''Prototaxites'' is a genus of terrestrial fossil fungi dating from the

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

until the Late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

periods, approximately . ''Prototaxites'' formed small to large trunk-like structures up to wide, reaching in length, made up of interwoven tubes around in diameter, making it by far the largest land-dwelling organism of its time. Whilst traditionally very difficult to assign to an

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

group of organisms, current opinion suggests a

fungal A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

placement for the genus. Its exact relationship with extant fungus lineages is uncertain. It was almost certainly a

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

organism that grew over multiple years. Several ecologies have been proposed, including that it was

saprotrophic Saprotrophic nutrition or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of decayed (dead or waste) organic matter. It occurs in saprotrophs, and is most often associated with fungi (f ...

like many modern fungi, or that it was a lichenised

autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

Morphology

left, upDawson's 1888 reconstruction of a conifer-like ''Prototaxites'' Prototaxm10

With a diameter of up to , and a height reaching , ''Prototaxites'' fossils are remnants of by far the largest organism discovered from the period of its existence. Viewed from afar, the fossils take the form of tree-trunks, spreading slightly near their base in a fashion that suggests a connection to unpreserved root-like structures. Infilled casts which may represent the spaces formerly occupied by "roots" of ''Prototaxites'' are common in early Devonian strata. Concentric growth rings, sometimes containing embedded plant material, suggest that the organism grew sporadically by the addition of external layers. It is probable that the preserved "trunks" represent the fruiting body, or "

sporophore {{Short pages monitor") be adopted, a move strongly against scientific convention. Dawson fought adamantly to defend his original interpretation until studies of the microstructure made it clear that his position was untenable, whence he promptly attempted to rename the genus himself (to ''Nematophyton'', "stringy plant"), denying with great vehemence that he'd ever considered it to be a tree. Despite these political attempts to rename the genus, the rules of botanical nomenclature mean that the name "''Prototaxites''", however inappropriate in meaning, remains in use today. Despite the overwhelming evidence that the organism grew on land, Carruthers's interpretation that it was a giant marine

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...

was challenged just the once, in 1919, when

Church Church may refer to: Religion * Church (building), a building for Christian religious activities * Church (congregation), a local congregation of a Christian denomination * Church service, a formalized period of Christian communal worship * Chris ...

suggested that Carruthers had been too quick to rule out the possibility of the fungi. The lack of any characters diagnostic of any extant group made the presentation of a firm hypothesis difficult; the fossil remained an enigmatic mystery and subject of debate. It was not until 2001, after 20 years of research, that Francis Hueber, of Washington's

National Museum of Natural History The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7 ...

, published a long-awaited paper which attempted to put ''Prototaxites'' in its place. The paper deduced, based on its morphology, that ''Prototaxites'' was a fungus. This idea was received with disbelief, denial and strong scepticism, but further evidence is emerging to support it. In 2007, isotopic analyses by a team including Hueber and

Kevin Boyce Kevin L. Boyce (born October 5, 1971) is an Americans, American politician of the Democratic Party (United States), Democratic Party. He currently serves as President of the Franklin County, Ohio, Franklin County Board of Commissioners. Formerly ...

of the

University of Chicago The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...

concluded that ''Prototaxites'' was a giant fungus. They detected a highly variable range of values of

carbon isotope Carbon (6C) has 15 known isotopes, from to , of which and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by t ...

ratios in a range of ''Prototaxites'' specimens; autotrophs (organisms such as plants and algae, that make a living via

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

) living at the same time draw on the same (atmospheric) source of carbon; as organisms of the same type share the same chemical machinery, they reflect this atmospheric composition with a constant carbon isotope trace. The inconsistent ratio observed in ''Prototaxites'' appears to show that the organism did not survive by photosynthesis, and Boyce's team deduced that the organism fed on a range of substrates, such as the remains of whichever other organisms were nearby. Nevertheless, the large size of the organism would necessitate an extensive network of subterranean mycelia in order to obtain enough organic carbon to accumulate the necessary biomass. Root-like structures have circumstantially been interpreted as ''Prototaxitess

rhizomorphs Mycelial cords are linear aggregations of parallel-oriented hyphae. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae. Cords may look similar to plant roots, and also frequently have similar function ...

, and could support the possibility of the organism transporting nutrients large distances to support its above-ground body. Other recent research has suggested that ''Prototaxites'' represents a rolled-up bundle of liverworts, but this interpretation has substantial difficulties. A similar genus, ''

Nematasketum ''Nematasketum'' (sometimes incorrectly spelt nematosketum) is a nematophyte with internally thickened tubes. It is thought to be terrestrial or freshwater, and seems to be aligned with the fungi. References Silurian plants {{siluri ...

'', also consists of banded and branching tubes in axial bundles; this seems to be a fungus. A 2022 paper suggested that ''Prototaxites'' was a fungal

rhizomorph Mycelial cords are linear aggregations of parallel-oriented hyphae. The mature cords are composed of wide, empty vessel hyphae surrounded by narrower sheathing hyphae. Cords may look similar to plant roots, and also frequently have similar functio ...

that grew on its side and likely at least partially underground, as opposed to the traditional view that it grew upright.

Species

''Prototaxites honeggeri'' is the oldest known described species ''Prototaxites'', known from the

Darriwilian The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species '' ...

age

Douglas Lake Member The Douglas Lake Member is a geologic unit of member rank of the Lenoir Limestone that overlies the Mascot Dolomite and underlies typical nodular member of the Lenoir Limestone in Douglas Lake, Tennessee, region. It fills depressions that are ...

of the Lenoir Limestone, at

Douglas Dam Douglas Dam is a hydroelectric dam on the French Broad River in Sevier County, Tennessee, in the southeastern United States. The dam is operated by the Tennessee Valley Authority (TVA), which built the dam in record time in the early 1940s to me ...

, Tennessee. ''P. honeggeri'' is a small species with simple erect trunk and 4-5 club-shaped terminal branches furrowed by transverse wrinkles.''P. honeggeri'' is similar to other species, but much smaller, about the size of a pencil. It was lichenized by small coccoids, probably green algal, attached to generative hyphae.

Ecological context

''Prototaxites'' would have been the tallest living organism in its day by far. In comparison, the plant ''

Cooksonia ''Cooksonia'' is an extinct group of primitive land plants, treated as a genus, although probably not monophyletic. The earliest ''Cooksonia'' date from the middle of the Silurian (the Wenlock epoch); the group continued to be an important comp ...

'' only reached 6 cm, and itself towered over the "moss forests".

Invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s were the only other land-dwelling multi-cellular life. ''Prototaxites'' became extinct as vascular plants rose to prominence. The organism could have used its tall columnar structure for spore dispersal. Alternatively, if ''Prototaxites'' contained photosynthetic structures, the height would have increased light capture. The

research team has it reconstructed as a branchless, columnar structure.Prehistoric mystery organism verified as giant fungus
Press release from

, April 23, 2007. The presence of bio-molecules often associated with the algae may suggest that the organism was covered by symbiotic (or parasitic) algae (making it in essence a huge

lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.δ¹³C between speciemens of ''Prototaxites'' suggest that it was heterotrophic. ''Prototaxites''

mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates ...

(strands) have been fossilised invading the tissue of vascular plants; in turn, there is evidence of animals inhabiting ''Prototaxites'': mazes of tubes have been found within some specimens, with the fungus re-growing into the voids, leading to speculation that the organisms' extinction may have been caused by such activity; however, evidence of arthropod borings in ''Prototaxites'' has been found from the early and late Devonian, suggesting the organism survived the duress of boring for many millions of years. Intriguingly, ''Prototaxites'' was bored long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved.

References

External links

Images and discussion of the classification of ''Prototaxites''
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{{Taxonbar, from=Q2061593 Late Devonian genus extinctions Devonian fungi Silurian fungi Pridoli first appearances Enigmatic fungus taxa Fossil taxa described in 1859 Paleozoic life of Ontario Paleozoic life of New Brunswick