The beautifully preserved filaments from a fungus that lived 440 million years ago are the oldest-known fossils of a land-dwelling organism yet found, according to a new study.

Key points: Analysis of tiny fossils indicates they are root-like filaments of fungus.

Analysis of tiny fossils indicates they are root-like filaments of fungus. Fungus appeared around 440 million years ago, just before moss-like plants.

Fungus appeared around 440 million years ago, just before moss-like plants. Likely that other organisms appeared on land earlier but fossil evidence is hard to find.

The early pioneer, known as Tortotubus, helped lay the foundation for more complex organisms to take root and thrive on land, said palaeontologist Dr Martin Smith of Durham University in the United Kingdom.

"It's hard to imagine the surface of the Earth before any vegetation — picture barren rocks, with any dust being quickly blown into rivers and the sea," said Dr Smith.

"The tiny threads of Tortotubus would have helped to consolidate and hold together early soils, allowing deeper soils to form.

"Tortotubus would also have broken down organic matter into a form where its nutrients were available to other organisms, providing a form of fertiliser to early land plants," he said.

Those land plants would have included moss-like plants that appeared shortly after.

Tiny fossils — each shorter than a human hair is wide — were first discovered more than 30 years ago on the Swedish island of Gotland, but it was unclear what they were.

It was while Dr Smith was trying to find another organism in the same fossil-rich rocks that he began to look at Tortotubus more closely.

"After a long time looking at the fossils I eventually realised that it'd be possible to reconstruct the way that the animal grew, and work out what its true affinity was," said Dr Smith, who conducted the work while at the University of Cambridge.

He discovered the fossils resembled cord-like filaments called mycelium that fungi use to extract nutrients from the soil.

"When a fungus grows it sends forwards teeny filaments, a single cell in width, that 'forage' to look for a food source ... then send out branches that grow back down the original filament, allowing the fungus to pipe nutrients and energy from food sources to other parts of the organism," he explained.

An electron scanning micrograph close-up of a fully grown Tortotubus "cord" or "pipe" that would have shuttled nutrients from the foraging frontier back to the heart of the organism. ( Martin R. Smith )

Fossil not likely to be first land-based organism

While Tortotubus may be the oldest-known fossil, it is unlikely it was the first organism on land, said Dr Smith.

"We don't have any direct fossil evidence, but it is likely that the Earth harboured very simple films of bacteria, alga-like or lichen-like organisms long before anything as complex as Tortotubus," he said.

But, said Dr Smith, it was very difficult to find fossils of early land-based organisms because they eroded easily.

"These early fungi seem to have lived in an environment that was very close to a sheltered shoreline," he said.

"They must have been washed a short distance to the coast then quite quickly buried, probably in fine clay-rich muds that sealed them off from the atmosphere and stopped them from decaying away."

Their structure also helped.

"It's only because the cell walls were so tough to begin with that they were able to survive the vagaries of fossilisation," Dr Smith said.

The research is published today in the Botanical Journal of the Linnean Society.