Funny fossil microbes
deutsche Version

Fossil microbes are mainly known from stromatolites, where they had grown as microbial slime and become mineralized layer by layer. Microbial layers are also found in various cherts formed by silicification of watery habitats. The filamentous cyanobacterium Croftalania, for example, forms tufts and wondrous shapes on submerged terrestrial vegetation preserved in the Lower Devonian Rhynie chert. Other microbes which do not seem to be filamentous, possibly also cyanobacteria, can form mm-size emergences occasionally growing out from abundantly present smooth layers.

microbial emergencesmicrobial emergencesFigs.1,2: Microbial layers with upward- growing emergences in Rhynie chert. Note the levels formed in water between silica gel.
Width of either picture 10mm.

Microbial layers as those shown here are seen in a minority of Rhynie chert samples. Often they are nearly horizontal and only slightly curved but they may be folded or broken and tilted after deformation of unknown cause in a partially silicified state.
Distinct emergences as those seen here are rare.

on the sequence of silicification can be drawn from the pictures : The microbes apparently triggered the early deposition of silica gel which is now seen as bluish chalcedony covering layers and emergences. Silica clusters formed in the water in separate compartments and settled into emulsions with horizontal surfaces now seen as level faces separating chalcedony layers of differential aspect. Judging from the stacks of levels, the deposition proceeded in several stages.
Gel formation seems to have been more homogeneous in Fig.3, where a tangle of hyphae of some aquatic fungus is vaguely seen below the stack of smooth layers on top. The presence of the fungus is most probably in no way related to the microbial colonies.
microbial emergences

Fig.3 (right): "Crazy performance": emergences between smooth microbial layers as seen on the raw outside of an old Rhynie chert fragment.
Width of the picture 17mm.

microbial emergencesFig.4 (left): Microbial formations, same sample as Figs.1-3, later formed chalcedony stained yellow. Width of the picture 6mm.

Fungus hyphae in the water became coated with clear silica gel which gave rise to the formation of separate whitish spherulites (poorly seen here in the yellow area) before the water in between turned into homogeneous gel and chalcedony. The cause of local yellow stains is unknown.

As already mentioned, the sequence of silicification steps can be deduced from the above pictures taken from one sample. Another sample provides additional evidence (Fig.5): A smooth layer has broken into pieces upon emergences from a layer below. This proves that both layer and emergences were solid when they got into contact but everything else nearby was liquid. (The white dots inside the plant section and elsewhere are tiny spherulites grown in silica gel.)
broken microbial layer
Fig.5: Peculiar assemblage of microbial columns and layers in Rhynie chert, inclined plant section above. Width of the picture 8mm.

cartoonFig.6: Did the artist anticipate the existence of microbial formations like those above ?
           Copy from:

H.-J. Weiss      2014


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