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Prototaxites
The genus ''Prototaxites'' describes terrestrial organisms known only from fossils dating from the Silurian and Devonian, approximately 420 to 370 million years ago. ''Prototaxites'' formed large trunk-like structures up to wide, reaching in height,〔 made up of interwoven tubes just in diameter. Whilst traditionally very difficult to assign to an extant 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.
==Morphology==
With a diameter of up to a meter, and a height reaching 8 m, ''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", of a fungus, which would have been fuelled by a mycelium, 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 μm across, have thick (2–6 μm) walls and are undivided for their length, and generative "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 the early plant ''Nematothallus'' has led to suggestions that the latter may represent leaves of ''Prototaxites''. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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