Window into the distant past - Palaeonitella
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Palaeonitella in Rhynie chertThis image may incite the imaginative onlooker to see a fairy-tale setting with caves and, if looked at attentively, a window into the past, seen right of the middle in Fig.1 and enlarged in Fig.2.
Palaeonitella, glossy
Fig.1: Rhynie chert with cavities and the charophyte green alga Palaeonitella preserved in chalzedony.
Width of the image 7mm.

Fig.2: Palaeonitella parts consisting of and embedded in clear chalzedony, providing the illusion of looking through a window pane with glossy tubes behind. Detail of Fig.1, width of the bigger tube 120Ám.

Behind the apparent glass front one can clearly see transparent tubes which seem to have been formed with surprising perfection. They have retained their transparency and smoothness, and the space between the tubes has been kept clean for 400 million years, which had been facilitated by silicification of the watery habitat of this alga. This must have gone on so quickly that there was not much time for decay processes except for the decay of the cell content.
An unexpected feature in the above images indicates that the cylindrical surfaces of the plant parts are even smoother and cleaner than immediately seen in Fig.2. The specular reflection from the surface of the cylinder distinctly seen through the chalcedony, and the dimmer reflection from the inner face of the hollow cylinder seen through the chalcedony and the wall, justify the conclusion that any roughness must be much smaller than the wavelength of the light, hence it seems to be well below 0.1Ám, as an estimate. Note that a statement of such precision referring to a fossil is usually not obtained with a light microscope. Also unusual is the life-like preservation. Live Nitella in water would not provide a better picture than Palaeonitella in this clear chalzedony does. Less smooth but nevertheless wondrous is the distinctly seen cylindrical part in Fig.3.
Palaeonitella free-standing

In order to appreciate the peculiarity of this fossil species, one should take notice of the following facts: Palaeonitella strongly resembles the extant charophyte Nitella, the latter being much bigger. It is rather exceptional that two similar plants are found with hundreds of million years in between, and that the extant species is much bigger than the related fossil one. They have in common that every stem and branch segment consists of only one big cell, with cell sizes up to 4mm in this chert sample. (This is uncommon for a fossil plant, and all the more so for an extant plant, with the length of the tube-like cells of Nitella eventually exceeding 10cm.)
Other than Fig.2, Fig.3 offers the rare opportunity to see a greater part of an individual plant cell standing on its own for 400 million years, having kept its position and shape without support by a surrounding medium. The cylindrical surface does not show specular reflections here, hence it must have become rough, with bumps larger than the wavelength of light, most probably by a thin silica coating deposited while the cavity was filled with water. On the left in Fig.3 there is another tube-like cell, enclosed in chalzedony. It appears thinner because what is seen is only the inner hollow filled with opaque matter.

Fig.3 (right): Strange case of an individual cell having retained its shape and position for 400 million years as a free-standing column: Palaeonitella axis segment
in a cavity, thickness 125Ám, another one in transparent chalzedony on the left. Height of the image 2.6mm.  
Palaeonitella whorls
Fig.4 (left): Opaque cast of hollow transparent Palaeonitella in chalzedony, seen here with three whorls, branches directed towards the observer cut off.
Height of the image 2.6mm.

The hollow of the tube-like cells may be filled with clear chalzedony as in Fig.2 or with opaque matter but often it has remained empty, except for a lining of quartz crystals. (See Rhynie Chert News 10 .) The empty cells are easily spotted if cut. If not cut, they may be recognized by the glitter of the quartz crystals shining through the surrounding chalzedony, as seen with the top branches in Fig.4.

The above pictures indicate that different modes of preservation were involved in the making of this chert sample. It is not known how and why silicification processes produced starkly differing results at positions only millimeters apart.

There would be a simple explanation for the separate column in Fig.3 if there were not the enclosed column only one millimeter away. Obviously, silica gel formed and solidified into perfectly clear chalzedony in some places but dissolved or never had formed in others. Sometimes the cell wall acted as a diffusion barrier keeping the silica out while the surrounding watery silica solution turned into gel and finally into chalzedony, and the quartz lining inside formed much later, but on other occasions it let the silica through and into the lower pH due to decaying cell plasma where it formed gel and chalzedony while perhaps the surroundings remained liquid and later became empty as they are now.
Other problems connected with this fossil, as the purpose of basket-like structures made of a whorl of oddly wound cells as described in Rhynie Chert News 10 , are beyond the scope of this contribution. The characteristic gyrogonites mentioned in [1] have not been seen here.

The photographs have been taken from one Rhynie chert sample of unusual pale aspect, 0.6kg, found by Sieglinde Weiss near Milton of Noth in 2001.

H.-J. Weiss  2015   

[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.

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