Croftalania images
deutsche Version

Picturesque fossil cyanobacteria are seldom seen. The Rhynie chert, which is famous for its often well preserved Lower Devonian organisms, offers such, too. The filamentous cyanobacterium Croftalania [1], when grown in dense bunches on substrates, can make fanciful appearances [2
,3].
Croftalania on decayed plant section
Fig.1: Croftalania tufts on agate in Rhynie chert.
Width of the picture 8.6mm.


Figs.2 and 3 (smaller one below): 
Croftalania tufts on plant cuticle.
Width
of the pictures 4.3mm, 0.9mm.
Croftalania grown on plant cuticle


Croftalania tufts on cuticle10

Although Fig.2 is less conspicuous than Fig.1, it is more instructive. In Fig.1 it is not quite obvious on which substrate
Croftalania had grown because the agate had formed later in the water-filled cavity. Inspection of Fig.2 suggests that in Fig.1, too,  it had grown on the cuticle of a decayed land plant, probably Aglaophyton, of which only a roughly elliptical shape is left. The instructive feature in this picture is the thin line detaching from the periphery in the upper half on the right and dangling loosely inside. It is the cuticle which had served as the substrate for Croftalania but is pried loose from it now.  (One should ignore the stained cracks which do not provide any information here.)
This configuration permits some conclusions concerning the mechanical properties of the components. The coating consisting of 
Croftalania has preserved the contour of the vanished plant tissue while the cuticle was peeled off, hence it must have been stiff. Such stiffness is compatible with the assumption that the cyanobacteria filaments were embedded in organic gel and that all tufts of the coating were fused into a continuous layer of gel. Patches of the layer broke off, one is now seen still adhering to the end of the detached cuticle, enlarged in Fig.3, where the (displaced) cuticle is clearly seen as a thin dark line. If no gel were involved and the filaments were simply dangling in the water, the tufts sticking to the end of the detached cuticle would probably have become deformed by the motion. The absence of any visible deformation is another argument for the presence of gel between the filaments.Croftalania on chlamydospores
Often the tufts show a textured appearance, as expected, owing to the directed growth of the filaments. The growth is usually more vigorous upwards to the sunlight, as seen in Fig.1 and in several pictures of [2]. In the present sample the texture due to growth is often superimposed by phenomena which might be degrading effects in the organic gel between and around the filaments. The radial growth direction with respect to the cylindrical cavities in Fig.4, still faintly seen here, has become overlaid with equally faint lines of other orientation and with small whitish spots.

Fig.4 (right): Croftalania grown on unidentified plant parts. The radial arrangement of
the filaments is apparently spoilt by secondary phenomena in the gel. Width of the picture 2mm.
Croftalania tufts degradedFig.5 (left): Croftalania tufts influenced by shaping after growth and by the formation of a network of narrow cracks of uncertain cause.
Width of the picture 1.4mm.


In the absence of apparent degradation, as in the sample described in [2], s
ilicification turned both the organic gel and the surrounding water into clear chalcedony. The phenomenon of clear substances turning white is always due to the formation of grains or holes with sizes comparable to the wavelength of light. Hence, the patches of whitish chalcedony in the present sample could have been brought about in different ways: The organic gel itself could have become whitish by degradation, or substances released in degradation could have triggered crystal growth during silicification such that the chalcedony became whitish. By the way, whitish chalcedony improves the visibility of the cyanobacteria filaments lying near the surface of the chert.
The club-shaped tufts in Figs.1,5 indicate the action of processes other than mere growth of filaments. With the presently available fossil material and facilities, only a minor part of the arising questions can be answered. The clear-cut outlines of the tufts which do not correspond to the bunch of aligned filaments, as seen on parts of Fig.5 and much more distinctly in [2],
had most probably been shaped by grazing aquatic creatures, an idea supported by the presence of the crustacean Castracollis on the "blue-green meadow".

All pictures have been taken from the cut faces of one chert sample of 0.19 kg obtained in 2014.


H.-J. Weiss       2015

[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.

[2]  H.-J. Weiss:  Croftalania venusta and other Lower Devonian microbes. Rhynie Chert News 56
[3]  H.-J. Weiss:  Rhynie chert - Implications of new finds.  European Palaeobotany and Palynology Conference 2014, Padua.
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