Varied nematophytes in Rhynie chert 
deutsche Version

Recent finds of nematophytes in the Rhynie chert, notably hitherto unknown ones, provide information which
may help to demystify these "Enigmatic Organisms" [1].  Nematoplexus [2] is the only one known to consist of screw-like wound tubes. The comparison with a screw is appropriate since the tubes are always wound in a right-handed way, with a definite pitch of the thread. Contrary to fossil evidence, the "spiral coils" have repeatedly been ascribed to all nematophytes [3,4], which is clearly erroneous.
The tubes of Nematoplexus from own finds are larger than the original ones from [2]: Rhynie Chert News 29.  Unexpectedly, straight or weakly curved tubes, too, have been found associated with Nematoplexus (Fig.1). As a remarkable fact, there are no tubes with shapes intermediate between the straight or weakly curved ones and the screw-like ones:
Rhynie Chert News 51Among the confusing tangle of tubes, several turns of one screw as in Fig.1 (right) are not often seen. Nematoplexus bows and spirals

Fig.1: Nematoplexus with slightly curved and screw-like wound tubes, width 11-12Ám.
          Image width 1.5mm.

The term "branch-knot" and its repeated use in connection with tube generation has led to the view that it is a dense tangle of tubes where the tubes profusely branch inside before they venture out. This notion has been refuted in
Rhynie Chert News 134, 136, 152. To avoid consolidation of erroneous views, the branch-knots may simply be called clots. They are no tangles of tubes but bodies of their own with a (not clearly defined) surface. The tubes arise from that surface or near-surface region.
big nematophyte clotNo clot related to the slightly curved tubes as in Fig.1 (left) has been found yet but, as a big surprise, a bunch of slightly curved big tubes attached to a big clot (Fig.2) provides another enigma: Slightly curved big tubes have only been found near the "normal" screw-like ones as if they were mutually related somehow: See Rhynie Chert News 135,
137. It appears that even the seemingly thoroughly inspected Nematoplexus still hides big secrets.

Fig.2: Slightly curved big tubes, 17-22Ám, and a few very thin ones, 4-5Ám, emerging from a big clot;
           nearly straight thin tube, 9Ám, passing by. Same scale as Fig.1, image width 1.3mm.

Since Nematoplexus is apparently the only nematophyte with screw-like wound tubes, the aspects of all others lately found in the Rhynie chert differ greatly. An impressive example of a stack of nearly straight aligned tubes with large diameters is seen in Fig.3.

Nematoplexus big tubes
Fig.3: Aligned big tubes of unknown nematophyte, up to 70Ám across.
           Same scale as Figs.1,2, image width 1.5mm.

A few conclusions can be drawn from details in Fig.3: The absence of mineral debris between the tubes seems to indicate that they did not grow separately in the swamp water which became the Rhynie chert but rather in a lump of organic gel produced by themselves to live in. This would be compatible with the irregularities seen above, which are possibly due to beginning decay, and with the beginning formation of a pseudo-cell pattern of shrinkage cracks in the gel below left: Rhynie Chert News 30 Also the different aspect of the chalcedony inside and outside the tubes indicates different conditions of silicification, as discussed in Rhynie Chert News 154.
The tiny precipitates seen as deposits inside two tubes below right must have formed while the tube content was fluid. Hence, it had decayed before silicification but the organic gel surrounding the tubes had not, keeping the tubes apart until silica gel stabilized the structure. Note that there are no tubes touching.
nematophyte shows a rather different structure: Fig.4. The chaotic arrangement of limp tubes seems to indicate a liquefaction of the organic gel before silicification but this interpretation is questionable.
Inside the lump of gel, the tubes are protected against exsiccation and microbial attack:  Rhynie Chert News 155.  Most probably, other nematophytes, too, made use of this two-fold protection.
Nematophyte big tubes

Fig.4 (left): Unknown nematophyte with big tubes up to 60Ám across, poorly aligned, irregularly curved. Same scale as above, width 2mm.

Gel turned into bluish chalcedony has drawn the attention to surprisingly narrow nematophyte tubes (Fig.5), which hitherto may have been overlooked or misinterpreted as less interesting fungus hyphae: Rhynie Chert News 86 Yellow mineral debris is seen above left and through the translucent gel on the right.  

Fig.5 (below): Unknown nematophyte with very narrow tubes about 6-8Ám across, irregularly curved. Same scale as above, width 1.4mm.

nematophyt small tubes

A clearly defined outline is missing with the nematophytes in Figs.1-5, where the lumps of gel with tubes inside are seen as fragments in the chert. Nematophytes with preserved outline are shown in other samples: Rhynie Chert News 99.
The pale spots of densely spaced tubes
in Fig.6, below left and above right, might be the expected centres of tube formation. With tube diameters ranging from well below 10Ám up to 27Ám and a width of the whole organism of hardly more than 2cm, this nematophyte resembles the flat one in Rhynie Chert News 46 but it lacks the clearly structured outer sheath seen on that specimen.
nematophyte tubes
Fig.6 (left): Unknown nematophyte with tubes up to 27Ám across. Same scale as above, width 1.3mm.

Fig7. (below): Nematophyte with layered structure and different types of tubes in the layers, thus resembling Nematophyton taiti. Same scale as above, width 1.5mm.
nematophyte margin

The nematophyte in Fig.7 is the only one among the own finds with distinctly different types of "tissue", as outlined in Fig.8. See also Rhynie Chert News 35, 46, 153.  
Fig.8: Attempted reconstruction of the nematophyte in Fig.7. Width 3.5cm.

Lately, the similar Nematophyton taiti has been assorted among the ascomycetes and renamed Prototaxites taiti [5]. Possibly the present fossil and other nematophytes, too, will have to be reconsidered.
The detail in Fig.7 has been chosen such that it is reminiscent of the outer sheath of Pachytheca. That spherical nematophyte is known from several locations but one exceptionally preserved specimen has been found in the Rhynie chert: Rhynie Chert News 1, 3644,  (No picture included here.)
Nematothallus in chert
Tubes with patterned walls occasionally seen among "normal" ones with Nematoplexus seem to be "out of place" there but are normal with Nematothallus
: Fig.9. Their obvious similarity with tracheids has given rise to reasonable suggestions concerning the evolution of land plants [6] but also to quite absurd ideas about water flow in cells [7]. 

Fig.9: Worm-like tubes about 20Ám wide with patterned walls, tiny tubes (<3Ám ?) in between, apparently grown in a lump of organic gel, possibly first
Nematothallus in 3D-preservation.
Width 2mm, same scale as Figs.1-7.

 To sum up, the nematophytes lately found in the Rhynie chert and briefly presented in Figs.1-9 provide evidence that ...
  - the only nematophyte with screw-like wound tubes, the allegedly well investigated Nematoplexus,
        remains highly enigmatic with its apparent affiliation to
unknown structures (Figs.1,2),
  - the clots called "branch-knots" are no tangles of branching tubes but lumps producing the tubes in obscure ways (Fig.2),

  - there are several hitherto unknown nematophyte forms and species in the Rhynie chert (Figs.1-8),
        among them species with remarkably narrow or wide tubes.
Nematoplexus, is not, as once assumed and numerous times repeated [1], a permineralised form of Nematothallus.

Samples shown in Figs.1-9: Rh9/86.1, Rh15/79.1, Rh2/81.1, Rh13/7.1, Rh3/9.1, Rh13/1.2, Rh2/7.6. 
H.-J. Weiss    2020   

[1]  T.N. Taylor, E.L.Taylor, M. Krings: Paleobotany, Elsevier 2009.
[2]  A.G. Lyon: On the fragmentary remains of an organism referable to the Nematophytales,
      Trans. Roy. Soc. Edinburgh 65(4)(1962): 79-87.

[3]  P. Selden, J. Nudds: Evolution of fossil ecosystems. Manson publ. 2012, p.84.
[5]  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.

[6]  P.K. Strother: Clarification of the genus Nematothallus Lang: J. Paleont. 67(1993), 1090-1094.
[7]   K.J. Niklas, V. Smokovitis: Evidence for a conducting strand in early Silurian plants, (1983).
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