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Pachytheca, a peculiar, vegetable little sphere

Sometimes small glossy spheres are found in sediments of the Upper Silurian and the Lower Devonian. They measure 1 to 6 mm and possess a characteristic internal structure. Hooker named them Pachytheca in 1853, meaning 'thick sporangium'. In the first half of the 19th century they were considered to be seedlike objects or parts of fish jaws. From 1889 on Hooker took the little spheres for algae or colonies of algae and nowadays they are positioned in the Nematophytes, a group of enigmatic organisms, consisting of variously shapes and sized tubes. Other members of this group are Prototaxites, Nematothallus and Nematoplexus.

Pachytheca from Gileppe

Pachytheca from Forfar

Pachytheca from Glasgow

Pachytheca from Brecon

Pachytheca is most frequently found in the countries of western Europe: Scotland, Wales, England, Belgium, France and Germany, but there are also finds from other parts of the world, like Canada and Australia.
The specimens of Lac de la Gileppe in Belgium are exceptionally well preserved. Broken specimens show a spherical inner zone, surrounded by a thick cortex. Using a lens the fibrous structure of this cortex and the radially arrangement of the fibers are visible.
Dr. P. Gerrienne from Liège (Belgium) has described these Pachytheca's in detail in 1991. The SEM-photos below are from his article (with permission).

SEM-photos of Pachytheca from Lac de la Gileppe, Belgium

Broken specimen

Contact between inner and outer zone

Base of the outer zone

Tube with a filament

The cortex consists of thin, straight tubes in which sometimes a kind of thread appears. Tubes occur also in the inner zone, but these are strongly curved and run in all directions. They are in most cases poorly preserved indicating a less solid structure. This could be the reason that many specimens of Pachytheca are found with an empty or sedimentfilled inner zone.
At the right is the reconstruction by Taylor (1988). Click the image to see the enlargement. Several times a channel has been observed and there is a possibility that this channel had something to do with the, still not elucidated, reproduction of the plant.

The specimens of Lac de la Gileppe in Belgium have been so well preserved that they often show the detailed internal structure. For that purpose a peel can be made from the fossil. A piece of rock with Pachytheca in it, is sawn through and polished. The polished surface is then etched with diluted hydrochloric acid. In this way a thin layer of chalk is removed whereas the organic parts remain. After rinsing and drying aceton is poored over the etched surface and a piece of cellulose acetate folium is put over it. This is pressed to the surface and pulled off after some drying time. The organic remains are now on the peel and they can be studied under the microscope.
Below a couple of photos of such peels. I have obtained these peels from mr. H. Hass of the Wilhelms University of Münster (Germany).

Photos of peels of Pachytheca from Lac de la Gileppe

Complete specimen

Detail of the preceding photo

Second specimen

Detail of the preceding photo

It looks as if the cortex grew relatively thinner as the organism became older (and more mature?). In young specimens the cortex is about as thick as the inner zone; in older and bigger specimens the cortex is relatively much thinner.
Some time ago Hans-Jürgen Weiss from Rabenau  (Germany) has found a well preserved specimen of Pachytheca in a piece of Rhynie chert. Click here to study this fossil.

Pachytheca belongs to a group of enigmatic plants, which conquered the land at about the same time as higher plants (like Cooksonia) did. Other members of this group are Nematothallus, Parka, Prototaxites and Nematoplexus. All plants of these group became extinct during the Devonian.