Nematophytes vs. liverworts
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Nematophytes, which literally means "filamentous plants" (Fig.1), are still so poorly understood that lately they have been tentatively listed under the heading "Enigmatic Organisms" [1]. This poses a challenge to look for explanations [2].
nematophyte with tube cross-sections and 2 clots of smaller filaments, Rhynie
Fig.1: Nematophyte in Rhynie chert with nearly parallel tubes in cross-section and two dark clots supposedly consisting of a tangle of much smaller tubes; undescribed species. Width of the picture 1.26mm.
(Annotation 2016: Closer inspection has shown that the big clots seen here do not consist of a tangle of tubes and thus differ from the "branch knots" known from other nematophytes. See Rhynie Chert News 92, This does not affect the below conclusions.)

Recently it was tried, with much effort, to gather evidence for the idea that some or most nematophytes are ancient liverworts [3]. This idea is based on two guesses:
  -  The tubes constituting the nematophyte are rhizoids of some liverwort.
  -  The cuticles with cellular patterns known as Nematothallus [4,5]
    and Cosmochlaina, often found together with coalified tangles
    of filaments or tubes, belong to ancient liverworts.

The idea of a liverwort connection has quickly spread among the scientific community and more or less been accepted without checking, judging from quotations like this: "One interesting hypothesis suggests that several of the enigmatic Cambrian to Devonian fossils traditionally included in the nematophytes may represent remains of ancient liverworts ... [1]." Apparently this hypothesis has never been seriously doubted, except for the present contribution based on a very few nematophyte specimens
recently found in the Rhynie chert [6].
The dark clots among the mass of tubes in Fig.1, which are a typical feature of some nematophytes, preclude an interpretation of the tubes as rhizoids. (The clots apparently consist of a close tangle of much smaller tubes whose cross-sections are seen as tiny bright dots. The existence of the clots is one of the enigmas connected with the nematophytes.)
nematophyte section with pseudocells
Fig.2: Same sample as Fig.1, with a cellular pattern interpreted here as boundaries between gel coats around the tubes.
See also Rhynie Chert News 30

The problem of how the nematophytes could possibly produce a cuticle with cellular aspect can be separated into two parts: How the pattern is brought about, and how the cuticle is made.
Fig.2 suggests an explanation for the origin of the pattern: Every one of the tubes surrounds itself with a coat of gel. The gel coats taken together make a continuous mass of gel, with the boundaries between them still there and, under favourable conditions, seen as a cellular pattern.

It remains to be explained how the pattern can be imprinted onto a cuticle. This is attempted with the drawing in Fig.3. Here it is assumed that the boundaries between the gel coats seen on a cut and polished chert face in Fig.2 make a network of grooves on the surface of the lump of gel, possibly deepened by drying.
Nematothallus cuticle formation, schematic drawing
Fig.3: Schematic drawing illustrating the possible formation of a cuticle with cellular pattern (Nematothallus aspect) on the gel surface of a nematophyte. (The shape of the tube ends is not known.)

The tubes may produce a cuticle precursor substance and release it into the gel where it moves by diffusion towards the surface where it accumulates, spreads, fills the grooves, and polymerizes into a protective cuticle with Nematothallus aspect, which is decay-resistant as known from the cuticles of land plants.
This is the proposed solution of the long-standing Nematothallus problem, without invoking the idea of a liverwort connection. It is essentially based on the presence of gel as a constituent of nematophytes which reveals itself in the Rhynie chert but not with the compressed specimens found elsewhere.
A similar solution is proposed for the Cosmochlaina problem.
Prototaxites formation as imagined by Graham et al.
Fig.4: Prototaxites forming from liverwort rolls as imagined by
Graham et al. (2010) [7].


The vanishing evidence for the liverwort interpretation of nematophytes affects the latest fancy hypothesis declaring the enigmatic huge Prototaxites trunks to be rolled-up and subsequently silicified liverwort mats as illustrated in Fig.4 [7]. 



The unique preservation of nematophytes in the Rhynie chert, non-compressed and suggesting the presence of gel between the filaments, allows the following conclusions to be drawn [6]:
 -  The idea of nematophytes being related to liverworts, brought up in 2004 and widely spread since, is not substantiated.
 -  The latest elaboration of the liverwort idea in 2010, which is the introduction of liverwort rolls as another
     interpretation of Prototaxites, might be remembered for its inventiveness but should better be forgotten.
 
H.-J. Weiss             2010  2016

[1]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009, p163.
[2]  H.-J. Weiss : "Enigmatic Organisms" – Insights derived from new finds. (Poster)
      8th European Palaeobotany - Palynology Conference 2010, Budapest.
[3]  L.A. Graham, L.W. Wilcox, M.E. Cook, P.G.Gensel :
      Resistant tissues of modern marchantoid liverworts resemble enigmatic Early Paleozoic microfossils.
      Proc. Nat. Acad. Sci., USA, 101(2004), 11025-29.  
[4]  P.K. Strother : Clarification of the genus Nematothallus.  J. Paleont. 67 (1993), 1090-94.
[5]  H. Steur, W. v.d.Brugghen : Nematothallus – een raadselachtige plant uit het Siluur en het
      Vroeg-Devoon.  Grondboor & Hamer (1998) Nr.2, 28-35.
[6]  H.-J. Weiss : Nematothallus: How the filaments produced a cellular cuticle. (Oral presentation)
      8th European Palaeobotany - Palynology Conference 2010, Budapest.
[7]  L.E. Graham, M.E. Cook, D.R. Hanson, K.B. Pigg, J.M. Graham: Structural, physiological, and stable
      carbon isotope evidence that the enigmatic Paleozoic fossil Prototaxites formed from rolled liverwort mats. 
      Am. J. Bot. 97(2010), 268-275.
Acknowledgement:  Thanks are due to Christopher Taylor , Curtin University, Perth,
for drawing attention to the above painting and related discussions by means of his blog, which instigated the presentation [6].

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