Australia: The Land Where Time Began
Prototaxites is to date the largest of the known Nematophyta. It has been found in rocks from the Silurian and Devonian in the form of axes that are compressed or silicified, which are sometimes called pseudostems, that are known to reach up to diameters of 1.25 m and more than 8 m long (Arnold, 1952b; Jonker, 1979; Bahafzallah et al., Schweitzer, 1983, 2000; Chitaley 1992; Hueber, 2001). Prototaxites has been reported from the Lower Devonian rocks of the Welsh borderlands in the form of charcoalified pieces that have been interpreted as the products of wildfires (Glasspool et al., 2006). It has a smooth or mildly ribbed outer surface. It has been shown by thin section microscopy to have an internal organisation that was pseudoparenchymatous or plectenchymatous which was constructed from tubes that were oriented longitudinally (hyphae, according to Hueber, 2001) of different types. Using the terminology used for hyphal types trimitic extant basidiomycetes, i.e., those with 3 types of hyphae, (Hueber, 2001):
Skeletal hyphae – thick walled, large, long straight or flexuous, aseptate and unbranched;
Generative hyphae – large, thin-walled, septate with an open or occluded pore, and branching profusely; and
Binding hyphae – small, thin-walled, with a pore in the septum, and branching profusely.
Arrangement of the “tissue” composed of these hyphae, as well as the presence of borders of growth increments that are well defined, marked by increased tissue density suggest to Taylor et al.1 some type of growth periodicity. As well as tubes many lacunar are scattered throughout the pseudoparenchyma, and the lacunae often contain tissue that has disintegrated and occasionally tubes, which have been interpreted as being the remains of some type of parasite.
At the time of the initial discovery of this very large type of nematophyte it was believed the specimens represented some type of conifer wood, the organism being named Prototaxites (Dawson, 1859). On closer examination the fossils were reinterpreted as algal like, transferred to another genus (Nematophycus), and formally classified with the Codiaceae (green algae). Prototaxites has also been compared to Lessonia, the brown alga (Carruthers, 1872b Kräusel, 1936). When P. southworthii tubes were examined ultrastructurally (Schmid, 1976), distinct septa (cross walls) were identified, each of which had an elliptical aperture or pore that was located centrally. Superficially these pores appear similar to the pores and pit connections seen in certain red algae, though corresponding structures also occur in fungi (e.g. basidiomycetes) in the form of dolipore septa. Small outgrowths in some of the tubes or hyphae occur close to the septa that remotely resemble basidiomycetous clamp connections (Hueber, 2001).
An affinity with algae was suggested by early chemosystematic data, whereas they are implied to have been terrestrial by the presence of cutin and suberin in the samples. It has been hypothesised that Prototaxites represents a failed experiment at terrestrialisation, based on evidence from other biomarkers (Abbott et al., 1998), as was originally suggested (Lang, 1937), or that it belongs with one of several algal groups that were in the process of adapting to a terrestrial habitat in the Devonian, though didn’t survive to the present (Niklas & Amocovitis, 1983). According to another interpretation (Schweitzer, 1983, 1990) that is based on a reinterpretation of Mosellophyton hefteri, a large axis that is irregularly branched found in Lower Devonian deposits in the Mosel Valley in Germany (Wehrmann et al., 2005), that was originally interpreted as a trachyeophyte (Schaarschmidt, 1974). M. hefteri was combined with Prototaxites (Schweitzer, 1983) and provided a reconstruction depicting a kelp-like alga which consisted of a monopodial trunk, Prototaxites, with a distal multi-branched crown, Mosellophyton. M. hefteri was combined with Prototaxites (Schweitzer, 1983) and provided a reconstruction that depicts a kelp-like alga hat consists of a monopodial trunk, Prototaxites, with a distal crown that was multibranched, Mosellophyton. This alga is believed to have inhabited shallow coastal marine waters that were tidally influenced where a root-like holdfast attached it to the substrate. A more recently discovered branched Prototaxites specimen from the Waxweiler quarry, Germany, was interpreted as a portion of the basal holdfast of this organism (Schweitzer, 2000). Taylor1 et al. suggest it should be noted that M. hefteri and Prototaxites have never been found in organic connection; therefore it remains questionable as to whether they were actually both parts of a single organism.
The comprehensive paper on P. loganii (Hueber, 2001) contains the most recent interpretation of Prototaxites, hypothesising that the large “logs” represent a giant terrestrial organism that was saprophytic that may have belonged to the basidiomycetes. So far no structures that are easily identified that would support their assignment to the basidiomycetes or any other fungal group have been found. Carbon isotope analyses of Prototaxites and land plants occupying the same environment indicate that there is a much wider variation in its C12/C13 ratio than would be expected in any plant, i.e., autotrophs, which indicates that Prototaxites was a heterotrophic organism (Boyce et al., 2007). According to Taylor et al.1 the structure of the typical ecosystems of the Late Silurian-Early Devonian, which were comprised of sparse vegetation in which the plants were very small, and probably also contained algal and cyanobacterial growth, makes it appear questionable whether there would have been enough carbon in the ecosystem to support a heterotrophic organism that grew to more than 8 m tall. It has also been suggested that Prototaxites may have been an example of an ancient mutualistic association of 2 or more different organisms combining both heterotrophy and a degree of lichen-like nutritional mode (Selosse, 2002). Taylor et al.1 suggest the hypothesis has some merit, though no direct evidence of an associated photobiont within the permineralised specimens has been found. It is suggested that the photobiont may have consisted of cyanobacteria associated with the outer surface of the organism and that may have not been observed or possibly not yet identified. Prototaxites must have been an imposing structure whatever it was.
The Rise of the fungi2
According to Moore1 in recent years a fossil record of fungi has accumulated, though there are not many unique morphological features and fungal hyphae do not generally fossilise well to be preserved over long time periods. Of the fungi that have been preserved from the Ordovician/Devonian the most impressive by far is the fossil genus Prototaxites, that were terrestrial organisms that have been found from the Ordovician, 460 Ma, to the early Devonian, which suggested they lasted for at least 40 million years (Boyce et al., 2007; Hueber, 2001). Among the earliest fossil evidence for terrestrial organisms to be found, termed the nematophyte phytodebris, contained these fossils. This term ‘nematophyte phytodebris’ has no relevance to the understanding of taxonomy of the present, having been assigned in the mid-19th century, as it does not necessarily indicate it was derived from plants; though it indicates that the problem with identification of the material has been continuing for more than 150 years (see discussion Hueber, 2001); Taylor et al., 2010). Known specimens of Prototaxites are generally large, being more than 1 m in diameter (Wellman & Grey, 2000) and up to 8 m tall (Hueber, 2001) and a colour illustration in Moore et al., 2001, pp. 33-34). Prototaxites was so common in these early terrestrial ecosystems, in terms of abundance and diversity, that it was a major component of these ecosystems. Some of the earliest examples that have been found were trunks that were tree-like and were constructed of tubes <50 μm in diameter, that were arranged concentrically in transverse sections, and the fossils had originally been interpreted as small coniferous trees, though it is now known that at the time Prototaxites was fossilised large vascular plants had not evolved.
In these ancient terrestrial habitats the largest organism present was Prototaxites, and these environments didn’t include vascular plants, being dependent instead on cyanobacteria (aka blue-green algae) eukaryotic algae, lichens and mosses, liverworts and the related bryophytes. According to Moore the most compelling evidence that Prototaxites was a fungus is the isotope ratio mass spectrometry of individual Prototaxites (Boyce et al., 2007; Hobbie & Boyce, 2010); Moore considers this evidence as conclusive. Carbon isotope ration of 12C:13C of individual Prototaxites fossils are shown by these analyses to have varied too much for them to have been photosynthetic primary producers of any type (Boyce et al., 2007), thereby indicating that Prototaxites was a heterotroph (saprotroph) which digested substrates that were isotopically heterogeneous, which suggested it was a consumer and a recycler. It has been demonstrated (Hobbie & Boyce, 2010) that a similar large range of carbon isotopes exists among extant fungi from an environment of the present that resembled the landscapes of the Early Silurian and Devonian where Prototaxites occurred. It has been found by a critical examination of the microscopic anatomy of Prototaxites (Hueber, 2001) that there were similarities with the trimitic system of hyphae evident in basidiomycetes of the present. Hueber has stated that his examination had a triple purpose:
1. To name, as neotype, a recognisable specimen of Prototaxites that had been collected by Dawson for which the locality and stratigraphic data are known,
2. To redescribe the genus as structurally composed of 3 forms of hyphae that were interactive, i.e. hyphae that were large, thin-walled, septate, branching, and binding, which all combine to form a gigantic, phototropic (though according to Moore more likely to have been gravitropic) amphigenous (a hymenial hyphal layer of extant Ascomycota and Basidiomycota extending over the entire surface of the spore-producing body, perennial sporophore with saprobic nutrition, and
3. To classify it in the Kingdom fungi.
(Hueber, 2001, abstract).
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