place. “The fossils we found show that the area was once a temperate, moderately humid rainforest and that life was rich and abundant here in the Central Tablelands,” said McCurry. However, indications of the onset of drought can already be seen: “The pollen that we found in the sediment suggests that there may have been drier habitats in the vicinity of the more humid rainforest, which indicates a change to drier conditions.” says McCurry. In addition, the researchers discovered remains of hard-leaved plants, which are also typical of arid regions.
Exceptionally preserved fossils
With the help of various microscopic techniques, including scanning electron microscopy, the researchers were able to depict the tiny structures of the fossils. That way, they got a glimpse of how the species interacted with each other. “For example, we evaluated the stomach contents of fish so that we can find out what they ate,” explains McCurry’s colleague Michael Frese. Among other things, they found remains of insect larvae and even a dragonfly’s wing in the fish’s stomachs. “We also found examples of pollen that is preserved on the bodies of insects so that we can determine which species have pollinated which plants,” says Frese.
The fossils were preserved in a ferrous rock called goethite, which is not normally known as a source of well-preserved fossils. But from the researchers’ point of view, the very process that turned the organisms into fossils could explain why they were so well preserved. According to this, the fossils were probably formed in a so-called billabong, a body of water that often forms from a branch of a river and, depending on the amount of rainfall, occasionally dries up. “Our analysis suggests that the fossils formed when ferrous groundwater flowed into a billabong. The iron minerals that settled included organisms that lived in or fell into the water, ”explains McCurry.
Past and future in a warmer world
According to the researchers, the fossils offer a previously unique insight into Australia’s ecological past and could even provide a glimpse into the future. “McGraths Flat’s plant fossils give us a glimpse into the vegetation and ecosystems of a warmer world, a world we are likely to see in the future,” says co-author David Cantrill of the Royal Botanic Gardens Victoria, Australia. “The conservation of plant fossils is unique and offers important insights into a period for which there are few fossils in Australia.”
Source: Matthew McCurry (Australian Museum Research Institute, Sydney) et al., Science Advances, doi: 10.1126 / sciadv.abm1406