Scientists find perfect fossils in rust beneath Australian farmland

Researchers from the Australian Museum Research Institute have uncovered an extraordinary collection of fossils here. Although the region is now dry and dusty, it was once a thriving rainforest. The fossils provide a detailed look at this ancient ecosystem, capturing life as it existed millions of years ago.

The rocks at McGraths Flat stand out for their deep red color. They are made entirely of goethite, a fine-grained iron-rich mineral. This iron has preserved plants, insects, spiders, fish, and even feathers with remarkable clarity.

A recent study published in the journal Gondwana Research highlights another surprising aspect of this site. It challenges long-standing assumptions about where exceptionally preserved fossils can form and the conditions required to create them.

Rethinking Fossil Preservation Beyond Traditional Rocks

Most well-known fossil sites are found in rocks such as shale, sandstone, limestone, or volcanic ash. These environments allow organisms to be quickly buried in fine sediments, which helps preserve not only hard parts like bones but also soft tissues.

Famous examples include Germany's Messel Pit and Canada's Burgess Shale. Fossils from Messel Pit, about 47 million years old, preserve details like feathers, fur, and skin. The Burgess Shale, which dates back around 500 million years, contains soft tissues from some of the earliest animal life on Earth.

In contrast, iron-rich rocks have not typically been associated with well-preserved fossils, especially those of land-based (terrestrial) plants and animals.

This is largely because iron-rich sedimentary rocks are best known from banded iron formations. These deposits formed roughly 2.5 billion years ago in ancient oceans that lacked oxygen, long before complex life evolved.

More recently, iron is usually seen as a product of weathering. It forms rust on land when exposed to oxygen, as seen in Australia's red outback landscapes, which preserve features ranging from millions to billions of years old.

The discoveries at McGraths Flat go against these expectations.

Fossils Preserved in Iron With Cellular Detail

The rock at McGraths Flat is a fine-grained, iron-rich material called ferricrete, which acts like a natural cement made of iron.

This ferricrete is composed of microscopic iron-oxyhydroxide particles, each measuring about 0.005 millimeters. When organisms died and became buried, these tiny particles filled their cells. This process allowed soft tissues to be preserved in extraordinary detail.

Fossil sites that preserve land-based ecosystems are already rare. Sites that capture soft tissues from terrestrial organisms are even more unusual. McGraths Flat stands out because it preserves features that are almost never seen.

Scientists have identified pigment cells in fish eyes, internal organs in insects and fish, and even fine structures such as spider hairs and nerve cells.

The level of detail rivals that of the world's best fossil sites formed in shale or sandstone. The difference is that these fossils are preserved in iron.

How the McGraths Flat Fossil Site Formed

The new study also explains how this unique fossil site developed, offering clues for locating similar sites in the future.

During the Miocene, the area experienced warm and wet rainforest conditions. Iron was released from weathering basalt and carried underground by acidic groundwater.

This dissolved iron eventually reached a river system that included an oxbow lake, which is an abandoned river channel. There, the iron formed extremely fine iron-oxyhydroxide sediments.

These particles quickly coated organisms that had settled on the lake floor, preserving their soft tissues at a microscopic level.

A New Way to Find Exceptional Fossil Sites

Understanding how McGraths Flat formed provides a useful guide for identifying other iron-rich fossil sites around the world.

Researchers suggest looking for very fine-grained, layered ferricrete in locations where:

• ancient river channels cut through older iron-rich landscapes, such as basaltic rocks from volcanoes
• warm and humid conditions once caused intense weathering
• the surrounding geology lacks significant limestone or sulfur-containing minerals (such as pyrite), which could interfere with iron-oxyhydroxide formation

The discoveries at McGraths Flat expand our understanding of how exceptional fossil sites can form.

Future breakthroughs in studying ancient terrestrial life may not come from traditional fossil-bearing rocks like shale or sandstone, but from iron-rich deposits hidden beneath the surface.

The study's authors acknowledge the traditional custodians of the land and waterways on which McGraths Flat is located, the Wiradjuri Nation people.