Nematoplexus - enigmatic ever more

Nematoplexus, one of the rare components in the Lower Devonian Rhynie chert, first described in 1962 [1] and included into the monograph "Paleobotany" [2] in 2009 as one of the "Enigmatic Organisms", has not become less enigmatic in the meantime. On the contrary, it became ever more enigmatic with own finds of spirally coiled and other tubes since 2001. Nematoplexus spiral tubes

Loosely coiled or screw-like wound tubes with the pitch of the thread roughly equal to the diameter of the thread seem to be a typical feature of Nematoplexus, by which it is easily recognized in the chert. (The statement that "nematophytes appear to generally comprise networks of intertwined spirally coiled tubular cells" [3] is simply not true.)
The first Nematoplexus seen on an own sample (0.28kg) looked like a larger version of N. rhyniensis, pictured and compared with the latter in
Rhynie Chert News 29, under the label Rh9/86. Two more pictures from the surface of this sample are shown here: Figs.1,2.

Fig.1: Wound tubes of Nematoplexus, bigger than those in [1], and one of the black "branch knots" which seem to produce the tubes in obscure ways. Picture width 2.4mm.

Closer inspection of this sample revealed an unexpected variety of shapes of nematophyte tubes: wound (spirally coiled) tubes of various size, long fragments of straight or slightly curved tubes with various curvature and diameter, black "branch knots" apparently producing the tubes. Some worm-like tubes
with their annular or spiral wall patterns (Fig.2) superficially looking like annelids or land plant tracheids, scattered in the chert or aggregated in lumps with a tangle of tiny tubes in between, resemble Nematothallus [4]. One rare case of extreme differences in spiral size data is vividly seen in Rhynie Chert News 106, there Fig.2. All this, beside the tubular moult parts of crustaceans and the tubular branches of charophyte algae, can be seen within a few centimeters of surface and cut faces of this sample, and it has to be untangled so that one can separate things into those possibly related to Nematoplexus and others.
The tubes in Fig.1 are of the wound (spiralling) smooth type with diameters 11-16Ám, except for only one short fragment of 22Ám width, which is of the patterned type. So it came as another one of the several enigmas connected with Nematoplexus when a multitude of the above-mentioned worm-like tubes of similar width showed up as a lengthy cluster only 4cm away from the tubes in Fig.1, without any spiralling smooth tube in between, as seen in
Rhynie Chert News 107, and as an enlarged detail thereof in Fig.2. 
Nematothallus worm-like tubes
Fig.2: Wall patterns of worm-like tubes
resembling land plant tracheids, located in the chert sample near the typical spiralling Nematoplexus tubes in Fig.1 and similar to the non-spiralling patterned Nematoplexus tubes. Picture width 0.5mm. Photograph by G. Schmahl.

The similarity between the
worm-like tubes resembling Nematothallus and the land plant tracheids is possibly not merely superficial but there may be an essential connection, as pointed out by P.K. Strother [4].

Very small wound tubes can have a smaller thread than the spiralling wall pattern of the big worm-like tubes as in Fig.2, which might cause confusion. (See
Rhynie Chert News 102.)
Apparently never seen before and most probably related to Nematoplexus are the big "branch knots" in another sample (Rh15/79) with big slightly curved tubes emerging, first described in Rhynie Chert News 71.

Only part of the available information on Nematoplexus has been put into the below plot: size data of possibly non-deformed wound tubes from [1] (picture in [3]) and from 5 own samples. (The plot includes the data of the table in
Rhynie Chert News 29. ) The uncertainty of measurement may be larger than indicated by the circles in the plot. The dashed lines mark the tube diameters (7)-8-9-(10) Ám in [1].
(Annotation: The term "spiral", which also applies to a plane curve, is meant here always in the sense of a screw-like wound curve.)
Nematoplexus spirals sizes

Fig.3 (right): Spiral tube data of Nematoplexus taken from images and samples, sorted accordingly.

This plot mainly shows the following remarkable facts:
 - The type specimen [1] is restricted to a small area of this parameter field.
 - Bigger spirals are oftener seen than smaller ones.
 - There are samples with a wide range of spiral sizes: here Rh9/86 and Rh15/79.
 - The very small spirals are very rare.

The observations give rise to questions:
 - Does Fig.3 indicate three separate size regions:  
    sizes near those of the type specimen, also many bigger ones and rare much smaller ones ?
 - How to interpret the much smaller spirals occasionally seen among the big ones ?

 - Are the straight and slightly curved smooth-walled tubes always associated with the bigger spirals ?
 - Is the presence of what seems to be Nematothallus (Fig.2)
near Nematoplexus incidental or not ?

More questions, not considered here, arise in connection with the patterned tubes and the "branch knots". Answers would probably pave the way to a better understanding of these enigmatic organisms.

H.-J. Weiss      2018 

[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 plates.    (Scale error on Plate I Fig.1: not x19 but x1.5.)
[2] T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009. 
[4]  P.K. Strother: Clarification of the genus Nematothallus Lang: J. Paleont. 67(1993), 1090-1094.

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