Water turned into Devonian chert

Palaeonitella, hollow
Palaeonitella, upright, hollow

Contrary to the widespread notion that chert is silicified sediment (after Wikipedia),
the most interesting fossiliferous cherts, like the famous Rhynie chert, had been formed via silica gel from water-logged or watery habitats with plants and creatures. As an advantage over sediments, cherts from watery locations may preserve things in their natural positions and shapes. Palaeonitella, a delicate water plant related to the extant stoneworts (Chara and Nitella), is abundantly found in upright position in the chert sample of which a 10mm wide area is shown in Fig.1. Most fragments are vaguely seen in the depth but incidentally a hollow one with three whorls of branches is positioned so precisely in the cut plane that the central cavity is cut into half lengthwise between the whorls (Figs.1,2).
As a general phenomenon with Palaeonitella, the tubular cells in the chert can be hollow, with tiny quartz crystals along the inside. Often the glittering quartz lining is seen in the depth but the outer cell wall is not, which may be more or less misleading concerning cell diameters. (Note that, like with extant Nitella, tube diameter = cell diameter.)
Swamp gas bubbles had been later filled with water, then with bluish chalzedony and quartz crystals, or with level deposits and white quartz powder, as seen above left.

Nematoplexus spiralNematoplexus spiral

Fig.1 (above):
Silicified watery habitat with Palaeonitella (Green Alga), upright in Rhynie chert, hollow specimen incidentally cut along its axis  (in the middle of the picture), horizontal layers in a former cavity above left indicating the orientation.

Fig.2 (above right): detail
of Fig.1, picture width 2mm.

Fig.3 (far left): Silicified water with enigmatic rugged inclusions, the larger ones hollow, often with stained walls, among them one tiny Nematoplexus spiral, seen in the middle of the picture of 4mm width.

Nematoplexus spiral from Fig.3, three full turns: rare sight.
Picture width 0.2mm, spiral width 0.09mm.

All pictures have been taken from the sample Rh5/3 found by Sieglinde Weiss 
   near Milton of Noth in 2001, Slab 2A.

is to show how confusing the aspect of the cut and polished chert face may be even though the chalzedony is rather clear so that enclosed objects are seen in 3D, like the solitary Nematoplexus spiral. It can be concluded that this chert sample serves an another piece of evidence that clear water can turn into clear chalzedony. (There is lots of more evidence, for example Rhynie Chert News 23 , 71 , 74.)  Here, the rugged objects in Fig.3 cannot even tentatively be interpreted. Their chaotically irregular outer and inner surfaces seem to be fine-grained with crystalline quartz.
Nematoplexus, first described in 1961 and affiliated with the enigmatic nematophytes, then found in a few own chert samples since 2001 and compiled recently, is still one of the rarest fossils in the Rhynie chert. Although the present sample has been thoroughly inspected on 10 cut faces, mainly for charophytes, no other Nematoplexus spiral than this one has been found.

H.-J. Weiss      2018 

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