A field of rust in Australia is preserving fossils better than almost anywhere on Earth. Scientists finally know why.

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A spider fossil in rural Australia is so well preserved that the individual hairs on its legs are still visible. A fish fossil still carries its original eye pigments. An insect is frozen mid-visit to a flower, still dusted with pollen. All of them are preserved in iron, a material scientists never thought could do this.

On a dry farm in central New South Wales, a farmer named Nigel McGrath was clearing rocks from his field when he noticed something in the iron-red stones he was moving. The rocks were full of leaves. Not impressions of leaves, not shadows of leaves, but leaves so perfectly preserved that their individual pores were still visible under a microscope.

McGrath had stumbled onto one of the finest fossil sites on Earth. What no one could explain, until now, was why.

A new study published in the journal Gondwana Research has finally worked out the chemistry behind McGraths Flat, the name given to the site. The answer overturns a basic assumption that palaeontologists have held for a long time: that the kind of exceptional fossil preservation usually only happens in shale, sandstone, limestone, or volcanic ash. At McGraths Flat, it happened in iron.

The fossils at the site are startling. In 11-to-16-million-year-old red rocks, researchers from the Australian Museum Research Institute have found spiders preserved down to the finest hairs on their legs. Fish are preserved with their eye pigment cells still intact, which means scientists can work out what colour they were in life. Insects are preserved with their internal organs visible. One sawfly was caught in the act of visiting a flower, still dusted with pollen. Another fish fossil contains a parasitic mussel larva attached to its tail fin, hitching a ride.

None of this should be possible in iron-rich rock. Iron has never been considered a medium for this kind of preservation, especially not for land animals and plants. The world’s famous soft-tissue fossil sites, like Germany’s Messel Pit or Canada’s Burgess Shale, all formed in the fine sediments of ancient lakes or seafloors. McGraths Flat breaks that pattern entirely.

The new research explains how it happened. About 15 million years ago, the area was a warm, wet rainforest. Rainwater soaked through volcanic rock nearby, picking up dissolved iron as it went. That iron-rich water eventually reached an oxbow lake, the kind of quiet, slow-water body that forms when a river bends back on itself and cuts off a loop.

Fossilization Process

When organisms fell into this lake and settled on the bottom, the dissolved iron began to precipitate out of the water. Microscopic iron particles, each only a fraction of a millimetre across, coated the cells of dead organisms before they could rot. The iron locked everything in place at a cellular level.

The result, after millions of years, is a fossil record of almost impossible detail. And because the iron itself makes the fossils easier to image under modern electron microscopes, without the chemical treatments often needed at other sites, the site keeps yielding more information with each visit.

The significance of the new paper goes beyond explaining McGraths Flat. By understanding exactly how the site formed, the researchers have produced a guide for finding similar sites elsewhere.

The key conditions are specific: fine-grained iron-rich crust, ancient river channels cutting through basaltic volcanic rock, a warm and wet climate at the time of formation, and the absence of limestone or sulphur minerals that would interfere with the process. Researchers now believe that other sites just as detailed as McGraths Flat may be waiting to be found in eastern Australia and possibly on other continents.

McGraths Flat is still being excavated. Researchers have so far found insects, spiders, plants, fish, feathers, and fungi. They expect to eventually find whole birds. Every fossil from the site is a species scientists have never seen in the Australian fossil record from this period, making it a significant window into what Australia looked like before its rainforests retreated and the interior dried into the landscape it has today.

The study is published in Gondwana Research. The researchers acknowledge the traditional custodians of the land, the Wiradjuri Nation people.

References

Djokic, T., Smith, P.M., Havig, J.R., Frese, M., Vasconcelos, P., Brocks, J., Ellwood, M., Cantrill, D.J., McGeeney, D., Pogson, R., Ryan, C., Wang, Y., Nguyen, A., Rosas, M.L. and McCurry, M.R. “Taphonomy of soft-tissue preservation in ferricrete at the McGraths Flat Lagerstätte.” Gondwana Research, vol. 149, pp. 429 (2026). DOI: 10.1016/j.gr.2025.08.012 https://doi.org/10.1016/j.gr.2025.08.012

McCurry, M.R., Frese, M. et al. “A Lagerstätte from the Miocene of New South Wales reveals exceptional preservation of soft tissues.” Science Advances (2022). DOI: 10.1126/sciadv.abm1406 https://www.science.org/doi/10.1126/sciadv.abm1406

Quotes in this article are drawn from the paper by Tara Djokic, Matthew R. McCurry, Michael Frese et al., published in Gondwana Research (2026), DOI: 10.1016/j.gr.2025.08.012, and from the associated article by the authors published in The Conversation.

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