Beneath farmland on the Central Tablelands of New South Wales lies one of Australia’s most remarkable fossil sites, known as McGrath Flat. The site dates back to the Miocene epoch, 11 to 16 million years ago, when many modern plants and animals first appeared.
Researchers from the Australian Museum Research Institute have discovered an amazing collection of fossils here. Although the area is now dry and dusty, it was once a rich rainforest. Fossils provide a detailed look at this ancient ecosystem, capturing life as it existed millions of years ago.
The rocks at McGrath Flat stand out with their deep red color. They are made entirely of goethite, a fine-grained iron-rich mineral. This iron preserved plants, insects, spiders, fish, and even feathers in amazing clarity.
Recent research published in journals Gondwana research It highlights another amazing aspect of this site. This challenges long-held assumptions about where exceptionally preserved fossils form and the conditions necessary to generate them.
Rethinking fossil preservation beyond traditional rocks
The most well-known fossil sites are found in rocks such as shale, sandstone, limestone, and volcanic ash. These environments allow organisms to quickly embed themselves in fine sediments, preserving hard parts such as bones as well as soft tissues.
Famous examples include Germany’s Messel Pit and Canada’s Burgess Shale. Messel Pit fossils, which are approximately 47 million years old, preserve details such as feathers, fur, and skin. The Burgess Shale dates back about 500 million years and contains soft tissue from some of the earliest animals on Earth.
In contrast, iron-rich rocks are not associated with well-preserved fossils, especially fossils of terrestrial (terrestrial) plants and animals.
This is primarily because iron-rich sedimentary rocks are best known for their banded iron layers. These deposits formed in ancient oceans lacking oxygen some 2.5 billion years ago, long before complex life evolved.
These days, iron is usually considered to be a product of weathering. As seen in Australia’s red outback landscapes, rust forms on land when exposed to oxygen, preserving landforms from millions to billions of years ago.
The McGrath Flat discovery contradicted these expectations.
Fossils preserved in iron containing cellular details
The rock at McGrath Flat is a fine-grained iron-rich material called ferricrete, which acts like a natural cement made of iron.
This ferricrete is composed of fine iron oxyhydroxide particles, each approximately 0.005 millimeters in size. When an organism dies and is buried, these tiny particles fill its cells. This process made it possible to preserve the soft tissue in great detail.
Fossil remains that preserve terrestrial ecosystems are already rare. Even more unusual are the locations where soft tissue is harvested from terrestrial organisms. McGraths Flat stands out because it preserves features that are rarely seen.
Scientists have identified pigment cells in microstructures such as the eyes of fish, the internal organs of insects and fish, and even spider hairs and nerve cells.
Its level of detail rivals the world’s best fossil sites formed in shale and sandstone. The difference is that these fossils are preserved in iron.
How the McGrath Flat Fossil Site was formed
The new study also explains how this unique fossil site developed and provides clues for finding similar sites in the future.
During the Miocene, the region experienced warm and humid rainforest conditions. Iron was released from weathered basalt and transported underground by acidic groundwater.
This molten iron eventually reached the river system, including Lake Crescent, an abandoned river channel. There, the iron formed very fine iron oxyhydroxide precipitates.
These particles quickly covered the organisms deposited on the lake floor, preserving their soft tissues at the microscopic level.
A new way to find great fossil sites
Understanding how McGrath Flat was formed provides a useful guide to identifying other iron-rich fossil sites around the world.
Researchers suggest looking for very fine-grained, layered ferricrete in locations such as:
- Ancient river channels carved through old iron-rich terrain, including basaltic rocks from volcanoes
- Warm, humid conditions once caused intense weathering
- The surrounding geology has little limestone or sulfur-containing minerals (such as pyrite) that could interfere with iron oxyhydroxide formation.
The McGrath Flat discovery deepens our understanding of how exceptional fossil remains are formed.
Future breakthroughs in the study of ancient land life may come not from traditional fossil-bearing rocks such as shales and sandstones, but from iron-rich deposits hidden beneath the earth’s surface.
The study’s authors acknowledge the Wiradjuri Nation people as the traditional custodians of the land and waterways on which McGrath Flat is located.
