Cyanobacteria, Cyanophytes
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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 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.
Generally it can be said that well-preserved cyanobacteria, also inconspicuous ones, seem to be rare fossils [3].
cyanobacteria on plant debris in Rhynie chert
Photographs: Fossil filamentous cyanobacteria grown in tufts on submerged and largely decayed land plants and trimmed into pointed shapes by means unknown. Croftalania venusta, Rhynie chert, Lower Devonian.

Heights of the pictures: 7mm (left), 11mm (right).

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[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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