McGraths Flat is in the middle of New South Wales’ tablelands. The land now looks pretty normal, with fields, scattered trees, and dry weather. But between 11 and 16 million years ago, during the Miocene epoch, this very spot was a lush rainforest that surrounded a slow-moving river.
Today, that ecosystem that is no longer there lives on as thin slices of deep red rock. Researchers used a hammer and chisel to break up the layers, and they found fish, insects, leaves, feathers, spiders, and more. The cells are so well-preserved that they can often be seen under a microscope.
The fossils at McGraths Flat are so well-preserved that you can still see pigment cells in fish eyes and delicate spider hairs.
This kind of preservation is rare in any rock, but it’s almost never seen in things made mostly of iron.
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What makes these fossils so surprising
Most fossils that are “perfect” come from rocks that are very different from each other.
Shale, sandstone, limestone, or volcanic ash are home to many famous fossil treasure troves called lagerstätten. Some well-known examples are:
Messel Pit, Germany, is about 47 million years old. It has oily shale that keeps the shapes of fur, feathers, and stomach contents.
Burgess Shale, Canada – roughly 500 million years old, with soft-bodied sea creatures from near the dawn of animal life.
In those places, fine muds quickly buried animals, stopping them from rotting and letting even soft tissues turn into fossils. On the other hand, rocks with a lot of iron have usually been thought to be bad places to keep land-based life.
Most of these deposits are old “banded iron formations” that were formed more than two billion years ago in oceans that didn’t have oxygen. This was long before forests, insects, or mammals evolved. Iron usually shows up on continents as weathered rust, which turns cliffs and deserts red but doesn’t show any delicate biological details.
Iron-rich sedimentary rocks were thought to be some of the worst places to look for soft-tissue fossils on land before McGraths Flat.
The odd case of ferricrete
That rule is completely broken by McGraths Flat. The rock here that has fossils in it is called ferricrete, which is like natural iron cement. It is mostly goethite, an iron-oxyhydroxide mineral that gives the slabs their bright brick-red colour.
This ferricrete is made up of pieces that are only about 0.005 millimetres wide when looked at under a microscope. That very small grain size let dissolved iron get into and around dead organisms, coating and filling cells before they broke down.
The iron turned the soft tissues into stone by locking into solid goethite. This is why scientists can see:
- Cells that give colour to the retina in fish eyes
- The insides of insects and fish
- Nerve cells that are too small to see and hairs on spiders
- Fragile plant tissues and leaf structures
The result is comparable to, and in some instances identical to, the detail observed in traditional shale-based fossil sites, despite significant differences in the chemistry and texture of the host rock.
Rebuilding a rainforest from the Miocene era
The McGraths Flat fossils paint a clear picture of a rainforest that used to be very wet. Plant remains show that the plants were thick and liked to be wet. There are a lot of insects and spiders, which means that the forest floor and canopy are very complicated. The presence of a slow-moving river and oxbow lakes is shown by fish fossils and aquatic insects.
The Miocene was an important time for the development of ecosystems as we know them today. Many groups of plants and animals that are common today were changing their ranges or diversifying in response to changes in the climate. McGraths Flat captures this moment in high resolution, giving us rare detail for a time and place that didn’t have many soft-tissue terrestrial fossils before.
The site is like a time-stamped picture of a rainforest from 11 to 16 million years ago, but it was made of iron instead of mud or ash.
How a lake that was rusty turned into a fossil vault
From rock from volcanoes to mud with a lot of iron in it
The new study says that the story goes back to the weathering of basalt, a dark volcanic rock that is common in the area. In the warm, wet rainforest, acidic water slowly broke down the basalt and pulled out iron.
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That iron-rich groundwater flowed through the ground until it hit an old river bend, which is an oxbow lake that was made when a river loop was cut off and left behind as a crescent-shaped pool.
In this stagnant body of water, the chemical conditions made it easy for iron-oxyhydroxide particles to fall out of the water quickly. These particles settled on the lake bottom as fine sediment, where dead leaves, insects, fish, and other organisms built up.
Fast mineral coats, slow decay
Timing is everything when it comes to great preservation. Decay starts right away after an organism dies, especially in warm, wet places. The iron particles at McGraths Flat seem to have coated and entered tissues quickly enough to keep them stable before microbes could break down the fine structure.
Step in the process: What it does to make fossils
| Step | What it does to make fossils |
|---|---|
| Basalt breaking down | Let a lot of dissolved iron into the groundwater. |
| Take to oxbow lake | Iron that is concentrated in a calm, low-energy body of water. |
| Goethite precipitation | Made very fine iron sediment that settled on living things. |
| Infiltration of tissue | Iron particles filled cells and copied soft anatomical features. |
| Solidifying into ferricrete | Over a long period of time, locked the tiny structures into hard rock. |
The study also talks about what was not there. The deposit has only a small amount of limestone and very few minerals that are rich in sulphur, like pyrite. These materials can change the chemistry of water in ways that make it harder for iron-oxyhydroxide to form. Their absence likely helped the ferricrete form in a consistent way and keep its detail.
A new map for finding fossil treasure
The effects go far beyond just one place. If iron-rich ferricrete can hold such beautiful fossils in Australia, then similar conditions could have done the same on other continents.
McGraths Flat is like a geological “how-to guide” for finding more fossil sites with iron in them that are hiding in plain sight.
Researchers say that, based on the Australian case, we should look for places with:
- Proof that old rivers flowed over iron-rich volcanic rocks
- Warm and humid climates in the past that caused strong chemical weathering
- It is finely layered and has very fine grains, not coarse ironstone.
- There aren’t many limestone or sulphur minerals in the rocks around it.
That combination could mean that there are other oxbow lakes or ponds where iron once slowly settled on dead plants and animals, turning them into minerals one cell at a time.
What scientists mean when they say “lagerstätte”
In German, the word “lagerstätte” means “rich deposit.” In palaeontology, it refers to places where fossils are not only plentiful but also very well preserved, often with soft tissues like skin, organs, or feathers.
McGraths Flat is now a lagerstätte, which means it is one of a small number of important fossil sites around the world that lets scientists study ancient ecosystems as if they were still alive.
Ferricrete and goethite, in simple words
Ferricrete is basically concrete made from iron that is found in nature. Over time, water with a lot of iron in it hardens and binds loose sediment into a solid mass. The rock gets a deep reddish-brown colour when most of its mass is made up of a certain iron mineral, like goethite.
Goethite is an iron-oxyhydroxide. It often forms in soils and on rusted surfaces. At McGraths Flat, it has formed into very small particles that perfectly captured the shapes of life from the Miocene era.
Why this is important for research on climate and biodiversity
McGraths Flat is more than just a geological curiosity for scientists who want to know how living things react to changes in the climate. During the Miocene, parts of Australia became cooler, the amount of rain changed, and more open habitats spread. Researchers can see how species reacted, which lineages thrived, and which ones faded by looking at a rainforest community that has been preserved in such detail.
Fossils can also help us test climate models. The shapes of leaves, the types of insects that live in them, and the types of freshwater fish that live in them all depend on temperature, humidity, and water chemistry. Researchers can improve their simulations of past Earth systems when biological signals match or contradict modelled climate conditions.
There is also a practical side to this. Knowing exactly what conditions make iron lock up organic matter can help researchers learn more about how carbon is stored in today’s soils and wetlands. Fossilisation happens over a long period of time, but the same basic chemistry helps control how much carbon landscapes can hold today.
For anyone who is interested in deep time, McGraths Flat is a great place to remember that the next big leap in understanding ancient life might not come from a well-known limestone cliff or shale quarry, but from a layer of rusty-red rock lying quietly under someone’s farm, waiting for a hammer blow to split it open.
