Nematophyte or fungus ?
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Now that the famous nematophyte
Prototaxites is interpreted as a huge fungus or lichen [1] and the less known small Nematophyton taiti [2] from Rhynie has been re-named Prototaxites taiti [3], the startling question arises whether or not more nematophytes are really fungi or lichens. This question will certainly be answered some day but for the present we will put it aside and delve into details related to it. 
So it may be appropriate to look at the own specimen of a fossil nematophyte resembling Nematophyton taiti [2], tentatively described in Rhynie Chert News 46 (and [4]), once more. It is readily assorted with the nematophytes by the tangle of curved tubes (Fig.1) in the middle layer of the sandwich structure seen in Rhynie Chert News 35
nematophyte tubes
Fig.1: Tangle of tubes with various diameter and curvature in the middle layer of a flat nematophyte. Width 2mm.

As seen here, the tubes do not seem to follow any rule, not even randomness. Their diameters vary between 10Ám and 25Ám but most of them seem to be in the range of 17-20Ám.
From the absence of mineral debris one may conclude that the tubes did produce organic gel as it is known from other nematophytes, with the purpose of keeping the tubes together in a lump of gel and to prevent exsiccation in case of dried-up surroundings.
It is not seen where the tubes come from. There are no conspicuous "branch-knots" thought to be related to tube generation
, as known from Nematoplexus. One may wonder whether the tubes are possibly generated in the clusters on the right in Fig.1.
flat nematophyte cross-sectionTube aggregates of a different type are rather puzzling than elucidating for several reasons. They appear as dark bundles of thick-walled tubes, mostly seen as cross-sections in Fig.2, which leads to the unexpected conclusion that this nematophyte specimen, or the part of it seen here, must have a preferred direction which is not apparent from the tangle of tubes in the inner layer (Fig.1). (Also it appears that the chert sample had been cut into slabs incidentally such that most of the mentioned tube bundles are seen as cross-sections.)

Fig.2 (left): Partial section of a flat nematophyte: tangle of tubes in bluish gel below, former gap with mineral debris, intermediate layer with very narrow tubes and a few bundles of big dark tubes, outer layer with pockets of pale aligned tubes above left. Image width 1.5mm.


There is no indication where the dark bundles come from or what they mean. They are found scattered in the middle layer of the nematophyte and also in the outer layer with very narrow tubes, as seen in Fig.2. The number of tubes per bundle varies between 1 (near the middle of Fig.2) and several dozens (on the right in Fig.2). The tube diameters in the bundles, 7-27Ám, are similar to those in the middle layer (Fig.1).
 
Apparently the specimen had been torn before silicification and muddy water with mineral platelets had filled the gap and became silicified along with the organic gel above and below.
nematophyte10nematophyte













Figs.3,4 (right): Nematophyte, tangle of curved tubes below and pockets of straight aligned tubes at the above surface,
unidentified structures in between, intermediate layer smaller than in Fig.2. Width 1.5mm.

Fig.5 (below): Nematophyte, straight narrow tubes arranged in a fan-like way.
        Figs.1-5: same scale. Figs.2-5: width 1.5mm.
nematophyte surfacen
The straight narrow tubes usually seen
aligned in pockets along the surface are occasionally arranged in a more or less divergent way, as shown in Fig.5 and Rhynie Chert News 46. Unlike the curved tubes in Figs.1-4, the straight tubes in the pockets (Figs.2-5) are distinguished by their definite lengths of about 0.5mm, less distinct small diameters, and by their densely packed arrangement with all tube ends together forming a common face integrated into the nematophyte surface.
Strings of beads, probably spores, are seen among the straight tubes in the pockets, apparently with one string per tube, thus differing from the specimen in [3], where the spores are produced within wider asci by the hundreds.


Fig.6 (right): Spores (?) of various sizes arranged in (mostly deformed) strings, bunch of empty (?) aligned tubes below.
Image size 0.75mm, magnification twice that of Figs.1-5.

Confusing as it may be, Fig.6 seems to offer the interpretation that strings of tiny spores had formed in the straight tubes directed towards the surface, then expanded and became deranged while the tubes decayed. There is still a bunch of orderly aligned tubes, directed downward and apparently without visible dots inside, vaguely seen in Fig.6 below.
As a less probable alternative interpretation, the dark clots might be no spores but alga cells which lived in a symbiosis with the nematophyte as a lichen [5].
The dispute on "nematophyte or fungus (or lichen)" concerning Prototaxites is analysed thoroughly and comprehensibly in [6]. An image of Nematophyton taiti in [7] is unsuitable for comparison with this specimen and with [3] because of a scale error (factor 2.4) and the mirror image.
Besides some differences between this specimen and the one called Nematophyton taiti [2] once but Prototaxites taiti [3] now, their structure is so similar that this one may be regarded
as an ascomycete fungus with "nematophyte aspect" as well.
Sample: Rh2/7 (2.1kg), obtained from Shanks in 2000.

H.-J. Weiss     2020  

[1] F.M. Hueber: Rotted wood-alga-fungus: the history and life of Prototaxites Dawson 1859. Rev. Palaeobot. Palynol. 116 (2001), 123–158.
[2]  R. Kidston, W.H. Lang : On Old Red Sandstone plants showing structure ...,
      Part V, Trans. Roy. Soc. Edinburgh 52 (1921), 855-902.
[3]  R. Honegger, D. Edwards, L. Axe, Ch. Strullu-Derrien: Fertile Prototaxites taiti: a basal ascomycete with inoperculate, polysporous asci lacking croziers.
      Phil.Trans. Roy. Soc. B 373 (2017): 20170146.
[4]  H.-J. Weiss: Enigmatic Organisms  -  Insights derived from new finds, Poster presentation, EPPC Budapest 2010.
[5]  G.J. Retallack, Ed Landing: Affinities and architecture of Devonian trunks of Prototaxites loganii. Mycologia, 106(6) (2014), 1143–1158.
[6]  H. Steur:  Prototaxites. Google: steurh.home.xs4all.nl/engprot/
[7]  www.abdn.ac.uk/rhynie

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