Prototaxites and Cosmochlaina: no liverwort connection
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A very peculiar hypothesis has been presented [1] as an explanation for the big enigmatic fossil called Prototaxites. This fossil has been included into the chapter Enigmatic Organisms in [2] since it looks like a tree trunk but does not consist of wood, and it lived in the Silurian and Lower Devonian when plants on land did not exceed heights of a few centimeters.
Fig.4: Prototaxites forming from liverwort rolls as imagined by GRAHAM et al. (2010)
In order to obtain, in imagination, a big trunk-like fossil from the humble vegetation, the authors [1] invoke the idea of rolled-up mats. Liverwort mats on sloping rock surfaces are supposed to curl in at the upper end and eventually start rolling down the slope as pictured with artistic skill in Fig.1. Leaving aside the question how all those carpet rolls could become so tightly wound that their cross-sections remained nearly circular while lying around awaiting silicification, one can challenge the hypothesis by starting from the liverwort side of the problem. Small nematophytes recently found in the Rhynie chert provide surprising insights.

Fig.1: Prototaxites forming from liverwort rolls as imagined by GRAHAM et al. (2010) [1].  

Fossil liverworts are known since the Upper Middle Devonian [2]. In order to have liverworts in the Silurian, the authors [1] had re-interpreted Silurian cuticles known as Nematothallus and Cosmochlaina (Fig.2), usually thought to belong to the nematophytes, as liverwort cuticles in a previous publication [3]. Recent finds of nematophytes in Rhynie chert provide contrary evidence [4]. A concise version of the presentation [4], focused on the explanation of the hitherto unexplained structure of the Nematothallus cuticle, is given in
Rhynie Chert News 38.
What remains to be done is to interpret the conspicuous pattern of Cosmochlaina without resorting to liverworts. (According to [1], the arrows in Fig.2 indicate "pores surrounded by cell rosettes", which is shown to be a misinterpretation in the following.)

Fig.2 (below) with detail: Silurian cuticle Cosmochlaina, re-interpreted in [1] as lower epidermis tissue of liverworts with broken-off rhizoids but quite differently interpreted here.
Silurian cuticle Cosmochlaina, most often pictured varietyCosmochlaina detail: no cells but cracks
The rather superficial similarity to the lower epidermis of liverworts with broken-off rhizoids misled Graham et al. [3] to the assumption that it really is such. The polygonal meshes, cautiously called "entities" in [5] but readily interpreted as cells in [3], should better give rise to suspicion. Rhizoids grow from one cell each so that a rhizoid emerging from 7 cells at once as in this  picture (see detail) would not make sense. By carefully inspecting Fig.2 one finds more features of the pattern which are hardly compatible with the notion of a cell sheet. Here it is useful to know that what looks like a cellular pattern could be a crack pattern, an idea which may suddenly remove apparent absurdities and open other vistas. (For another case of this type, see Rhynie Chert News 8.)

According to the interpretation proposed here, Cosmochlaina is a cuticle belonging to a nematophyte as originally assumed. The round spots are the cross-sections or ends of the tube-like filaments embedded in gel which make up the nematophyte. Hence, the old problem of how a nematophyte could produce a cell sheet has vanished: There are no cells but only cracks. The cracks could be due to shrinkage of the drying surface of the lump of gel. The nematophyte could have released an organic substance which accumulated on the gel surface, polymerized into a decay-resistent cuticle as a protection against exsiccation, and thus preserved a replica of the surface crack pattern.
Unexpectedly, an early paper on Cosmochlaina [5] supports the interpretation of Fig.2 as a crack pattern. There, specimens are shown whose aspect differs much from Fig.2, the most often reproduced picture of this fossil: See Fig.3.
Cosmochlaina, seldom pictured variety without mesh pattern
Fig.3 (right): Cosmochlaina as it is seldom pictured [5]: no polygon pattern but early stage of crack propagation.

Remarkable is the absence of the polygon pattern which is often thought to be an inherent feature of Cosmochlaina but is not. Apparently the cuticle in Fig.3 shows a very early stage of crack pattern formation, with only a few short cracks emerging from the tube cross-sections. The transition from individual cracks to polygon patterns is not yet completely understood in fracture mechanics but the phenomenon as such is well known from various substances undergoing shrinkage.
Annotation 2020: A tendency of growing shrinkage cracks to form polygon patterns has been demonstrated with fracture mechanical computations in [7].

Now that no evidence is left for Silurian liverworts in the disguise of Cosmochlaina and Nematothallus, the liverwort hypothesis of Prototaxites lacks its main ingredient. Without any other vegetation available for making carpet rolls from, Prototaxites remains as enigmatic as ever. (See also [6].)

H.-J. Weiss         2010    2020
[1] L.E. Graham, M.E. Cook, D.T. 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.
[2] T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009, p163.
[3] L.E. 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] H.-J. Weiss: Nematothallus: How the filaments produced a cellular cuticle. (Oral presentation)
      8th European Palaeobotany - Palynology Conference 2010, Budapest.
[5] D. Edwards : Dispersed cuticles of putative non-vascular plants from the Lower Devonian of Britain,
      Bot. J. Linnean Soc. 93(1986), 259-75.
[6] H. Steur:  
M. Hofmann, R. Andersson, H.-A. Bahr, H.-J. Weiss, J. Nellesen: Why Hexagonal Basalt Columns?
      Phys. Rev. Lett. 115, 154301 (2015)

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