Nematoplexus surprisingly polymorphic
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Nematoplexus lumps
The enigmatic organism Nematoplexus has been known since 1961 as an assemblage of screw-like wound tubes, diameters 7-10Ám, emerging from dark lumps named "branch knots", in one sample of Rhynie chert [1], then discovered as a smaller patch of such tubes in a separate stratum of the Rhynie chert called Windyfield chert, published in 2003 [2]. Also in 2003, samples were found with similar-shaped but larger tubes, 10-15Ám, also with slightly curved or straight ones near the wound ones.
What seemed to be a different species,
big tubes of about 20Ám radially grown from a big dark lump, had first been seen in a sample found in 2014 and described in Rhynie Chert News 71. Then, this combination of big tubes and big lump had unexpectedly been seen in a second sample, and shown, among other lumps, in Rhynie Chert News 126. The results of closer inspection of this same sample are presented here. 

Fig.1: Nematoplexus tubes and "knots" on the raw surface of a small sample of 30g. Picture width 1mm.

Closer inspection of Fig.1 suggests that the "normal" wound tubes are not related to the big lump which serves, although inconspicuously in the present case, as a center of much bigger tubes only. This is revealed by stacks of pictures taken and combined into more distinct images by Gerd Schmahl: see Figs.2,3. 
Nematoplexus tubesNematoplexus tubes
Fig.2 (left): Right part of the big lump in Fig.1 with hollow big tube (right) and patterned tube (left).
Picture width 0.25mm.

Fig.3 (right),
same scale as Fig.2: Lower part of the big lump in Fig.1, slightly turned to the left.
Picture width 0.4mm.


 Most conspicuous in Fig.2 is the inclined cut of the hollow big tube with yellow deposit, 23Ám outer diameter. Another tube of about 30Ám is vaguely seen as a broad dark strip in the depth below, more clearly seen on another picture of the stack.
A stub of a big tube of 30Ám is seen in Fig.3. It seems to begin on a kind of surface, quite similar to a same-size tube apparently attached to a big lump in
Rhynie Chert News 135.
A curved 10Ám-tube in Fig.3 above right represents the "normal" tubes making the "
Nematoplexus-Aspekt" with the "normal" lumps traditionally called branch-knots. The small lump in Fig.1 is probably one of such.
The big lump in Fig.1 has got something in common with other big lumps shown in Rhynie Chert News 71, 126, 135: It is the presence of big tubes (like those hidden in Fig.1 and made visible in Figs.2,3) and the absence of adhearing "normal" Nematoplexus tubes. This distinguishes them clearly from the so-called "branch knots", and since no branching has been seen, they may simply be called "lumps with big tubes".
Also remarkable are the tubes with patterned wall. The broken 17Ám-tube in Fig.2 with irregular annular or spiral pattern appears surprisingly similar to the tubes of Nematothallus and also to the tracheids of vascular plants. Nematoplexus irregular

Irregular-shaped tube with patterned wall and variable diameter, detail of Fig.3 below left. Picture width 0.1mm.

The unusual structure in Fig.4 is particularly interesting: It is an irregular-shaped tube with patterned wall and variable diameter: 13Ám in the depth and 23Ám at the sample surface. Possibly it had been hidden within the dark lump and is seen here only because of partial decay of the lump. It seems to turn into a regular tube with patterned wall while emerging from the lump on the left.
The slightly curved or straight tubes mentioned above have been found in this small sample, too: Figs.5,6.
Nematoplexus curved tube
Fig.5 (left), same scale as Fig.1: Slightly bent
Nematoplexus tube (about 14Ám) and charophyte branch section (0.15mm). 
Picture width 1mm.

Nematoplexus straight tubes

Fig.6 (right),

same scale as Fig.1:

Straight Nematoplexus tubes (20Ám, 31Ám) and charophyte parts.Picture width 1.2mm.

As another unexplained phenomenon, apparently parallel straight Nematoplexus tubes with their origin hidden in the depth of the sample have been seen before: Rhynie Chert News 136. The tubes in Fig.6 have grown right through the branches of a charophyte alga, probably after the latter had been partially decayed, except for the well-preserved dark capsule tentatively interpreted as an oogonium. For more details of this quite uncommon charophyte discovered in 2015 and found together with Nematoplexus in the presently considered chert sample see Rhynie Chert News 90.

Disregarding the not yet understood details (and the tubes with patterned walls), the sparse fossil evidence justifies some conclusions to be drawn:
  -  The screw-like wound tubes have got diameters of mostly 7-15Ám and are associated with their kind of lumps known as "branch knots".
  -  The narrow weakly curved or straight tubes have got diameters of mostly 7-15Ám. Their related lumps have not yet been seen.
  -  The big
weakly curved or straight tubes have got diameters of mostly 17-30Ám and are associated with the hitherto unknown "lumps with big tubes".
Fig.7: Detached
Nematoplexus tube fragment, 12Ám, of non-typical shape; crustacean Castracollis, main body part seen from above. Picture width 2.8mm.

Disregarding separate tube fragments which do not fit into the above types, like the one in Fig.7, the following can be stated:
No two of these three types of tubes have been seen transforming among themselves or adhering to the same lump. In
particular, the winding tubes of 7-15Ám are never seen emerging from the "lumps with big tubes", and the big tubes are never seen attached to the "branch knots".
The subject is made even more confusing by tubes with patterned walls and a wide range of diameters, known to arise from branch knots. They can also arise from "lumps with big tubes", as shown here for the first time.

With these observational facts one might be tempted to assume that there were two or more separate
Nematoplexus species if there were not the unexpected facts that the weakly curved tubes are always near or between the screw-like wound tubes and that the big and narrow weakly curved or straight tubes are mostly found together in one sample. 
If the tubes represented separate species, it would be highly improbable to find tubes of two or three different types nearly always together in one sample. Since they are found together in small chert samples like the present one, there must be a reason. Possible reasons are hard to imagine, and any one would be highly unusual: some kind of symbiotic relation between two nematophyte species, or Nematoplexus with an ability to bring forth various forms, for whichever reason. Anyway, Nematoplexus can be expected to provide more surprises.
All pictures have been taken from the natural surface of one chert sample of 30g, Rh6/102, found by Sieglinde Weiss 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]  S.R. Fayers, N.H. Trewin: A review of the palaeoinvironments and biota of the Windyfield chert.
      Trans. Roy. Soc. Edinburgh Earth Sci. 94(2004 for 2003), 325-39.
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