Nematophytes in gel
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Fossil nematophytes, like any fossils preserved in chert, underwent transient stages of being embedded in and permeated by silica gel, governed by processes more or less well understood, which will not be considered here. This contribution is about organic gel produced by the organisms themselves for multiple protection. After silicification, the organic gel is usually no more seen but its former presence may be deduced from details seen in the chert (Fig.1).
nematophyte in chert
Fig.1: Two types of chert combined in one sample: nematophyte (upper half of the image), silicified earlier than the swamp matter including degraded plant cross-sections below. Width of images: 4.3mm.


This image may be confusing and enigmatic at first sight. It is a natural combination of two quite different parts of  chert. The upper half is a nematophyte consisting of randomly arranged but partially aligned tubes in bluish chalzedony. The nearly horizontal line in the image indicates a smooth fracture face unlike that of a felt of tubes or of a fibrous composite material. Hence, the nematophyte had broken like a compact material, which implies that it had been in some advanced state of silicification. The fragment got into the muddy water with mineral debris and flooded land plants below, now largely degraded. Silicification proceeded until finally all had turned into solid chert.
Apart from the fossil content, another difference between the halves of the picture is obvious: Contrary to the abundant debris and mineral precipitates below, the space between the tubes above is rather clean. The small white specks are no inclusions but the result of recrystallisation in chalzedony. The absence of debris between the tubes indicates the presence of organic gel which precluded any dirt from entering.
Essentially the same is seen in Fig.2. Here, the crack goes athwart the texture of the felt. Again the crack face is smooth, without tube ends sticking out. Hence the nematophyte as a whole must have been mechanically homogeneous when it broke, with tubes and matrix thoroughly silicified so that the felt-like structure did not affect the crack propagation.
There is no indication concerning the cause of the fracture of the nematophyte while the swamp matter was still fluid. It is known that organic matter gets silicified faster than the surrounding water and thus can break when bent or torn. Perhaps the organic gel assumed to fill the space between the tubes triggers the early formation of silica gel which tends to become more and more solid by uptake of silica so that it can break when strained.



nematophyte in chert
Fig.2: Two types of chert combined in one sample: fragment from brittle fracture of early silicified nematophyte (right) next to fluid swamp matter with degraded plants (left) silicified later.

In addition to the advantages of organic gel for life in water, which is keeping things together, keeping dirt away, and protecting against attack, gel serves as a means of survival of watery organisms under subaerial conditions. The present sample offers evidence that nematophytes may have applied this survival technique, as suggested by Fig.3. This nematophyte had lived as a felt of tubes in a lump of gel with unknown overall size and shape, either in water or on moist ground, or in a periodically changing habitat. During a prolonged dry period the lump with tubes dried up at the surface and shrunk, which proceeded into some depth. Thereby the limp tubes collapsed so that they are seen as mere narrow streaks in the affected region of the lump which forms a kind of cortex protecting the bulk from drought. No signs of exsiccation are visible below.
The protective effect seems to have been limited by internal stresses causing the layer to detach and peel off. In Fig.3 it adheres to the bulk on the right but becomes increasingly detached towards the left.
Finally the slightly damaged lump must have become inundated and silicified while lying in the muddy water together with mineral debris and degraded land plants, as also seen in Fig.3.

Fig.3 (below): Nematophyte as a lump of gel dried and shrunk to some depth, thus forming a dark protective layer against exsiccation of the bulk.
nematophyte in gel, dried outside

Among numerous own samples of Rhynie chert found since 1998 there were only 9 with nematophytes, among them the well-known spherical Pachytheca (one sample with one specimen) and the spiralling Nematoplexus, with tubes distinctly larger than originally described, in 4 chert samples. Less certain is the interpretation of a rather well preserved nematophyte in another sample as Nematophyton taiti [1].

Three more samples contain nematophytes differing much from the known ones so that they might represent 3 new species. Obviously they are very rare, otherwise they would have been found in more than one chert sample each.
With the exception of Pachytheca, branching of tubes has not been clearly seen in these samples.
Apparently there had been organic gel between the tubes in every one of the nematophytes mentioned here.
The lump of gel in Fig.3 with a dried-up, shrunken, and possibly hardened layer along the surface seems to provide the most convincing evidence that nematophytes might have been able to survive or temporarily live under subaerial or dry conditions, as proposed in [2,3]. Now that we see them thoroughly silicified, we can conclude that they must have been flooded with silica-rich water. There they became silicified sooner than the water since they broke like a solid material (which could as well have been in a state of gel) while the surrounding water was still fluid and got silicified later, as suggested by Figs.1,2.
The pictures have been taken from the sample described in Rhynie Chert News 13, found by Sieglinde Weiss in 2005.

H.-J. Weiss     2016

[1]  R. Kidston, W.H. Lang : On Old Red Sandstone plants showing structure ...,
     Part V, Trans. Roy. Soc. Edinburgh 52 (1921), 855-902.

[2]  P.K. Strother: Clarification of the genus Nematothallus Lang, J.Paleont. 67(1993), 1090-94.
[3]   www.abdn.ac.uk/rhynie
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