Fossil site in Australia indicates abundant life during the Miocene

Australia has not always been a land with desert and dry, scrubby vegetation at is center. In fact, at the start of the Miocene (23 million years ago), after the continent had broken away from South America and Antarctica and was drifting slowly northwards, it was covered with verdant rainforest. The Miocene was a time of enormous richness and biodiversity in Australia, equal to that found today in the rainforests of Borneo and the Amazon.

The animals and plants from this time are not well represented from fossil deposits in Australia, but a new fossil site known as McGrath’s Flat has now changed all that. Located in the Central Tablelands in New South Wales near the town of Gulgong, the site contains superb examples of fossilized animals and plants from the Miocene epoch. 

A team of Australian and international scientists led by Australian Museum (AM) and University of New South Wales paleontologist Dr. Matthew McCurry, and Dr. Michael Frese of the University of Canberra have been excavating fossils from the site in secret during the past three years. The experts have discovered thousands of fossilized specimens, including rainforest plants, insects, spiders, fish and a bird’s feather.

Dr. McCurry said the fossils formed between 11 and 16 million years ago and are important for understanding the history of the Australian continent. 

“The fossils we have found prove that the area was once a temperate, mesic rainforest and that life was rich and abundant here in the Central Tablelands, NSW,” McCurry said.

“Many of the fossils that we are finding are new to science and include trapdoor spiders, giant cicadas, wasps and a variety of fish,” McCurry said.

“Until now it has been difficult to tell what these ancient ecosystems were like, but the level of preservation at this new fossil site means that even small, fragile organisms like insects turned into well-preserved fossils,” McCurry said.

The exceptional level of preservation from the site has led to its being termed a ‘Lagerstätte’, a term used to denote a sedimentary site that exhibits extraordinary fossils with detailed preservation. The fossils were formed in a type of iron-rich rock called goethite, which is not usually a source of exceptional fossils. 

“We think that the process that turned these organisms into fossils is key to why they are so well preserved,” explains McCurry. “Our analyses suggest that the fossils formed when iron-rich groundwaters drained into a billabong, and that a precipitation of iron minerals encased organisms that were living in, or fell into, the water.”

Associate Professor Michael Frese, who imaged the fossils using stacking microphotography and a scanning electron microscope (SEM), said that the fossils from McGrath’s Flat show an incredibly detailed preservation. 

“Using electron microscopy, I can image individual cells of plants and animals and sometimes even very small subcellular structures,” Frese said.

“The discovery of melanosomes (subcellular organelles that store the melanin pigment) allows us to reconstruct the color pattern of birds and fishes that once lived at McGrath’s Flat. Interestingly, the color itself is not preserved, but by comparing the size, shape and stacking pattern of the melanosomes in our fossils with melanosomes in extant specimens, we can often reconstruct color and/or color patterns,” Frese explained.

“The fossils also preserve evidence of interactions between species. For instance, we have fish stomach contents preserved in the fish, meaning that we can figure out what they were eating. We have also found examples of pollen preserved on the bodies of insects so we can tell which species were pollinating which plants,” Frese added.

Dr. McCurry said that the fossilized plants and animals are similar to those found in rainforests of northern Australia, but that there were signs that the ecosystem at McGrath’s Flat was beginning to become more arid.

Around 14 million years ago a sudden change in climate known as the “Middle Miocene Disruption” caused widespread extinctions of Australian fauna and flora. Subsequently, throughout the latter half of the Miocene, Australia gradually became more and more arid, and rainforests turned into the dry shrublands and deserts that now characterize the landscape.

“The pollen we found in the sediment [at McGrath’s Flat] suggests that there might have been drier habitats surrounding the wetter rainforest, indicating a change to drier conditions,” McCurry said.

Professor David Cantrill, Executive Director, Science, at the Royal Botanic Gardens in Victoria said that the variety of fossils preserved, together with an extraordinary fidelity of preservation, allows for unprecedented insights into an important time in Australia’s past, a time when mesic ecosystems still dominated the continent.

“The McGrath’s Flat plant fossils give us a window into the vegetation and ecosystems of a warmer world, one that we are likely to experience in the future. The preservation of the plant fossils is unique and provides important insights into a time period for which the fossil record in Australia is rather poor,” Cantrill said.

Australian Museum Chief Scientist and director of the AM’s Research Institute, Professor Kristofer Helgen said that the fossil site brings to life a picture of outback Australia that we can now barely believe existed.

“Australia is a unique continent biologically, and this site is extremely valuable in what it tells us about the evolutionary history of this part of the world. It provides further evidence of changing climates and helps fill the gaps in our knowledge of that time and region,” Helgen said.

The results of the paleontological investigation by the Australian scientists is published today in the journal Science Advances.

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By Alison Bosman, Earth.com Staff Writer