Pachytheca – a nematophyte propagule ?
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The pea-size spheres known as Pachytheca, found in the Silurian and Lower Devonian worldwide, had been interpreted in various ways [1,2], most recently as  nematophytes, and put under the heading "Enigmatic Organisms" [3], which adds to the fascination emanating from this fossil which seems to defy proper classification.
Pachytheca sphere, central section, in Rhynie chertIn this situation, even vague details, if combined, may provide some insight. Three possibly useful observations are listed here:

(1) A peculiar coating consisting of short "hairs", probably tubes in some state of decay and embedded in a layer of organic gel, is seen on part of the surface of the only well preserved Pachytheca specimen hitherto found in the Rhynie chert. (See Figs.1,2 and Rhynie Chert News 36.)
(2) Tubes arranged such that they may represent a poorly preserved nematophyte (Figs.3,4) are seen near the Pachytheca sphere in the Rhynie chert sample.    (See "Annotation 2016" below.)
(3) Pachytheca and nematophytes are found together in Lower Devonian non-chert rock at the Lac de la Gileppe [1,2]. The nematophyte fragments from there have been assigned to Prototaxites but own observations suggest that there are smaller nematophytes involved. (Evidence will be presented later.)

Fig.1 (right): Pachytheca in Rhynie chert, central section. Note the "hairy" coating along the circumference and the very vaguely seen remains of nearby tubes (?) above left. Diameter 3.8mm.

Fig.2 (below): Pachytheca in Rhynie chert, off-centre section. Note the preferential inclination of the "hairs" towards the right.
Pachytheca with hairy coating in Rhynie chert
patch with poorly seen tube-like filaments near Pachytheca in Rhynie chertFig.3 (right): Irregular-shaped whitish layer (above left to below right) with poorly preserved tubes, diameter about 40-80Ám, some with dark fill, vaguely seen in the chert sample within 2cm distance from Pachytheca.
Width of the picture 6mm.


Fig.4 (below): Bounded patch in the chert sample at 1cm distance from Pachytheca with poorly preserved tubes whose presence is indicated by discontinuous dark fills. Width of the picture about 8mm.
bounded patch with tube-like filaments near Pachytheca in Rhynie chert
The observation that the patches with tubes are set off from the surrounding chert by colour and some kind of rugged boundary (Figs.3,4) is compatibel with the assumption that what is seen here is a nematophyte consisting of tubes within a lump of organic gel. (It must be mentioned that poorly preserved early growth stages of Palaeonitella might be mistaken for nematophytes.)
The hairy coating seen in Figs.1,2 , taken together with the other observations, suggests the idea of an intimate connection between Pachytheca and filamentous structures typical for flat nematophytes. Taking this for granted, one arrives at the conclusion that Pachytheca possibly is a small compact part of a larger nematophyte "body" with less well defined shape and structure, which leads to the question what could be the reason why a nematophyte invented a body part of such highly symmetrical shape and structure.
An answer may be found by combining more observations and facts:

(4) Nematophytes, or at least some of them, consist of loosely arranged tubes embedded in organic gel. (See Rhynie Chert News 13, 29.)
(5) Nematophytes, or at least some of them, protect themselves against exsiccation by a cuticle, as the well-known Nematothallus cuticle, whose pseudo-cellular structure had been enigmatic ever since but has been tentatively explained now as a replica of a shrinkage crack pattern in organic gel. (See Rhynie Chert News 30, 38 .)
(6) There is evidence that Pachytheca has got a smooth protective cuticle even under the comparatively thick fuzzy coating. (See Rhynie Chert News 1, 36 .)

Apparently, the mechanically most stable and drought-resistant structure which can be obtained with the few components available to nematophytes, namely tubes, gel, and cuticle substance, is the one realized by Pachytheca: a thick spherical shell consisting of radially arranged tubes of one size, glued together by gel, forking such that they keep densely packed, and forming a nearly perfect sphere with a cuticle covering the whole.
Such compact structure differs much from that of nematophytes with loosely arranged tubes. Since physiological processes are based on the exchange of substances by diffusion, the compact structure seems unfavourable in a similar way as seeds and bulbils are unsuitable for sustaining physiological activity. They are adapted to standing hardships while dormant, which makes them suitable for survival and distribution. Pachytheca could have served the same purpose, as a hardy nematophyte propagule, mechanically so strong that it usually remained spherical while everything else in the sediment became compressed. It can be assumed that such strength had been advantageous. Hence, it seems worthwhile looking for propagules of other nematophytes. Also the idea suggests itself that the different types of Pachytheca, if they really are propagules, represent different nematophyte species.

Annotation 2016: Although nematophytes may appear as tangles of tubes in whitish clouds with dark boundary
(see Rhynie Chert News 98), new observations suggest that the present Figs.3,4 probably show some undescribed alga incidentally grown near Pachytheca.

H.-J. Weiss 2011

[1]  H. Steur : Pachytheca, een vreemd, plantaardig bolletje uit het Devoon,
      Grondboor & Hamer 58(2004), 52-57.
[2] H. Steur:  Pachytheca, a strange, vegetable little sphere.
       http://www.xs4all.nl/~steurh/
[3]  T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009.
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