On the origin of the pseudo-cellular pattern on the surface of nematophytes
deutsche Version

Nematophytes are enigmatic plants consisting of a felt of tubes, probably kept together by some kind of gel. With some species, like Nematophyton taiti from the Rhynie chert, the felt is covered by a layer consisting of a different type of tubes, aligned perpendicular to the surface [1].
Nematophyte in Rhynie chert revealing a relation between tubes and meshesIf preserved as coalified patches, the nematophytes often show a pseudo-cellular surface pattern referred to as "Nematothallus cuticle" [2]. As such pattern seems incompatible with the felt structure, the phenomenon has been repeatedly discussed, apparently without a conclusive result [3]. It has even been proposed that the joint ocurrence of pseudo-cellular cuticle and felt of tubes is due to mere coincidence [4]. The latter can be rejected by evidence from a recent find in a sample of Rhynie chert (Figs.1,2).

Fig.1:  Nematophyte in Rhynie chert, cross-section of a stack of aligned tubes revealing a relation between tubes and meshes; width of the picture 1.2mm.

Nematophyte in Rhynie chert revealing a relation between tubes an pseudocellsFig.2:  Nematophyte in Rhynie chert, cross-section of a stack of aligned tubes with conspicuous meshes forming a pseudo-cellular pattern
("Nematothallus cuticle") as often seen on the surface of coalified nematophytes; width of the picture 0.82mm.

The relation between tubes and meshes is obvious: There is a tube cross-section in every mesh. Apparently every one of the parallel tubes produces its own coat of gel. The coats become squeezed by mutual contact and thus make the pseudo-cellular pattern seen on the surface and occasionally in the bulk of the nematophyte. How the pattern possibly gets onto a cuticle is indicated in
Rhynie Chert News 38 .

H.-J. Weiss     2009    2015

[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,
      J. Paleont. 67 (1993), 1090-94.
[3]  H. Steur, W. van der Brugghen: Nematothallus, een radselachtige plant ...,
      Grondboor & Hamer (1998) Nr.2, 28-35.
[4]  D. Edwards: Fragmentary non-vascular plant microfossils from the Late Silurian of Wales,
      Bot. J. Linnean Soc. 84 (1982), 223-56.

Addendum 2013
Originally, "Nematothallus" meant a stack of tubes or filaments, often covered with a cuticle with a pattern as if there had been an epidermis. As it was not obvious what kind of organism could have produced both tubes and cuticle, Nematothallus gave rise to various interpretations, as revisited in a recent publication [5], where also a new diagnosis is introduced. According to [5], Nematothallus denotes nematophytes with cellular surface layers between cuticle and filaments, including Silurian fossils with a rather spongy aspect. Since such diagnosis excludes some nematophytes traditionally named Nematothallus, it must be mentioned that the present contribution makes use of the old concept of Nematothallus without cellular layers.
As a remarkable development, recently discovered Siluran fossils resembling Nematothallus and therefore named Nematothallopsis [6] (also without intermediate cellular layers) may help to unravel the nematophyte enigma: "... the most appropriate phylogenetic placement of Nematothallopsis – along with Nematothallus and Cosmochlaina [*] – is in the ... Coralline Red Algae ..." [6].  
  * (not in the sense of [5])

[5]  D. Edwards, L. Axe, R. Honegger:  Contributions to the diversity in cryptogamic covers in the Mid-Palaeozoic: Nematothallus revisited.
      Botanical Journal of the Linnean Society, .. (2013) ... .
[6]  M.R. Smith, N.J. Butterfield:  A new view on Nematothallus: Coralline Red Algae from the Silurian of Gotland.
      Palaeontology 56(2013), 345–357.

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