A Devonian alga with peculiar features (Part 2)
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3 alga tops
This bluish lump in yellowish chalcedony (Fig.1) is conspicuous for a detail usually not seen in the Rhynie chert: It is the ellipsoidal capsules, circular in top view, and surrounded by alga cells. The alga represents the same unknown type which has been described recently for the first time [1]. This alga has been found unexpectedly in another sample from which this picture has been taken. Its distinctive feature, the capsules borne on long stalks emerging from a central columella within a kind of basket made of lateral branches, better seen with proper magnification as in Fig.2, superficially resembles a flower with stamens. Three "flowers" are seen in Fig.1: below, near the right boundary, and above.

Although not immediately apparent because of the scarcity of the fossil material, branching of the main axis can be inferred from the observation that the "flowers" have been found arranged  in small umbels or parts thereof, as in Fig.1. This is compatible with the branching growth mode of charophytes.
All axes and branches consist of only one or a few big cells in a row, as it is known from Palaeonitella and extant Nitella. Quite unlike charophytes as they have been known hitherto is the presence of capsules on stalks. They differ much from the organs of known charophytes. Probably they are oogonia.

Fig.1: Alga of probably charophyte affiliation, apparently branched into an umbel composed of "flowers", part of which is seen here on a cut face of Rhynie chert.
Width of the picture 4mm.

Fig.2: Section of the "flower" uppermost in Fig.1, revealing essential details of the structure: lateral branches arranged like a lopsided basket with central columella with stalks bearing capsules.
Width of the picture 1mm.

alga floweralga flower drawing
Fig.3: Contours of "flower" details in
Fig.2, including those in the depth and therefore out of focus.

The structure pictured in Figs.2,3 is the most instructive one seen on cut faces of the two only chert samples with this alga type found up to now. It can be helpful for the identification of similar but less well preserved fossil remains, apparently not noticed in the chert hitherto, as components of alga "flowers".
Despite of the clearly seen details there is something confusing in Figs.2,3: The lower part of the image seems to show a near-axial cut of the 
baskets but the branch sections above indicate some gross asymmetry. Apparently the "flower" can be axially symmetric as shown in [1], Fig.1, or more or less asymmetric, perhaps corresponding to a more central or a more marginal position in the umbel.
The shapes of the conspicuous capsules are close to rotational ellipsoids. They can be transparent or (partially) filled with dark or whitish matter. The dark fill may look smooth or irregularly granular. It may be shrunken and thus give the illusion of a thick capsule wall. No wall is seen in the transparent empty capsules. The different fills or stains are often seen together in the same basket, and their origin is not known.
At least three capsules are cut off in the foreground of Fig.2, and more are expected to be in the depth and out of focus. Hence, the total number may well have exceeded 15.
The columella of this specimen is 60Ám wide above and broadens near its base. Possibly a few more small cells not resolved here contribute to the structure.
The stalks are tapering gradually from the base outward, then suddenly to half its width near the capsule. This narrow part of the stalk is as short as about 10Ám and therefore hardly visible. Its small cross-section means easy detachment. Columellas have been found where nearly all capsules had been detached [1].
This and more fossil evidence of the peculiar details of this alga to be reported in another contribution might enable to place it on the phylogenetic tree.
Independent of any interpretation of the capsules, the following idea suggests itself: The compact archegonia and antheridia of the charophytes could have evolved from less compact structures derived from branching axes, not vice versa. This would imply that the present fossil is a primitive predecessor of charophytes ancient and recent.
Sample: found by 
Sieglinde Weiss in 2011. Fossil seen in 2015.

H.-J. Weiss       2015,   slightly modified 2016

[1]    Rhynie Chert News 73


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