Another weft of tubes - another nematophyte ?
deutsche Version

There are peculiar fossils consisting of a weft of tiny flexible tubes. The various species found worldwide have been compiled under the name nematophytes. They are of particular interest as they are thought to belong to the earliest land plants and have not yet got a position in the phylogenetic tree.  
Such kind of fossil has been found recently in a small sample (0.25kg) from Rhynie, where it appears as a "composite material" consisting of tubes in a matrix of siliceous or organic gel. Fragments thereof had lain in the swamp, together with land plants of the Rhynie flora. All silicified into chert.
The outer boundary of the weft  was either not on the present fragment or had been eroded away before silicification.
The tubes are rather evenly
arranged on a large scale and randomly in detail (Fig.1) except for the presence of a few tangled "knots" as in Fig.2, whose purpose and inner structure are enigmatic. The orientation of the tubes is not random but shows a tendency towards a texture.
Nematophyte, Rhynie chert, undescribed species
Fig.1:  Weft of flexible tubes in Rhynie chert, rather evenly distributed except for tangled knots as the one seen below right. Width of the picture 5.5mm.
Nematophyte, Rhynie chert, undescribed species, knot of tubes
Fig.2:  Tangled knot of tubes on the right, detail of Fig.1.
Width of the picture 2mm.

From the deformation of some tubes at positions of mutual contact, as in Fig.2, above left, it can be concluded that the tubes were rather limp before silicification.

The diameter of the tubes is about 50 to 70Ám. None of the tubes shows the spiral or annular wall pattern often seen in nematophytes. The tubes do not seem to be branched or subdivided into cells, perhaps with rare exceptions as in Fig. 3, where a tube seems to branch and form a larger bud**. However, this particular tube is exceptionally thin (35Ám) and runs athwart the texture, so it is not certain whether it belongs to the weft.
Nematophyte, Rhynie chert, undescribed species, differential tubes
Fig.3 (right):  Weft of tubes with one particular tube deviating in size, shape, and orientation (above centre). Note the outgrowth resembling a bud on a stalk.
All pictures have been taken from the natural surface of the sample.

With these features, the fossil differs distinctly from the two nematophyte species found at Rhynie before [1,2,3] and also from several others found elsewhere [4,5], and thus the question arises whether it is a nematophyte at all. It is roughly similar to the dense tufts of rhizoids below the rhizome of Horneophyton. Those rhizoids, however, are much thinner (25Ám) and more straight and aligned. An interpretation as a tuft of rhizoids of a larger, still unknown plant can be ruled out with the argument that it would be against the nature of rhizoids to form tangled knots as in Fig.2.
This loose aggregate of limp tubes could hardly be mechanically stable like a felt: When moistened it would likely collapse under the action of surface tension like a fluff of cotton. However, this would not mean that the delicate structure must have grown under water. The space between the tubes could have been filled with a gelatinous substance providing stability,* as it is known from various algae and cyanobacteria. This would also explain why there were no tiny bits of debris like spores or microbial clots between the tubes although such dirt was omnipresent in the swamp water where the Rhynie chert formed. (A granular structure faintly seen in some places may be an artifact due to crystallization.) So it seems that the statement “habitat probably subaerial” made in [5] for all nematophytes might well apply to this organism, too.

* More evidence for gel in nematophytes has become available from own finds.
** Up to now (2013), only one more clearly seen example of lateral branching has been found:
Rhynie Chert News 40, Fig.5.

H.-J. Weiss
    2005,  emended 2013

[2]  R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure,  Part V.
      Trans. Roy. Soc. Edinburgh 52(33)(1921): 855-902.
[3]  A.G. Lyon: On the fragmentary remains of an organism referable to the Nematophytales,
      Trans. Roy. Soc. Edinburgh 65(4)(1962): 79-87.
[4]  P.K. Strother: New species of Nematothallus, J.Paleont. 62(1988), 967-982.
[5]  P.K. Strother: Clarification of the genus Nematothallus Lang, J.Paleont. 67(1993), 1090-94.

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