A Devonian alga with peculiar features (Part 4)
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aDevonian alga whorl
The hitherto overlooked Devonian alga, first described under this headline in Parts 1 to 3, offers more details worth reporting. The whorl of 10 branches, cut near its base (Fig.1), would have been taken for Palaeonitella [1] if there were no other whorls like baskets of branches with ellipsoidic capsules inside (Fig.2).

Fig.1: Charophyte alga whorl with 10 upward-bent branches (including a dark one on the left). Slightly inclined cross-section, seen from above.

Fig.2:
Charophyte alga whorl with 10 upward-bent branches forming a basket, capsules inside. Inclined cross-section, seen from below.



Size of all images 1mm2.

Two capsules in Fig.2, each
with a dimly visible stalk, are attached to a central columella, better seen in previously shown pictures, here concealed by a light-coloured spot. The branches are bent into the picture plane, hence the cut basket is seen here from below. Apparently the cut plane is sufficiently high above the branching point so that several capsules have been cut.
The absence of capsules in Fig.1 could mean that either there were none or the cut plane was so close to the branching site that none are seen on it. 
In addition to the figures in Part 3, this Fig.2, too, can possibly explain why the peculiar features of this alga have been overlooked hitherto. Most often the capsules appear with low or vanishing contrast, and with their width of usually 130Ám they are easily mistaken for branch sections.
While inspecting the samples with the "alga flowers", the attention was drawn to another conspicuous structure, shown in the figures below.
Devoonian alga antheridium (?)Devonian alga, antheridium (?)Devonian alga, antheridium (?)Devonian alga, antheridium (?)
Figs.3-6: Sections of globular organs thought to be antheridia of the Devonian alga distinguished by "alga flowers" described in previous contributions.
Devonian alga, antheridium (?)
Fig.7: Antheridium with faint radial streaks and a few alga branches close by.

The wall of the organs in Figs.3-6 seems to be made of tightly fitting cells which are much smaller than the cells forming the baskets with the capsules. The fit implies that the cell cross-sections are not circular like those of the branches. The fill of the globules appears granular in these images. The overall aspect suggests an interpretation as early antheridia. They seem to be evolutionary precursors of the charophyte antheridia with more elaborate wall structure involving a smaller number of well fitting cells. 
The wall cells may have collapsed or decayed before silicification so that a wall is not distinctly seen, as in Fig.7. 
The faint radial streaks are compatible with the filamentous structure of the charophyte antheridia.

The recent observations reported here and in the preceding three parts pose a veritable problem. With the globules of this "alga with peculiar features" being quite similar to the antheridia of the charophytes, and the alga itself looking like a charophyte, and being found in the Rhynie chert where also the charophyte
Palaeonitella is found, it might well be included into the charophytes. Now that the male organs are known, most probably the "alga flowers" with their capsules are the female organs, a conclusion which is incompatible with the charophytes as we have known them, with the characteristic gyrogonites as the female organs.
This apparent incompatibility has to be overcome by suggesting an evolutionary path from the "alga flower" to the gyrogonite. It is thinkable that a succession of evolutionary steps reduced the number of capsules to one while the basket became narrower until the branches wrapped around tightly and fused with the capsule into a gyrogonite. This would have been a rather long path from the charophyte shown here to the conventional ones with gyrogonites.
The subject is complicated by the fact that there were already several charophyte species with gyrogonites in the Lower Devonian [2]. Hence the evolutionary path to the gyrogonite must have been trodden a long time before, which lets the alga with "flowers" appear as a relic from those times which has survived in the Rhynie habitat.
As an intriguing fact, the ellipsoidal capsule wall appears so evenly smooth that one wonders where a sperm cell could enter. Did the alga shed the capsules so that they germinated and formed archegonia ?
The presently available fossil evidence on charophytes from the Rhynie chert justifies another unconventional conclusion: Since gyrogonites have never been seen attached to Palaeonitella [1], some or most charophyte fossils hitherto assigned to Palaeonitella might really belong to this alga whose supposedly ancient features had not been noticed in those finds.
Samples found by: S. Weiss in 2011 (Figs.1-4), 2013 (Figs.5,6), H.-J. W. in 2003 (Fig.7). Fertile organs first seen in 2015 and 2016.

H.-J. Weiss       2016

[1]  R. Kelman, M. Feist, N.H. Trewin, H. Hass : Charophyte algae from the Rhynie chert,
      Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 445-455.
[2]  T.N. Taylor, E.L.Taylor, M. Krings: Paleobotany, Elsevier 2009.

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