Nematoplexus  -  thrice enigmatic
deutsche Version
Nematoplexus is properly listed under the heading "Enigmatic Organisms" in [1] since this strange creature consists essentially of tubes in several
varieties: ...screws
 (1) nearly straight, slightly curved, screw-like wound,
or irregular,
 (2) with smooth or patterned walls and a wide range of diameters,
 (3) emerging from dark lumps.
Most peculiar, several of these features may be found combined in various ways: weakly curved thick and thin tubes
side by side, regular screw-like spirals with smooth wall beside crooked tubes with spiral wall patterning [2], small spirals beside big spirals, and more of such.
All this versatility is restricted by rules: The thread is always right-handed, and its diameter is roughly equal to the 
pitch. Along the tubes, their type does not change: Spirals remain screw-like, diameter mostly below 16Ám, and tubes with about 20Ám never form spirals.
These rules are to be considered preliminary since they are based on
the few recovered samples 
The inner workings of the dark lumps, the so-called branch knots supposed to produce the tubes, is an enigma in itself. Their sizes vary between about 0.04mm and 0.25mm.
Nematoplexus big knot
Fig.1 (right): Nematoplexus, usual aspect with screw-like wound tubes and dark lump. Width of Figs.1,2: 0.7mm.

Fig.2 (left): Slightly curved big tubes and very thin ones emerging from "branch knot",
also two unrelated straight ones, found near spiralling Nematoplexus .

One would never think of assigning the two quite different structures seen in Fig.1 and Fig.2 to one species. Nevertheless, one can suspect that these structures might be mutually related in some hitherto unknown way,
being different manifestations of the same organism. This suspicion is based on the observation that the big slightly curved tubes with their knots are never far away from the more slender spirals with their knots, at distances of about 2cm or less. They are very rare fossils, and if they represented independent organisms, it would be highly improbable to find the different forms closely together in one small chert sample.
Nematoplexus big knot
Fig.3 (right):
Nematoplexus big tubes emerging from the dark lump with obscure structure in Fig.2. Width of the image 0.2mm.

"Branch knot" alludes to the notion that "Branching of the tubes occurs in ...very tightly coiled knots of tubes showing repeated and closely spaced branching" [3]. This is not substantiated by the samples available here. In Figs.2,3 it is seen that the tubes are club-like where they emerge from the dark lump
and that there is no indication of branching. The club-like shape is also obvious from the cross-sections of tubes cut near the lump, with diameters up to 34Ám near the lump in Fig.3 but 17...20Ám farther out in Fig.2. Thus it seems that the surface of the lump, although not well defined, serves as a reference site for the tubes. (See also Rhynie Chert News 134.)  The inclined cut on the right in Fig.3 looks as if the tube simply sticks to the surface of the lump.
Nematoplexus narrow tubes
Fig.4 (left): Nematoplexus narrow tubes (5Ám) with patterned wall, detail from Fig.2, turned 90░ to the right. Width 0.15mm, same scale as Fig.3.

Another
remarkable feature in Fig.2 is the narrow filaments attached to the lump. Two of them are faintly seen where an unrelated straight narrow tube (8Ám) crosses a cut-off big tube. The irregular mixed annular and spiral pattern as known from tube walls in the type specimen [2] is seen here on the walls of the narrow tubes of 5Ám, with spacings of 2...3Ám between the dimly seen structure elements (Fig.4). All other nematophyte tubes in this sample seem to have got smooth walls. 
The apparently radially extending big tubes in Fig.2 are really slightly curved towards the observer, which is out of the picture plane, and accordingly are cut off. Nematoplexus tube slantingThe granular aspect of the big tubes in Fig.2 is possibly due to microbial deposits or to decay products of the wall substance, as suggested by Fig.5, which shows the tube bent out of the picture plane near the lower edge of Fig.2. The outside of the tube enclosed in chalzedony is seen on the right in Fig.5. A clean slanting cut would have produced a parabolic boundary of the tube wall towards the left. Instead, a crack had run across the wall and fragments must have fallen off. Farther left, the rear wall with its probably insignificant random pattern is exposed.

Fig.5: Slanting cut
 of a big tube in Fig.2 with a probably irrelevant random pattern on the rear wall. Width of the image 0.1mm.

From the samples available here it can be concluded that Nematoplexus is more complex than a mere meshwork of "spirally coiled tubular cells" [3] and that it is not confined to the parameter region stated in [2] for the type specimen of Nematoplexus rhyniensis. This might be due to an extremely wide variability of the species or to quite another cause: Nematoplexus might be a symbiotic organism composed of several partners contributing tubes of various sizes and curvatures, with one or other of them occasionally dominating for reasons unknown, forming shapes as in Figs.1,2.
More surprises can be expected from new finds.
All pictures have been taken from one sample labelled Rh15/79, obtained from Barron in 2014, cut into 4 parts. Those with higher magnification, Figs.3-5, have been taken from Part 1 by Gerd Schmahl.in 2018.

H.-J. Weiss  2018

[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, from the Rhynie chert, Nematoplexus rhyniensis. 
      Trans. Roy. Soc. Edinburgh 65(1961-62), 79-87, 2 tables.
[3]  www.abdn.ac.uk/rhynie/nemato.htm  
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