Enigmatic cm-size blobs in Rhynie chert
deutsche Version

Most samples of the famous Lower Devonian Rhynie chert show silicified plant parts which are doubtless recognized as belonging to one of the known early land plant species, often with lots of fungus hyphae in between. Only a few samples contain distinctly preserved algae, lichens, nematophytes, or cyanobacteria. More common are indefinite layers and clouds probably indicating the presence of microbes of unknown affiliation. Less common are clouds with more or less well defined shapes or outlines, as the ones pictured here. It may be worth trying to explain them.

blobs in Rhynie chertFig.1: Two faces of one cut through a layer fragment of Rhynie chert. Width of one picture 6cm.
        (Figs.1-9:  taken from perpendicular cuts of one sample.)

With a cut gap of about 2mm, and the two halves of Fig.1 not much differing from mirror symmetry, it is obvious that the rounded shapes extend into depth much farther than 2mm. As the two arrangements of the 6 blob-like sections in Fig.1 are not seen on the rear of the 5mm slabs, it can be concluded that they are sections of roughly globular or slightly elongate blobs, hence no remains of land plants and thus worth being looked at closely. Their lower position in the chert layer indicates that they had been there before Rhynia grew or became flushed on top of them.

enigmatic blobFig.2 (left): One of the apparent blobs in Fig.1 (left half, 2nd) revealing a confusing structure: gray chalcedony with a former cavity inside and surrounded with "glued" mineral debris. Width of the picture 7mm.
black dotsblack dots
Figs.3,4: Details of Fig.2, successively enlarged: globules with dark or pale wall, transparent (or white) inside, of various sizes up to 35Ám, scattered along a strip of bluish chalzedony.

Figs.5,6 (below): Areas near the margin of the blobs with dark globules and indistinct bunches of dark streaks.

black dots + streaksblack dots and streaks

It seems as if aligned filaments or tubes had been involved in the formation of the blobs but had mostly decayed and vanished from sight (Figs.5,6,7).
There are other peculiar features involved: black clots of various shape, size, and arrangement. They are conspicuous when concave on the margin as in Fig.8. The clots may be arranged along a boundary, at some distance of it, or apparently at random. The tiny dark dots in Fig.8 are quite different from the dark globules in Figs.2-6. They seem to be the debris from a former structure. The texture in Fig.7 is so poorly preserved that it is hard to tell whether it originates from tubes or from aligned (bunches of) filaments. Whatever it was, it did not let the silt grains enter, hence they are kept beyond the margin: Figs.2-8.
(A similar formation seen on another sample, with tubes oriented towards the margin, has been discussed in connection with nematophytes.)
blob with faint texture

Fig.7 (far left): Area with a faint texture oriented towards the margin. Width of the picture 3mm.

Fig.8: Upper part of a blob with light-coloured chalcedony and dark substance of various shape. Width of the picture 6mm.

Fig.9 (below): Blob with black ribbon-like (?) remains of some structure. Height of the picture 2mm.

It seems difficult to make sense of these observations. At natural size the blobs are seen with an apparently clear outline. Magnification reveals that there are several not so well defined boundaries, more or less rugged ones.
What seems to be black globules in Figs.2-6 turns out to be hollow ones, now filled with clear chalzedony. If cut into halves it is seen that their wall is not black but dark and transparent. The black aspect against the background of light-coloured chalzedony appears only if the light is absorbed by both the front and rear wall of the whole globule.
Apparently the wall is covered with tiny dark dots about 6Ám apart. These globules differ clearly from the chlamydospores of the fungi whose hyphae are present in the chert above the blobs in Fig.1.
The smallest globules tend to be not dark but pale. A few globules in Figs.3,4 are of white aspect, which only indicates that they became filled with fine-grained quartz.
Various tentative interpretations have to be considered, including colonial microbes or algae, preferably arranged in peripheral positions around some of the blobs as if they formed a kind of symbiosis. So it may turn out that the small globules are no characteristic feature of the cm-size blobs. Unfortunately, the same can be suspected of the dark inclusions in Figs.5-9, too.
Without a practicable clue from the details, one can try to derive a tentative interpretation of the blobs from the overall aspect in Fig.1: Roughly globular or half-spherical cm-size blobs attached to the bottom under water or to moist soil are known from several cyanobacteria families. The blobs are usually kept in shape by organic gel. Those considered here might have grown in clear water or on land, then become flooded, with the silt particles sticking to their envelope of gel where they later became silicified together with everything else. Inclusions seen inside might either have been overgrown by the cyanobacteria or grown into the blob later, then degraded. Large tubes found in this sample, 80...120Ám wide and thus being comparable to Palaeonitella, may look like the debris in Fig.9 when crushed.
The attempted interpretations has to remain uncertain here because it is not known whether the observed details belong together or have got
together on the images only incidentally.

H.-J. Weiss       2014, modified 2015
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