Croftalania upgrowth shaped by grazing creatures ?
deutsche Version

As shown before, the blue-green alga Croftalania venusta [1] can appear in surprisingly fancyful shapes as if mimicking higher plants. Its cells adhere in a chain-like way to form filaments which may arrange themselves into tufts resembling bushes or trees. The tufts of filaments grown in water, as seen in Fig.1, look quite natural and do not give rise to wonder but they do when shaped like those in Figs.2,3 (same scale). Really, pointed shapes with smooth boundary are seen more often than the ones without such attributes requiring an explanation.

Fig.1: Blue-green alga Croftalania,
         filaments grown in water arranged in tufts resembling bushes or trees. Image width 2mm.

Figs.2,3 (below Fig.1): 
Croftalania tufts with distinct contour. Image width 2mm.

CroftalaniaA moult part of the small crustacean Castracollis seen close to trimmed Croftalania has led to the proposal that the creature could have nibbled away the loose ends of the filaments, thus making a smooth contour (
Rhynie Chert News 56). This seems to be confirmed by another sample of Rhynie chert, presented here, with a similar constellation of such parts (Fig.4). Here, a large moult part from a rather big specimen of Castracollis, with the typical segmentation, 14 segments visible, is found among large amounts of shaped Croftalania , part of which is seen in Fig.4 (in the upper left corner) and in Fig.3, where Croftalania invokes the illusion of a mountain range. The smooth contours in Figs.2-4 suggest the presence of a binding organic gel between the filaments, keeping the whole in shape.
The gray spots partially obstructing the sight of the filaments in Figs.2,3 seem to be due to secondary phenomena related to decay
A few specimens of another small crustacean, Ebullitiocaris, which is smaller than Castracollis, have been found in the same chert sample (Fig.5).



Fig.4 (left): Crustacean Castracollis moult part with typical segmentation; Croftalania (dark) with pointed contour nearby.
Image width 6mm.

Fig.5 (left): Crustacean Ebullitiocaris (far left), segmentation hidden inside, not seen;
(dark) nearby.
Same scale as Fig.4.

The fossil evidence leads to the conclusion that the smooth contour with cusps had most probably been brought about by crustaceans grazing on Croftalania filaments embedded in organic gel.
Samples:  Fig.1: Same sample as in Rhynie Chert News 65, own find 2001;   Figs.2-5: Collection Steffen Koehler, Meissen.

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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