Prototaxites
Encyclopedia
The genus Prototaxites describes terrestrial organisms known only from fossils dating from the Silu
-Devonian
, approximately 420 to 370 million years ago. Prototaxites formed large trunk-like structures up to 1 metres (3 ft) wide, reaching 8 metres (26 ft) in height, made up of interwoven tubes just 50 um in diameter. Whilst traditionally very difficult to assign to an group of organisms, current opinion is converging to a fungal
placement for the genus. It might have had an algal symbiont, which would make it a lichen
rather than a fungus in the strict sense.
An opposing view has been presented that Prototaxites was not a fungus but consisted of enrolled liverwort mats with associated cyanobacteria and fungal tubular elements.
With a diameter of up to a meter, and a height reaching 8 m, Prototaxites fossils are by far the largest 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 "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", of a fungus, which would have been fuelled by a net ("mycelium") 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 μm across, have thick (2–6 μm) walls and are undivided for their length, and "filaments", which are thinner (5–10 μm diameter) and branch frequently; these mesh together to form the organism's matrix. These thinner filaments are septate – that is to say, they bear internal walls. These septa are perforate - i.e. they contain a pore, a trait only present in the modern red algae and fungi.
The similarity of these tubes to structures in early plant Nematothallus has led to suggestions that the latter may represent leaves of Nematothallus. 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.
First collected in 1843, it was not until 14 years later that John William Dawson
, a Canadian scientist, studied Prototaxites fossils, which he described as partially rotten giant conifers, containing the remains of the fungi which had been decomposing them. This concept was not disputed until 1872, when a rival scientist named Carruthers poured ridicule on the idea. Such was his fervour that he rebuked the name Prototaxites (loosely translated as "first yew") and insisted that the name Nematophycus ("stringy alga") 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 clout that he'd ever considered it to be a tree. Despite these political attempts to re-name 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' interpretation that it was a giant marine alga was challenged just the once, in 1919, when Church 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, and so 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
, 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 faced 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 of the University of Chicago
concluded that Prototaxites was a giant fungus. They detected a highly variable range of values of carbon isotope ratios in a range of Prototaxites specimens; autotrophs (organisms such as plants and algae, that make a living via photosynthesis
) 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 deduce that the organism fed on a range of substrates, such as the remains of whichever other organisms were nearby.
Other recent research has suggested that Prototaxites represents a rolled-up bundle of liverworts, but this interpretation has substantial difficulties.
(pictured in navigational box below) only reached 1 m, and itself towered over the "moss forests"; invertebrate
s were the only other land-dwelling life. Prototaxites became extinct as shrubs and trees rose to prominence. The organism could have used its raised platform for spore dispersal, or, if Prototaxites really did form leaves, in competition for light. The University of Chicago
research team has it reconstructed as a branchless, columnal structure. 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
), or even that it was an alga itself.
Prototaxites mycelia (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 is bored long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved.
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
-Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
, approximately 420 to 370 million years ago. Prototaxites formed large trunk-like structures up to 1 metres (3 ft) wide, reaching 8 metres (26 ft) in height, made up of interwoven tubes just 50 um in diameter. Whilst traditionally very difficult to assign to an group of organisms, current opinion is converging to a fungal
Fungus
A fungus is a member of a large 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, Fungi, which is separate from plants, animals, and bacteria...
placement for the genus. It might have had an algal symbiont, which would make it a lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
rather than a fungus in the strict sense.
An opposing view has been presented that Prototaxites was not a fungus but consisted of enrolled liverwort mats with associated cyanobacteria and fungal tubular elements.
Morphology
The similarity of these tubes to structures in early plant Nematothallus has led to suggestions that the latter may represent leaves of Nematothallus. 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.
History of research
John William Dawson
Sir John William Dawson, CMG, FRS, FRSC , was a Canadian geologist and university administrator.- Life and work :...
, a Canadian scientist, studied Prototaxites fossils, which he described as partially rotten giant conifers, containing the remains of the fungi which had been decomposing them. This concept was not disputed until 1872, when a rival scientist named Carruthers poured ridicule on the idea. Such was his fervour that he rebuked the name Prototaxites (loosely translated as "first yew") and insisted that the name Nematophycus ("stringy alga") 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 clout that he'd ever considered it to be a tree. Despite these political attempts to re-name 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' interpretation that it was a giant marine alga was challenged just the once, in 1919, when Church 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, and so 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
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. Admission is free and the museum is open 364 days a year....
, 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 faced 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 of the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
concluded that Prototaxites was a giant fungus. They detected a highly variable range of values of carbon isotope ratios in a range of Prototaxites specimens; autotrophs (organisms such as plants and algae, that make a living via photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
) 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 deduce that the organism fed on a range of substrates, such as the remains of whichever other organisms were nearby.
Other recent research has suggested that Prototaxites represents a rolled-up bundle of liverworts, but this interpretation has substantial difficulties.
Ecological context
This organism would have been the tallest living thing in its day by far; the plant CooksoniaCooksonia
Cooksonia is an extinct grouping of primitive land plants. The earliest Cooksonia date from the middle of the Silurian ; the group continues to be an important component of the flora until the Early Devonian, a total time span of...
(pictured in navigational box below) only reached 1 m, and itself towered over the "moss forests"; invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s were the only other land-dwelling life. Prototaxites became extinct as shrubs and trees rose to prominence. The organism could have used its raised platform for spore dispersal, or, if Prototaxites really did form leaves, in competition for light. The University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
research team has it reconstructed as a branchless, columnal structure. 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
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
), or even that it was an alga itself.
Prototaxites mycelia (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 is bored long before plants developed a structurally equivalent woody stem, and it is possible that the borers transferred to plants when these evolved.