Questionable Devonian charophyte Palaeonitella
deutsche Version

The apparent scarcity of Palaeonitella gyrogonites has been puzzling, all the more so since the fruits with the characteristic spiral aspect are well known
as index fossils from almost every one of the numerous fossil and extant species of stonewort-like charophyte algae.
It took the authors [1] some effort to find
a small number of gyrogonites, previously not known in the Rhynie chert, however poorly preserved and detached. They made use of them for their emended diagnosis [1] of Palaeonitella cranii [2], including a reconstruction.
The statement that "examples of the reproductive organs of
Palaeonitella cranii are very rare" ([1] p.446) may explain the fact that no gyrogonites have been found in own Rhynie chert samples but there is a more interesting explanation:
The discovery of a 
peculiar charophyte with a quite different type of oogonia (see Rhynie Chert News 73, 89, 90), not seen before for possible reasons listed below, gave rise to suspicion: Some or most of the specimens assigned to Palaeonitella might belong to the new charophyte species lacking gyrogonites. Once raised, the suspicion may even encroach upon the emended diagnosis [1] based on poorly preserved  gyrogonites.
Putting the question aside how distinctly the spiral pattern should be seen in order to serve as evidence of a gyrogonite, let us turn to the much simpler case of capsules without any spiral pattern at all, as those in the three contributions listed above, and in Fig.1 below.
There, the oogonia are completely smooth, ellipsoidal to bean-shaped, and borne on long slender stalks. They may be so clearly visible that one wonders why they have not been seen all the time.
Here are some possible answers:
 - Well-preserved charophyte (stonewort) fossils with smooth oogonia had not been available before.
 - Possibly they had been seen but not recognized as such and hence not published.
 - If less well preserved, the capsules are easily mistaken for branch cross-sections or fungus vesicles of similar sizes.
 - If very poorly preserved, the capsules are not noticed at all unless specially looked for.
charophyte oogonium
charophyte oogoniacharophyte
Fig.1: Oogonium, apparently bean-shaped, with evidently smooth surface, attachment site below, within a poorly visible basket
            made of a whorl of upward-bent charophyte branches. Image width 1.2mm.
Fig.2: Partially decayed charophyte "flower", top view with cross-sections of branches and oogonia. Image width 1.2mm.
Fig.3: Largely decayed charophyte "flower", top view with thin stalks on a central column. Image width 1mm.


Figs.1-3 are to show that it would be difficult to recognize the smooth oogonia in the chert if there were not the much better preserved specimens in the above-mentioned contributions for comparison. One would not expect the black object in Fig.1 to be one of more than a dozen similar ones normally present at the same whorl but not seen here for reasons unknown. (Unrelated but interesting: The straight tubes below left belong to an enigmatic nematophyte.)
The cross-sections of oogonia and branches can often but not always be distinguished by their sizes, as in Fig.2. Such distinction is less clear in Fig.3. Clearly seen in Fig.3 are more than a dozen stalks emerging from the central column, left there after the oogonia had been shed. (See oogonia on stalks in
Rhynie Chert News 73, Figs.2,4,5.)

The problem with stoneworts in the Rhynie chert has turned out to be complex and hence interesting:
According to [1], the hitherto described specimens [1,2], although never seen with adhering gyrogonites, are thought to belong to Palaeonitella cranii in the sense of the emended diagnosis [1] involving gyrogonites at whorls (as with extant Tolypella). Own finds of stoneworts distinguished by "alga flowers" on branch ends with a multitude of smooth oogonia had been interpreted as a peculiar alga differing largely from any known stonewort, thus deserving a special place on the phylogenetic tree. This species must be a relic from times before the number of oogonia in the "alga flower" had reduced to one and the surrounding branches of the whorl had closely wrapped around the only oogonium in a spiral way to form the gyrogonite in the course of evolution. Therefore, Palaeonitella cranii [1] and all other algae with gyrogonites are advanced with respect to the Peculiar Alga.
It is thinkable that some or most alleged Palaeonitella finds represent this Peculiar Alga at a juvenile stage, without an "alga flower" at the top. This applies also to stonewort-like fossils in own samples which had been described under this name for their similarity with Palaeonitella in [2]: Rhynie Chert News 10, 74.

H.-J. Weiss   2019


[1] R. Kelman, M. Feist, N.H. Trewin, H. Hass: Charophyte algae from the Rhynie chert.
      Trans. Roy. Soc. Edinburgh 94 Part 4 (2004 for 2003), 445-455.
[2]
R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure ... Part V...
      Trans. Roy. Soc. Edinburgh 52 (1921), 855-902.

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