Nematoplexus with controversial aspects
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Nematoplexus knotsThe enigmatic organism Nematoplexus had become known from apparently only one sample of Rhynie chert collected decades ago [1]. Since 2003, a few more samples have provided evidence that
Nematoplexus is even more enigmatic than previously thought [2]. The new evidence gave also rise to doubts concerning the dark lumps called "branch-knots". They are supposed to produce the tubes (Fig.1) and, according to [3], to consist of "very tightly coiled ... tubes showing repeated and closely spaced branching". The latter part of this quotation refers to [1] (there Plate 1, Figs.7,10) with tubes of about 2...8Ám diameter, hence it does not concern the coiled tubes of 9...15Ám seen here. The first part of this quotation, "very tightly coiled", is an unsubstantiated addition since nothing like this is seen in [1] or in the own samples.

Fig.1:  Nematoplexus as seen on the raw surface of a Rhynie chert sample, with two dark lumps called branch knots among spiralling tubes. Image width 0.6mm.
For separate images of the lumps taken by Gerd Schmahl with higher resolution, other illumination,
         and other orientation of the sample, see Figs.2,3.

It would be hard to imagine that the tubes seen emerging from the dark lump in Fig.2 had lain "tightly coiled" and branched inside.  
The tiny lump in Fig.3 is less confusing than the bigger ones in Fig.2 or elsewhere in this sample. It shows two spiralling tubes emerging from the lump. The right one is seen bending towards the observer, then turning into the depth, vanishing from sight, then emerging and vanishing again. Hence, nearly two turns with right-hand thread are seen. With the uneven surface of this sample and other things precluding a proper visual impression, the tube on the left may provide the illusion of left-hand thread but comparison with the differently oriented sample in Fig.1 shows that the thread is right-handed as expected.  A third tube in Fig.3 is seen as a cross-section on the raw sample surface.

Again it appears that there is not much room for coiling and branching of the three tubes in the small "branch-knot", hence the tubes must have formed there somehow more immediately. Their way of formation remains obscure but there are  indications from other samples that they may form at or near the surface of the lump. (See Rhynie Chert News 135.) Anyway, the lumps or knots are not to be compared with the buds of plants containing tightly folded leaves waiting to get unfolded.    
Nevertheless one may rightly suspect that tube formation is caused or mediated by what is inside the lump. In Figs.1-3 there is an indistinct structure on parts of the lumps which might be sections of narrow tubes as more clearly seen in [1] and Rhynie Chert News 133. There is no idea in which way this structure could possibly be related to the spiralling or other big tubes.
Nematolexus lumpNematoplexus lump

Fig.2 (far left): Nematoplexus lump with tubes of 8...15Ám, cut off at the sample surface so that no full turns of spirals are seen. Detail from Fig.1, other orientation.
Width of Figs.2,3: 0.3mm.

Fig.3: Uncommonly small Nematoplexus lump with two spiralling tubes, 9Ám and 12Ám, another one cut off at the sample surface (= old fracture face), seen as a circular cross-section. Detail from Fig.1, other orientation.

Nematoplexus straight tubes

As one of the several peculiarities encountered in connection with Nematoplexus, tubes appearing perfectly straight may unexpectedly be found near wound tubes. The positions of Fig.1 and Fig.4, for example, are only a few centimeters apart. While the screw-like wound tubes known as spirals are produced by lumps called branch-knots, the origin of the slightly curved tubes had been unknown until evidence appeared which indicated that the bigger ones are attached to another kind of lumps. (See Rhynie Chert News 71, 135.) Hence, on can guess that the thinner slightly curved tubes and the straight tubes come from some not yet seen type of lump or knot.
The idea that the tubes with distinctly different curvature might represent different species is not favoured here since it would be improbable to find them mostly near each other.
The conspicuous arrangement of straight tubes, parallel and in bundles, requires an explanation, and so does the presence of straight big tubes and curved narrow tubes close together. The 5 parallel tubes arising from the depth in Fig.4 meet the sample surface where they end with elliptical sections. A 14Ám-tube is positioned behind a larger one, 2nd from the left, so that its section is seen higher up in the picture. A 10Ám-tube fitting nowhere incidentally ends where a 12Ám-tube is lying in the depth, providing the illusion of rectangular branching.

Fig.4: Nematoplexus, 5 parallel straight tubes 14...21Ám, slightly curved tubes 8 Ám, 10Ám, 12Ám. Picture taken at the raw sample surface, width 0.6mm, same scale as Fig.1.

So it has appeared again that Nematoplexus offers details which make one wonder in which way they could possibly fit into a consistent picture of this enigmatic organism. The problem is made even more confusing by the non-spiralling tubes with patterned walls resembling those of Nematothallus, not shown here but often found along with the spiralling smooth-walled tubes.
Comparison with several own finds of other nematophyte species in the Rhynie chert have been of no much help.

All pictures have been taken from the raw surface of one Rhynie chert sample of 0.28kg, Rh9/86, found in 2003.

H.-J. Weiss   2019

[1]  A.G. Lyon: On the fragmentary remains of an organism referable to the nematophytales,
        from the Rhynie chert, Nematoplexus rhyniensis. Trans. Roy. Soc. Edinburgh 65(1961-62), 79-87, 2 tables.
[2] T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009.
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