Nematothallus and Nematoplexus in one chert sample
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Nematothallus in chert
This fossil
resembles the nematophyte  Nematothallus, which had been found as compresssions only but never in chert. The similarity has been pointed out by P.K. Strother [1,2], hence it is called Nematothallus here, for simplicity.
It is also shown in Rhynie Chert News 107 and 122

Fig.1: Formerly separate fragment of gel with Nematothallus, here in bluish chalcedony from silicified water of the swamp containing Nematoplexus (see Fig.4).
Image width 1.38mm.

The unforeseen discovery of Nematothallus in a chert sample incidentally containing the rare fossil  Nematoplexus seemed to support the assumption that Nematoplexus "may represent the permineralized equivalent of Nematothallus" [3]. This is one of the reasons why the disputed nematophytes are considered here once more.  The statement in [4] that "Nematophytes appear to generally comprise networks of intertwined spirally coiled tubular cells" is not true.

Nematothallus in chert


Fig.2 (left):
Nematothallus, detail from Fig.1: tube wall patterns consisting of rings and spiral fragments of 19-23m diameter. Image width 0.97mm.
Photograph by G. Schmahl.
 

The lump in Fig.1 seems to have separated from a bigger lump or layer of Nematothallus in gel and got near Nematoplexus also grown in that habitat. All together silicified such that the two nematophytes are found now within a mutual distance of mere 3cm.
Part of the left end of the
Nematothallus lump is seen magnified in Fig.3.

Nematothallus in chert
Fig.3:
Nematothallus parts of various aspect, enlarged part of Fig.2 above left.
Image width 0.3mm.  Photograph by G. Schmahl.

A few details are worth mentioning: Not the tube walls but the reinforcing patterns have been preserved. The former diameter of the tubes had probably been about the same size. 
Conspicuous is the variable diameter of one of the tubes. As seen above right, the tube wall pattern is not always ring-like or spiral-like but can be rather rugged. The nature of the bubble on the left is not known.
A narrow tube, diameter about 4m, indicates that there may be more hidden structure. Tubes of 1.5-3m among the big ones have been reported from Nematothallus compressions [1,2], and possibly the thin line fragments in Figs.1,2 are the remains thereof. Small tubes with sizes of 3m and below, in addition to the larger ones, are also found close to Nematoplexus "branch knots", as seen in Rhynie Chert News 133 . Despite of these similarities it is not justified to regard Nematoplexus as a form of  Nematothallus.

Nematoplexus knot
Fig.4 (right):
Nematoplexus "branch knot" with two types of spiralling tubes emerging, diameters 10m and 16m, also one patterned tube of dubious affiliation, 24m.
Image width 0.6mm, same scale as Fig.2.

The conventional term "branch knot" in the literature on Nematoplexus suggests an origin of the tubes by branching. The contribution Rhynie Chert News 134 (there Fig.2) favours a different view: There, two 10m-tubes, each one with a slightly broadened "foot", seem to stick to the poorly defined surface of a central clot with about 75m diameter. There is no indication that the tubes had been produced by branching inside the clot. In this Fig.4, the central clot of the "knot" is of about the same size. Although its surface is less well defined, it can be assumed that here, too, the tubes emerge from the surface. Also there would  hardly be enough room for many branching tubes inside.
Near the knot in Fig.4, the tubes have not yet formed their ideal spiral shape. The bigger tube in Fig.4 emerges from the depth below the knot in a half circle towards the cut plane where it is cut off and hence no more seen, then dives down below the cut plane above left and thus is in the picture again. Nearby there is a big tube with patterned wall, which looks surprisingly similar to the Nematothallus tubes in the above pictures. (For easy comparison, Figs.2 and 4 have got the same scale.) It might be a stray relic from decayed Nematothallus, or else it could be one of the less abundant patterned tubes seen emerging from "branch knots" of Nematoplexus [4,5].
As a peculiar fact, the patterned tubes in these pictures are transparent, without a visible wall, but the spiralling tubes
from the same sample (Fig.4) appear dark, with more or less translucent well-defined walls. 
Even though the suggestion in [3] that Nematoplexus "may represent the permineralized equivalent of Nematothallus" is not at all substantiated, intriguing questions remain:  
Is there a reason why it is not unlikely that the two rare nematophytes are together in one piece of chert ?

Why are the worm-like tubes with patterned wall occasionally emerging from Nematoplexus "branch knots" surprisingly similar to those of Nematothallus ?
Sample: Rh9/86, 0.28kg, 2003.

H.-J. Weiss      2020

[1]  P.K. Strother: Clarification of the genus Nematothallus Lang: J. Paleont. 67(1993), 1090-1094.
[2]  W.H. Lang: On the plant-remains from the Downtonian ... . Phil. Trans. Roy. Soc. London B 227(1937), 245-291.
[3]  T.N. Taylor, E.L.Taylor, M. Krings: Paleobotany, Elsevier 2009.        (Scale error on Fig.6.9: scale bar not 100m, rather 20m ?)
[4]  www.abdn.ac.uk/rhynie/nemato.htm  
[5]  
Rhynie Chert News 137
151

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