Pachytheca in Rhynie chert, total

Enigmatic little sphere 
deutsche Version

Among the numerous circular sections of the well-known Rhynie chert plants usually seen on cut and polished faces of the chert samples, one particular circular section on a sample found in 2003 looks different (Fig.1): It does not show the usual structure of cellular tissue with a central strand but radially arranged tubes instead. It has turned out to be not a cross section of a plant shoot or axis but of a little sphere of about 4mm diameter incidentally cut right through the centre.
Fossil spheres of this type, called Pachytheca, have been known for a long time as carbonaceous fossils in Upper Silurian and Lower Devonian sediments from several places [1] but never in chert.
Pachytheca in Rhynie chert, forking tubes
Fig.1: Cross-section of the enigmatic sphere Pachytheca made up of radially arranged tubes. Diameter 3.8mm. Photographs: H. Sahm.
Note the cracks not entering but running along the surface of the sphere,
possibly indicating the presence of a waxy layer or cuticle. 
Nothing is left of the tangle of tubes usually present in the centre.

Fig.2: Cross-section of Pachytheca, detail from Fig.1.
Note the well-defined smooth surface, the concentric bands below it, and the forking tubes.

In early publications, the nature of Pachytheca had been a subject of wild speculation. More recently, it was regarded as an unsuccessful attempt of Green Algae trying to adapt to terrestrial life. This would make sense in view of the fact that the structure is optimized for minimum water loss. However, the alga connection apparently proved unsubstantiated so that it was cautiously placed under the heading "Enigmatic Plants". As it has some features in common with the nematophytes, which, still more cautiously, are called "Enigmatic Organisms" now, it has been placed there [2], which makes it all the more fascinating.

Among that much uncertainty it seems to be certain that Pachytheca is a terrestrial organism. This assumption is confirmed by peculiar crack paths in the present specimen: Cracks propagating towards the sphere do not traverse it but are deflected into a path along the surface. From this observation it can be concluded that the surface is a mechanically weak face in the chalcedony as it is often brought about by the presence of a waxy coating or cuticle covering the surface of land plants as a protection against exsiccation.
Like the filaments of other nematophytes, those of Pachytheca
appear as tubes not subdivided into cells (Fig.2). The tubes are clearly seen forking, a feature not distinctly seen with other nematophytes.
Pachytheca in Rhynie chert, damage in gel
Fig.3: Cross-section of Pachytheca, detail from Fig.1.
Note the well-defined smooth surface, straight and forking tubes, and irregular-shaped cavities apparently due to damage in gel between the tubes.
The thick coating on the surface is probably a microbial layer.

The aspect of the concentric bands faintly seen in the photographs, particularly in some places in Fig.2, suggests that these bands may not be a feature of the tubes but of the space in between. This and the cavities seen in Fig.3 seem to indicate the presence of gel between the tubes. This would be the first evidence for the presence of gel in Pachytheca. Observations on other nematophyte species in the Rhynie chert also suggest a consistence as jelly with filaments inside. Hence, jelly inside Pachytheca is compatible with the interpretation as a nematophyte. Also the presence of gel would easily explain the smooth surface of Pachytheca, and the concentric bands could be interpreted as the positions of older gel surfaces which became overgrown, which means they could be called growth rings, or rather growth spheres in this case.
Despite of some information derived from this well preserved specimen in the Rhynie chert, Pachytheca remains an enigmatic organism.

H.-J. Weiss     2004, updated 2010

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

Annotation: This specimen was first presented at the 3rd Chert Workshop, Chemnitz 2004, and a first version of this contribution, based on the now obsolete view of a relationship to Green Algae, was submitted to then. In connection with the current interpretation of Pachytheca as a nematophyte and the highly probable presence of gel inside, the idea of water uptake by capillary action put forward in the first version is retracted herewith. See also Rhynie Chert News 36, 44.
A few more photographs of this specimen by H. Sahm  have been attached to a more comprehensive presentation on Pachytheca by H. Steur [1].
For more references see [1,3].
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