Before trees ruled, the Earth was covered with giant fungi.

When I first heard that the Earth was once covered in giant fungi, the image above popped into my head: mushrooms with giant caps. But in fact, the fungi that dominated the world at that time were "Prototaxites" , different from the fungi we know today.

The mystery lasted more than a hundred years

According to The University of Chicago Magazine  , the genus Protomycota is believed to have lived between 430 million and 350 million years ago. "At that time, the land was inhabited only by insects and small plants. So protozoans were probably the largest land creatures on Earth at that time ," said C. Kevin Boyce, a geophysicist at the university.

Fossils of the genus Protomycota were not discovered until the mid-19th century. Since then, its fossils have been discovered in Saudi Arabia, Australia, Canada, and elsewhere. According to research, they can even reach a maximum height of 8 meters and have a trunk structure of about 1 meter wide.

Prototaxites is a genus of terrestrial fossil fungi that dates from the late Silurian to the late Devonian. Prototaxites formed large, stem-like structures up to 1 meter wide and 8 meters long, made up of interwoven tubes about 50 micrometers in diameter, making it the largest living terrestrial organism of its time. (Photo: ZME).

In fact, for the past 150 years, scientists have debated exactly what  the genus Protomycota  is. They were initially classified as conifers and then thought to be lichens or algae. Eventually, most people began to realize that they were actually fungi.

The main problem with protists, Boyce says, is that "when you look closely at their anatomy, it evokes so many different things. Their size also makes it hard to imagine . "

It wasn't until 2007 that Boyce figured out how to use geochemistry to study these mysterious creatures. Even though it was a fossil, its carbon isotopes could be analyzed.

Like a giant rhizome

In a 2022 academic paper published in  the Canadian Journal of Microbiology  , a team of scientists compared samples of primitive fungi from quarries in Germany and Canada with common fungi of our time. Each was studied using a variety of powerful, modern microscopes.

They found that the structures in the fossils looked very similar to modern root-like bodies in some fungi they grew in the lab.

These root-like appendages allow fungi to transport nutrients over long distances. Additionally, these fungi feed on other organisms.

The researchers explain that protomycetes, both fossil and modern fungi, 'consist of tubular structures of similar length and diameter that fuse together to form tissue-like material.' In addition, in their lab, some microscopic images of the root-like bodies in which the fungi grow.

From a distance, the fossils appear to be trunk-like, slightly spreading near the base in a manner that suggests a connection to root-like structures. Prototaxites were probably the tallest living organisms of their time. At the microscopic scale, the fossils consist of narrow, intertwined tube-like structures. They come in two varieties: skeletal 'tubes,' which are 20–50 μm across, thick-walled (2–6 μm) and undivided along their length, and are capable of reproduction. (Photo: Zhihu).

However, they have one major disagreement: the team doesn't believe that primary fungi grow vertically . In their view, fungi grow horizontally . And there's actually some logic to back up their idea.

First, there are no recorded specimens of them standing upright on the ground. It is clear from the fossils that they are all lying down. Additionally, nutrient gathering would be better if these fungi were lying horizontally, especially in real-life environments. The researchers note:

'At that time, there was an abundance of living plants and an accumulation of dead plant matter available for the fungi to decompose.' Apparently, placing it horizontally would allow the protozoan to spread over greater distances in search of nutrients and water.

Although most people support the idea that the protist mushroom was an upright, tree-like organism due to its stem structure, this association gives us a different picture of the world long ago.

First collected in 1843, it wasn't until 14 years later that Canadian scientist John William Dawson studied the Prototaxite fossils , which he described as partially rotted giant conifers. (Photo: ZME).

Although for us humans, ten thousand years of existence may seem like a long time, this period of time is but a blip in the history of the Earth.

We don't know what the Earth looked like hundreds of millions of years ago. And our understanding of this world still seems very shallow.