Laura Smith
Were mushrooms bigger in the past? It seems so. Giant mushrooms, or fungi, called Prototaxites, are believed to have dominated the Earth over 400 million years ago. These towering organisms reached heights of up to 24 feet (8 meters) and had trunks with diameters of up to 3 feet (1 meter). With their branchless, columnar structures, Prototaxites were the tallest living organisms of their time, towering over the moss forests and contemporaneous plants. The debate over the classification of Prototaxites has raged for over 150 years, with suggestions ranging from conifers to lichens, algae, or even an ancient type of tree. However, chemical analyses and fossil evidence suggest that Prototaxites were indeed giant fungi, becoming extinct over 350 million years ago.
| Characteristics | Values |
|---|---|
| Were mushrooms bigger in the past? | Yes, mushrooms were bigger in prehistoric times when there was more oxygen in the Earth's atmosphere. |
| Size of prehistoric mushrooms | Some mushrooms grew to be more than 20 feet tall, towering over everything else in the landscape. |
| Largest modern-day mushroom | A colony of honey mushrooms in Oregon is believed to be the world's largest known organism, covering nearly four square miles of forest. |
| Size of modern-day mushrooms | Some modern mushrooms can still grow to impressive sizes, such as the giant puffball, which may not even fit in a wheelbarrow. |
| Reasons for decrease in size | Habitat destruction, mushroom pickers, competition from other species (including trees), and changes in atmospheric conditions are possible factors. |
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What You'll Learn
Prototaxites fossils indicate giant mushrooms
For over a century, scientists have been baffled by a giant tree-like stump fossil. Was it a plant, an alga, a lichen, or something else entirely? The mystery was finally solved in 2007 when researchers from the University of Chicago and the Smithsonian National Museum of Natural History published a paper in the journal Geology. The fossil, known as Prototaxites, was identified as a giant fungus.
Prototaxites fossils are the remnants of a large macroscopic eukaryote dating from the Late Silurian to the Late Devonian periods. These fossils take the form of tree trunks, with a diameter of up to 1 metre (3 ft 3 in) and a height reaching 8.8 metres (29 ft). They are made up of tiny interwoven tubules around 50 micrometres in diameter. Prototaxites are widely considered to be a type of fungus, although the debate about their exact taxonomy is ongoing.
The identification of Prototaxites as a fungus was made through chemical analysis, specifically by examining the levels of carbon isotopes in the fossils. Plants tend to have similar carbon-12 and carbon-13 isotope ratios because they obtain their carbon from carbon dioxide in the air. Animals, on the other hand, consume other living things, so their carbon isotope ratios tend to be similar to what they eat. The carbon isotope ratios found in Prototaxites differed significantly from those of plants, indicating that it was not a photosynthetic organism. Instead, it likely fed on a range of substrates, such as the remains of other organisms.
The giant size of Prototaxites may be explained by the different atmospheric conditions during the Late Silurian and Early Devonian periods. During these times, there was more oxygen in the atmosphere, which allowed many living things to grow bigger. However, the rise of vascular plants during the Late Devonian period may have contributed to the eventual extinction of Prototaxites, as they competed for resources and space.
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The tallest living organisms
While mushrooms may have been bigger in the past, the tallest living organisms on Earth today are not fungi. The title of the tallest organism goes to organisms that are multicellular, like plants and trees.
The tallest tree in the world is Hyperion, a 379.7-foot-tall redwood. The General Sherman is a 2,000-year-old giant sequoia that is only middle-aged, with other sequoias believed to be more than 3,220 years old.
The tallest flowering plant is Rafflesia arnoldii, also known as the corpse flower. This plant boasts the largest bloom in the world, measuring 3 feet wide with blossoms that weigh 15 pounds.
The tallest animal is the blue whale, which can weigh up to 190 tons with a length of 91 feet. The African bush elephant is the largest land animal, reaching heights of 13 feet and weighing 11 tons.
The tallest mammal is the giraffe, which can grow to heights of 19 feet and weigh 2,800 pounds.
In terms of non-multicellular organisms, the largest are slime molds, which can reach a diameter of over 12 inches.
While not the tallest, it is worth noting that some of the largest living organisms are fungi. The Armillaria ostoyae, or honey mushroom, in Oregon's Blue Mountains covers 965 hectares or nearly four square miles of turf. It is estimated to be 2,400 years old and is believed to be the largest fungal colony in the world.
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Debate over classification
The existence of giant mushrooms has been a topic of debate for scientists for over 150 years. The debate revolves around the classification of certain giant, mysterious spires known as Prototaxites, which are estimated to be between 350 million and 420 million years old. These spires, which had "trunks up to 24 feet high and as wide as 3 feet", differ anatomically from modern mushrooms and do not resemble any existing organisms.
Initially, Prototaxites were believed to be kelp-like aquatic algae, fruiting bodies of fungi, or lichens, which are partnerships between fungi and algae. However, in 2007, a study by Hueber, Boyce, and others provided evidence that supported the hypothesis that Prototaxites were giant mushrooms, or the fruiting bodies of fungi. The study examined the ratio of carbon isotopes in Prototaxites fossils and found that this ratio was significantly different from that of plants, indicating that Prototaxites obtained their carbon from sources other than atmospheric carbon dioxide, as plants do.
Despite this evidence, not everyone is convinced that Prototaxites were early fungi. Some scientists question the idea of a 20-foot-tall fungus, arguing that such a size does not align with our understanding of fungi or algae. Additionally, it is acknowledged that extant fungi are already very poorly understood relative to other organisms, and extinct fungi are even less understood. This lack of understanding extends to their ecology, biology, and the specific factors that led to their extinction.
While the classification of Prototaxites remains a subject of debate, it is clear that giant fungi have existed and continue to exist in the present day. In 1992, a team of researchers discovered a colony of honey mushrooms in Michigan that weighed 10.5 tons and stretched across 37 acres. Later, in 1998, an even larger colony was found in Oregon, covering approximately 2,384 acres of soil in the Blue Mountains and estimated to weigh between 7,500 and 35,000 tons. These discoveries confirm that giant fungi, while rare, are not necessarily an anomaly, and there may be more undiscovered giant fungi in existence.
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Carbon isotope analysis
Mushrooms are likely to have been a part of the human diet in ancient times, but they are rarely preserved at archaeological sites and are often overlooked. Stable isotope analysis can provide insight into their consumption in the past. Mushrooms have a low carbon footprint, with an overall footprint of 0.12 kg (0.27 lb) of CO2e per pound of mushrooms, which is very low compared to other foods. This is mainly due to the few resources required to grow them and the fact that they are mostly grown domestically in the US.
Fungi play a crucial role in the carbon cycle and have a significant impact on carbon storage in the soil. They form symbiotic relationships with plants, increasing the carbon content in the soil by inhibiting decomposition and outcompeting soil microbes. This has important implications for understanding and mitigating global climate change.
Stable carbon isotope analysis has been applied to study methane (CH4) production by saprotrophic fungi, which play a role in nutrient recycling in ecosystems. By cultivating oyster and sulphur shelf mushrooms on different substrates, researchers measured the δ13C values of CH4 and carbon dioxide (CO2) emitted by the fungi. This helps understand the carbon isotope signature of methane produced by fungi and its impact on the atmosphere.
Additionally, carbon isotope analysis has been used to investigate carbon assimilation by marine fungi. By using 13C-labeled carbon from diatom extracellular polymeric substances (13C-dEPS), researchers traced fungal marine organic matter assimilation and studied the role of fungi in the marine carbon cycle. This involved combining the stable isotope probing (SIP) approach with community gene expression and metagenomic sequencing.
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The extinction of Prototaxites
The taxonomic classification of Prototaxites has been a subject of debate for over 165 years. Initially, it was believed to be a giant conifer or marine alga by J.W. Dawson, a Canadian scientist, in 1857. However, this interpretation was challenged by W. Carruthers in 1872, who suggested the name Nematophycus ("stringy alga"). Despite Carruthers' suggestion, the name Prototaxites remains in use due to botanical nomenclature rules.
The debate continued, with some classifying Prototaxites as a fungus, while others proposed it belonged to an entirely extinct lineage. In 2001, Francis Hueber published a paper arguing that Prototaxites was indeed a fungus based on its morphology. This conclusion was supported by carbon isotope analysis, which showed that Prototaxites had a wider-ranging isotope ratio than plants, indicating it obtained carbon from the atmosphere like fungi.
However, recent studies have cast doubt on the fungal hypothesis. In 2025, a molecular composition analysis revealed that Prototaxites lacked chitin and chitosan, compounds common to all fungi. This suggests that Prototaxites may indeed represent an entirely extinct lineage or even a new kingdom of life.
While the exact cause of Prototaxites' extinction is unknown, one theory suggests that the activity of animals inhabiting Prototaxites may have contributed to its demise. Evidence of arthropod boreholes has been found in early and late Devonian specimens, indicating that Prototaxites survived for millions of years despite this stress. Additionally, the rise of vascular plants in the Late Devonian may have also played a role in its extinction.
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Frequently asked questions
Yes, there is evidence that mushrooms were bigger in prehistoric times.
In prehistoric times, there was more oxygen in the atmosphere, allowing many living things to grow bigger.
Yes, there are still some giant mushrooms today. For example, in 1992, a team of researchers discovered a colony of honey mushrooms in Michigan that weighed 10.5 tons and stretched across 37 acres.
Mushrooms are now smaller due to habitat destruction, mushroom pickers, and competition from other species, such as trees.
