Sydney, Oct 13 (The Conversation) Hidden beneath the farmlands of New South Wales’ central tablelands lies McGraths Flat, one of Australia’s most extraordinary fossil sites. This location dates back 11 to 16 million years into the Miocene epoch, a period when many of today’s ubiquitous plants and animals came into existence.
Palaeontologists and geologists from the Australian Museum Research Institute have uncovered remarkable fossil discoveries in McGraths Flat. Presently an area torn by dust and drought, this site was once a flourishing, lush rainforest. The fossils found here depict this primordial ecosystem with stunning ecological detail.
McGraths Flat's strikingly red sedimentary rocks are entirely made up of goethite, a fine-grained mineral rich in iron. This iron has done an extraordinary job of preserving a wide range of plant, insect, spider, fish, and feather fossils.
Our recent study, published in the journal Gondwana Research, demonstrates another reason these rocks are fascinating. They challenge existing theories about the locations and potential of exceptionally well-preserved fossil sites around the planet.
Traditionally, the most remarkably preserved fossil sites are found in shale, sandstone, limestone, or volcanic ash. Consider places like Germany’s Messel Pit or Canada’s Burgess Shale, where organisms were rapidly buried in fine-grained sediments, allowing soft tissues’ preservation alongside hard parts.
The Messel Pit has protected fossils that are about 47 million years old, showing the outlines of feathers, fur, and skin. Meanwhile, the Burgess Shale holds soft tissues from some of Earth’s earliest animal life, dating back around 500 million years.
Contrastingly, one would not expect to find well-preserved remains of terrestrial animal and plant life within sedimentary rocks composed entirely of iron. Such iron-rich sedimentary rocks are typically known from banded iron formations, which formed roughly 2.5 billion years ago in ancient oceans absent of oxygen, prior to the evolution of complex animal and plant life.
In modern history, iron usually manifests as a byproduct of weathering, forming rust on continents when exposed to our oxygen-rich atmosphere. This results in Australia’s iconic red-rocked outback landscape that encases these ancient features. Yet, McGraths Flat defies these expectations significantly.
McGraths Flat is composed of a very fine-grained, iron-rich rock known as ferricrete, essentially an iron cement. The ferricrete consists almost entirely of microscopic iron-oxyhydroxide mineral particles, with a diameter of only 0.005 millimeters. When creatures died and were encapsulated in the sediment, this minute scale permitted the iron particles to occupy every cell, ensuing in extraordinarily preserved soft tissue fossils.
Fossil sites that conserve terrestrial life are exceptionally rare when compared with marine life, and sites that preserve terrestrial soft tissues even more so. The remarkable detail captured in the McGraths Flat fossils unveils new glimpses into our distant past.
These fossils are preserved so thoroughly that individual pigment cells in fish eyes, insects’ internal organs, and even the delicate hairs and nerve cells of spiders can be discerned. This level of preservation rivals that of other well-preserved fossil sites, such as shale or sandstone, but here, it’s all enfolded in iron.
Our new study illuminates how McGraths Flat formed, a pivotal step toward locating more terrestrial fossil troves encased in iron. McGraths Flat began taking form during the Miocene when iron leached from weathering basalt under warm, wet rainforest conditions.
Acidic groundwater ferried the dissolved iron beneath the surface until meeting a river system with an oxbow lake, an abandoned river channel. Here, the iron transformed into ultra-fine iron-oxyhydroxide sediment, quickly coating dead organisms on the lake floor and replicating their soft tissue structures down to the cellular level.
Understanding McGraths Flat’s formation paves a new path for discovering similarly iron-rich fossil sites globally. The red rocks of McGraths Flat begin anew chapter in our comprehension of how exceptionally well-preserved fossil sites evolve.
The subsequent breakthroughs in understanding ancient terrestrial life might emerge not from traditional shale or sandstone beds, but from the rusty-red rocks hidden beneath our feet. (The Conversation) RD RD
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