Devonian charophyte alga forming lumps

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charophyte tubescharophytes densely grown
Charophyte algae are well-known components of the watery habitat preserved in the Lower Devonian Rhynie chert. One species growing whorls of branches, thus resembling the extant charophyte Nitella, is known as Palaeonitella cranii [1], see Rhynie Chert News 10, 74. An un-named form, eventually branching at its base only, irregularly and without whorls, has been described in Rhynie Chert News 48. Unexpected recent discoveries of assumed charophyte specimes with a whorl of branches at the top and possibly more whorls below have been reported in Rhynie Chert News 73 89,  90, 93, 106, 129, 138, 139. They have been recognized as a "Peculiar Alga" by means of their ellipsoidal oogonia on stalks, an ancient trait which sets them apart from all stonewort-like charophytes as we know them.
A charophyte alga growing in dense lumps of tubes (Figs.1,2), apparently without whorls of branches, has been found in this sample, and less clearly in a few others. Fig.1 suggests a dense arrangement of tubes, not random but more or less correlated over some distance, as right of the middle, where mainly cross-sections are seen.

All images from Sample Rh2/175, obtained from Shanks in 2012.

Fig.1: Bluish chalzedony cloud in Rhynie chert, with densely grown charophyte tubes, some with white fill; no whorls of branches. Image height 5.2mm.

Fig.2 (far right): Aligned charophyte tubes below, two separate tubes of 4 cells above, see also Fig.3.
Image height 2.6mm, magnification twice that of Fig.1.
peculiar charophyte tubes
Disregarding the confusing structure in the middle of Fig.2, there are two areas on this cut face where the structure is easily recognized. The tubes in an area below are so closely packed that they mimic a coherent tissue,
with cell walls hardly visible but lengthy objects clearly seen inside, possibly the shrunken content of the cell.
In the upper third of the picture there are
two separate tubes of definite length, seen enlarged in Fig.3. The slender tube on the right is clearly seen to consist of four cells, apparently grown to the left. Less clearly seen is the tube with four bulky cells on the left.

Fig.3 (left): Detail of Fig.2 (upper part), two separate alga tubes of different aspect, consisting of 4 cells each. Image width 1mm.
        Same scale as Figs.4-9.

charophyte tubescharophyte tube topcharophyte tube topcharophyte tube topcharophyte tube topcharophyte tube top

Charophyte tube tops,
all same scale,
image height 1mm.

The diameter range of the tubes and the articulated structure support the assumption that the alga-like object in this sample is a charophyte alga. It is not known whether an alga with this particular type of growth, with densely packed tubes, apparently without whorls of branches, almost certainly without gyrogonites, and with articulated tube tops as seen here, is only a variety of the charophytes mentioned above, including the Peculiar Alga, or another charophyte species. With this uncertainty kept in mind, it may be useful to describe the facts nevertheless.
As an intriguing fact, top ends of alga tubes in this sample (Figs.3-9) have nearly always no more than 4 cells. This agrees with the number of cells
in the branches of some charophyte whorls, which may be incidental or not. Possibly one more cell is seen very faintly in Fig.6 on the left tube below.
Enigmatic are 
Figs.3,8, where the end wall of the biggest cell suggests the question where it could have broken off or how it could have formed otherwise.
As seen in Figs.3-9, the tube tops are of variable shape, with slender or bulky cells (Fig.3 or Figs.4,5). Very faint contours of bulky tube top cells are also seen in Fig.9 below.
The dark matter enclosd in the cells, diffuse as in Fig.5 or clearly shaped as in Figs.2,9, must be the variously decayed and shrunken cell content.
The search for hidden branchings and whorls in lumps of densely packed tubes as in Figs.1,2 and their surroundings may contribute to better understanding the whole.
Finally it is hoped that more details will enable this alga to be assorted to the
whorl-bearing charophyte algae mentioned above or to a new species.
Annotation 2019: The fact that the articulated tube top parts mostly consist of 4 cells, with the end wall of the biggest cell clearly
seen in Figs.3,8, suggests that these parts deliberately separated from the tubes to serve as propagules.

 H.-J. Weiss     2019   
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