Aspects of Devonian microbes
microbial structure
One cannot deny that there is something mysterious in the fact that cyanobacteria can aggregate in large numbers and form shapes serving a higher purpose rather than existing as mere separate unicells. Thereby they may produce even easthetically appealing shapes, as seen in Fig.1. Apparently three curved microbial layers converge to a spot on the left where there had been another club-shaped object which had got lost. No explanation is given here for the formation of the smoothly curved layers. All pictures have been taken from 6 samples of the Lower Devonian Rhynie chert.

Fig.1 (right):  Structure of unknown purpose brought about by a coordinated action
        of a multitude of unicells. Image width 5.5mm. Figs.1-4 with same scale.

Fig.2 (below): Similar as with Fig.1. The club-shaped protrusions are apparently
        related to the big globular object in the centre with multi-layered sheath. Image width 6mm. microbial formation

microbial formation

Clots within a microbial envelope.
Image width 4.7mm.

Fig.4 (below): Horizontal layer with small dark clot below and closed envelope above obviously made of the same microbes.
Image width 5.5mm.
microbial formation

Envelopes with one or more clots inside have been seen repeatedly but their formation is still enigmatic. There is no indication that they had been formed as bulky shapes squeezed flat.
From common features in Figs.1-4, the presence of (multiple) sheaths forming envelopes around clots, one may guess that these structures were formed by the same kind of microbes, or similar ones, which apparently have not yet been described.

One can distinguish a few different microbes from the Rhynie chert without going into details of the cellular structure. For example, Croftalania venusta [1] is easily recognized if grown in conspicuous tufts of filaments as in Fig.5 and in
Rhynie Chert News 56, 72.

Fig.5 (left): Tufts of the filamentous microbe Croftalania venusta [1] grown from a temporary silica gel surface towards the light.
Image width 1.7mm.
microbial coating on gel

Fig.6 (right): Black coating on the surface of bluish silica gel around Aglaophyton.
Image width 8.6mm.

Often seen are microbial coatings on plant parts and surfaces of silica gel. They may have begun yellow but turned brown or black with time.
From the aspect of Fig.6 it can be concluded that the silica gel had got a well-defined boundary before the microbes settled there in a coating of about 40Ám thickness, which makes a black line in cross-section. Higher magnification reveals that the black line is made up of tiny black dots. (Same sample as with
Rhynie Chert News 64.) Apparently these microbes, too, have not yet been described.

Another distinctly different kind of microbe is seen in
Rhynie Chert News 23 as a unique spherical colony being attacked by a unique creature, the only specimen ever seen of a Devonian rotifer. By an unbelievably lucky coincidence, the polished face only touches the rotifer and cuts right through the middle of the sphere of about 500 cells, probably cyanobacteria. It is recommended here to look for more of these elusive spheres and rotifers on Rhynie chert samples stored in collections so that eventually they become established components of this Lower Devonian habitat.
The larger microbial formations as seen in Figs.1-6 are not rare in the Rhynie chert.

H.-J. Weiss       2018

[1] M. Krings, H. Kerp, H. Hass, T.N. Taylor, N. Dotzler:  A filamentous cyanobacterium showing structured colonial growth from the Early Devonian Rhynie chert.
      Rev. Palaeobot. Palyn. 146(2007), 265-276.

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