Cyanobacteria, Cyanophytes
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cyanobacteria on plant debris in Rhynie chert
Cyanobacteria (cyanophyta, cyanophytes), commonly known as blue-green algae and as an occasional nuisance in lakes and pools, "are arguably the most successful group of microorganisms on earth" [1], which even have brought forth true multicellular species [2] in the sense that their cell plasma is interconnected. They do not store their genetic material in a nucleus: a primitive trait which they have in common with bacteria and the less well known but likewise numerous species of archaea. The lack of a nucleus controlling the processes of life in the cells is considered so essential that the blue-green algae are placed far apart from the green algae, other algae, fungi, land plants, and animals on the Tree of Life which represents the descent of the organisms.
cyanobacteria on plant debris in Rhynie chert
So it is all the more fascinating to see that the lowly beings which seem to have come up to us right from the dawn of life on Earth can stick to each other to form filaments and can form tufts of filaments into wondrous shapes as if mimicking higher life forms. Although this is not typical it is worth mentioning since it may draw attention to the fact that fossil evidence of these organisms is not restricted to bulky stromatolites, huge deposits if banded iron ore, or Ám-size unicells but can occasionally show up as cm-size objects as seen here. Perhaps the leaf-like formations serve the same purpose as real leaves do: easier access to sunlight and solutes, compared to configurations with all cells in a globular lump.

Seen here is the fossil filamentous cyanobacterium
Croftalania venusta, grown in tufts on submerged and largely decayed land plants and trimmed into pointed shapes by means unknown.
Heights of the pictures: 7mm (left), 11mm (right). Rhynie chert, Lower Devonian.
For more microbes including Croftalania see
Rhynie Chert News 23  56  64 67 68 72 80 112 113 121 125 144 146 .

Well-preserved cyanobacteria, also inconspicuous ones, seem to be rare fossils [3]. So it is interesting that the conspicuous stacks of thin sheets stained red or yellow, found in the red cherts of the Permian Doehlen basin, are most probably the result of sheet-like spreading 
cyanobacteria precipitating dissolved iron by oxygen production.
stack of stained sheetsThe thin laminae in the stacks had been loose at first, as indicated by the disintegrating torn stack in this picture on the left:
Permian Chert News 8. With progressing silicification, the stacks became more gel-like and stiff so that deformation caused them to break like a solid, as seen in Permian Chert News 18, 27, 28 .
Stacks of thin microbial sheets are seldom seen in the Rhynie chert: Rhynie Chert News 160.

[1]   I. Stewart, I.R. Falconer: Cyanobacteria and cyanobacterial toxins, in: Oceans and human health,
         Eds: P.J. Walsh, S.L. Smith, L.E. Fleming, Academic Press 2008, 271-296.
[2]  Bettina E. Schirrmeister, A. Antonelli, H.C. Bagheri: The origin of multicellularity in cyanobacteria.
        BMC Evolutionary Biology 2011, 11:45
[3]  T.N. Taylor, E.L. Taylor, M. Krings: Paleobotany, Elsevier 2009, 115-117.

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