Palaeonitella whorls -- young and old
deutsche Version

On the occasion of the recent centenary of the Devonian charophyte alga Palaeonitella [1] it may be appropriate to cast a brief look on its whorls of branches once more, although a lot of images has been offered before. Compared to the recent Nitella with spacings between the whorls of branches up to several centimeters, Palaeonitella is tiny, with spacings of 0.5...3mm.

alga top
hart face
Fig.1: Palaeonitella top, branches of upper whorls incurved, protecting the growing tip, filled with bluish chalcedony.

Fig.2: Palaeonitella whorl, hollow or filled with mineral grains.

Image sizes 1.6mm,
same chert sample.

The different aspect in these pictures can be ascribed to age in this case. Fig.1 seems to show "young" whorls with incurved branches protecting the growing alga top, filled with freshly formed silica gel turned into bluish chalcedony now. The whorl in Fig.2 looks old with the yellowish stain whose deposition on the walls and inside the hollow tubes must have taken some time. Also this picture serves as evidence of the alga cell wall acting as a diffusion barrier separating spaces of different silicification regimes within and without. This is compatible with the observation that silica gel may turn into chalcedony as in Fig.1 or dissolve, leaving behind mineral grains as in Fig.2.
Sample: Rh5/3, Part 2 , found in 2001 near Milton of Noth by S. Weiss

H.-J. Weiss     2022

[1] R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure ... Part V
      Trans. Roy. Soc. Edinburgh 52 (1921), 855-902.
Site map
Rhynie Chert News
Chert formation
Rhynie chert