| Prototaxites | |
|---|---|
 | |
| Branching apex ofP. loganii, "Schunnemunk tree" | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Stem group: | Ascomycota |
| Family: | †Prototaxitaceae Hueber |
| Genus: | †Prototaxites Dawson 1859 |
| Type species | |
| Prototaxites loganii Dawson, 1859 | |
| Species | |
| |
| Synonyms | |
| |
Prototaxites/ˌproʊtoʊˈtæksɪtiːz/ is anextinctgenus of terrestrialfungi dating from theLate Silurian until the LateDevonian periods.[1][2]Prototaxites formed large trunk-like structures up to 1 metre (3 ft) wide, reaching 8 metres (26 ft) in length,[3] made up of interwoven tubes around 50 micrometres (0.0020 in) in diameter, making it by far the largest land-dwelling organism of its time.
Thetaxonomy ofPrototaxites has long been the subject of debate. It is widely considered a fungus, but the debate is ongoing.[3] Its exact relationship with extant fungus lineages is uncertain. It was almost certainly a perennial organism that grew over multiple years. Severalecologies have been proposed, including that it wassaprotrophic like many modern fungi, or that it was alichenisedautotroph.[1]
With a diameter of up to 1 metre (3 ft 3 in), and a height reaching 8.8 metres (29 ft),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.[4] Infilled casts which may represent the spaces formerly occupied by "roots" ofPrototaxites are common in early Devonian strata.[5] Concentric growth rings, sometimes containing embedded plant material,[6] suggest that the organism grew sporadically by the addition of external layers. It is probable that the preserved trunk-like structures represent the fruiting body, orsporophore, of a fungus, which would have been fuelled by amycelium, a net of dispersed filaments (hyphae). On a microscopic scale, the fossils consist of narrow tube-like structures, which weave around one another. These come in two types: skeletal tubes, 20–50 micrometres (μm) across, have thick (2–6 μm) walls and are undivided for their length, andgenerative filaments, which are thinner (5–10 μm in diameter) and branch frequently; these mesh together to form the organism's matrix. These thinner filaments areseptate—that is, they bear internal walls. These septa areperforate—i.e. they contain agerm pore, a trait only present in the modernred algae and fungi.[7]
The similarity of these tubes to structures in the earlyplantNematothallus has led to suggestions that the latter may represent leaves ofPrototaxites. Unfortunately for this hypothesis, the two have never been found in connection, although this may be a consequence of their detachment after the organisms' death.[8]
First collected in 1843,[9] it was not until 14 years later thatJohn William Dawson, a Canadian scientist, studiedPrototaxites fossils, which he described as partially rotten giantconifers, containing the remains of the fungi which had been decomposing them.[10][11][6] This concept was not disputed until 1872, when the rival scientistWilliam Carruthers poured ridicule on the idea.[12] Such was his fervour that he rebuked the namePrototaxites (loosely translated as "firstyew"[13]) and insisted that the nameNematophycus ("stringyalga"[14]) be adopted,[6] a move strongly against scientific convention.[15] 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, calling itNematophyton ("stringy plant"), and denying with great vehemence that he had ever considered it to be a tree.[6] Despite these political attempts to rename the genus, therules 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,[16][17] Carruthers's interpretation that it was a giant marine alga was challenged just once, in 1919, whenArthur Harry Church suggested that Carruthers had been too quick to rule out the possibility that it was a fungus.[18] The lack of any characters diagnostic of any extant group made the presentation of a firm hypothesis difficult;[6] the fossil remained an enigmatic mystery and subject of debate. It was not until 2001, after 20 years of research, that Francis Hueber, of theNational Museum of Natural History inWashington, D.C., published a long-awaited paper which attempted to putPrototaxites in its place. The paper deduced, based on itsmorphology, thatPrototaxites was a fungus.[6]
This idea was received with disbelief, denial, and strong scepticism, but further evidence is emerging to support it.[19] In 2007,isotopic analyses by a team including Hueber andKevin Boyce of theUniversity of Chicago[3] concluded thatPrototaxites was a giant fungus. They detected a highly variable range of values ofcarbon isotope ratios in a range ofPrototaxites specimens. Autotrophs (organisms such as plants and algae that make a living viaphotosynthesis) living at the same time draw on the same (i.e.,atmospheric) source ofcarbon; as organisms of the same type share the same chemical machinery, they reflect this atmospheric composition with a constant carbonisotope trace. The inconsistent ratio observed inPrototaxites appears to show that the organism did not survive by photosynthesis, and Boyce's team deduced that the organism fed on a range ofsubstrates, such as the remains of whichever other organisms were nearby.[3] Nevertheless, the large size of the organism would necessitate an extensive network of subterraneanmycelia in order to obtain enoughorganic carbon to accumulate the necessarybiomass. Root-like structures have circumstantially been interpreted asPrototaxites'srhizomorphs, and could support the possibility of the organism transporting nutrients large distances to support its above-ground body.[20]
Other recent research has suggested thatPrototaxites represents a rolled-up bundle ofliverworts,[21] but this interpretation has substantial difficulties.[22]
A similar genus,Nematasketum, also consists of banded and branching tubes in axial bundles; this seems to be a fungus.[23]
In 2021,Gregory Retallack described the new speciesPrototaxites honeggeri from theDarriwilian stage of the MiddleOrdovicianDouglas Lake Member of theLenoir Limestone, atDouglas Dam,Tennessee, which marks the earliest known appearance of the genusPrototaxites.[24] While an Ordovician origin of the genus is mentioned by some studies,[2]paleobotanistDianne Edwards commented, referring to Retallack′s study, "When diagnostic features are absent, such fragmentaryorganic materials can be misinterpreted, leading to implausible attributions".[25] Nelson and Boyce (2022), referring to Retallack′s study, maintained that the first appearance of the genus was in the Late Silurian.[1]
A 2022 paper suggested thatPrototaxites was a fungalrhizomorph that grew on its side and likely at least partially underground, as opposed to the traditional view that it grew upright.[2]
A 2025 preprint argued thatPrototaxites was not a fungus but instead a member of an entirely extinct lineage of multicellular terrestrial eukaryotes, due to the lack of fossil chitin traces in chemical samples.[26]
TheUniversity of Chicago research team has reconstructedPrototaxites as a branchless, columnar structure.[27] It was the tallest living organism in its day by far; in comparison, the contemporary plantCooksonia only reached 6 centimetres (2.4 in) in height and itself towered over the "moss forests" that grew beneath it, andinvertebrates were the only other land-dwelling multi-cellular life.Prototaxites could have used its tall columnar structure for spore dispersal. Alternatively, ifPrototaxites contained photosynthetic structures, the height would have increased its ability to capture light.[4] The presence ofbiomolecules often associated with algae may suggest thatPrototaxites was covered bysymbiotic orparasitic algae, making it in essence a hugelichen, or even suggesting that it was an alga itself.[4][28][29] However, the variability in the ratios in theisotopic signatureδ13C between specimens ofPrototaxites suggest that it washeterotrophic.[1]
Prototaxites mycelia (strands) werefossilised as they invaded the tissue ofvascular plants.[6] In turn, there is evidence of animals inhabitingPrototaxites: Mazes of tubes have been found within some specimens, with the fungus re-growing into the voids, leading to speculation that the extinction ofPrototaxites may have been caused by such activity.[6] However, evidence ofarthropod boreholes inPrototaxites has been found from the early and late Devonian, suggesting the organism survived the stress of boring for many millions of years.[30] Intriguingly, boreholes appeared inPrototaxites long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved.[30]Prototaxites became extinct in the Late Devonian as vascular plants rose to prominence.[4]
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