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Australia: The Land Where Time Began |
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Prototaxites
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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |