Rhynia aspects
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Rhynia cross-section

 Rhynia gwynne-vaughani had been known as the less abundant and smaller one of the two early Devonian land plants named Rhynia until the bigger one had got a new name, Aglaophyton. Among the mostly deformed or partially decayed sections seen in the Rhynie chert, Rhynia is more likely to be seen with a few well-preserved specimens like the one in Fig.1. It is an uncommonly large one, 2.6mm across. Most diameters are well below 2mm. The smallest ones in Fig.2 could be Rhynia gametophytes. Fig.1 may serve as an introductory image here while the following ones taken from another sample are to draw attention to various other observations.
Note also the vaguely seen crack entering at the top of Fig.1, being deflected at the cuticle on the plant surface, running there along for a quarter of the circumference, and departing from the surface on the right. The thus debonded part of the surface is marked by a row of tiny bright dots. The waxy cuticle, which covers all land plants as a protection against exsiccation, provides an easy crack path in the silicified state.

Fig.1: Rhynia gwynne-vaughani well preserved among deformed shoots,  cross-section 2.6mm, 
2 Rhynia in hollow bubble in chert
Fig.2: Surface of a Rhynie chert sample with a former bubble in the swamp, now seen as a cavity with thickly coated Rhynia.
Image width 17mm.

Most conspicuous on the natural surface of this Rhynie chert sample of 0.3kg (Fig.2) is the cavity with two thickly coated Rhynia shoots seen inside. Incidentally they are also seen as inclined sections in the compact chert above. The cavity had been a bubble in the swamp water, possibly oxygen produced by algae or swamp gas, trapped among plant shoots and microbial  layer stacks, some of the latter hidden in the bright-coloured areas above and elsewhere.
Former bubbles now seen in chert had become stabilized by silica gel formation around them, then filled with silica-rich water while the gas had escaped by diffusion before silicification.
There are more than one indications that silicification processes had been going on within the water-filled bubble. First came a thin dark lining, not well seen here, possibly 
from a thin microbial lining on the cavity wall. The small yellow deposit at the bottom is probably from tiny silica grains raining down from the water where they had formed. (The grains could have grown suspended in gel which liquefied later under changing parameters like temperature or pH.)
Then, possibly triggered by substances released from
the decaying Rhynia shoots in the bubble, thick coatings of silica gel formed around them. With time, the silica gel gradually turned into chalcedony. A crack and small displacement in the shoot on the left indicate fracture while the whole was not yet fully hardened. The remaining water escaped by diffusion so that part of the former bubble is empty space now.
What looks like bulging eyes on a downward creeping snake above left in Fig.2 is the typical warts often seen on Rhynia shoots, whose purpose is not quite obvious.

Rhynia with levelsFig.3 (right): Raw lateral face of a Rhynie chert layer fragment with former bubbles stuck among Rhynia and microbial sheets; upright Rhynia with levels inside. Image width 17mm.

Fig.4 (below): Enlarged part of Fig.3, with peculiar details clearly visible: levels inside Rhynia, agate inside bubble, microbial sheets, and others. Image width 7mm.
Rhynia with levels, agate, microbial layers
What may appear confusing in Fig.3 can be discussed more easily in Fig.4. More conspicuous but less problematic is the agate fill of a small former bubble. One may only wonder why it is the only one of its kind in this chert sample.
More problematic is the sequence of silicification stages forming the various levels inside the Rhynia shoot. Levels of this kind,
indicating the horizotal direction during silicification, are usually the result of settling emulsions or suspensions in cavities. Judging from Fig.5,  decaying plant tissue did not much interfere with the process. From the fact that the levels are confined to the interior of plant shoots (or bubbles) it can be concluded that the silicification processes inside those compartments went on independent of what had been going on outside. Microbial sheets formed in the swamp water are clearly seen in cross-section as thin dark lines in Figs.4,5. The brown spot on the left in Fig.4 must be some kind of stain which had got there later since it does not interfere with the microbial sheets. It remains unexplained here.

Rhynia and deformed microbial sheets
Fig.5: Cut face of the same Rhynie chert sample as above; two u
pright Rhynia shoots of remarkably differing diameters, 0.5mm and 2.2mm, with a stack of microbial sheets suspended between them, forming a trough later filled with muddy water apparently flooding the partially silicified swamp. Image width 7mm.

Like some other phenomena revealed in these pictures, the conspicuous trough in Fig.5 does not suggest an obvious explanation. The variable contact angle on the left precludes the simple explanation as microbial sheets grown on a meniscus of a liquid. As a
possible but not quite consistent explanation, a sinking water level or drying silica gel caused the microbial sheets grown between upright Rhynia shoots to sag, and later all became flooded with muddy water from elsewhere, judging from the stack of yellow mica platelets and other debris.
It is hard to imagine how the trough might be shaped behind the Rhynia shoots in Fig.5 and how it could be compatible with the small trough-like sheets on the left of the smaller Rhynia. Instead of grinding Fig.5 away in order to see what lies behind, samples with similar phenomena will be inspected.
Fig.1: Fragment of a chert layer of 13cm, 0.7kg, 1998 obtained from  Margaret Shanks, labelled Rh2/5, here cut face of Part1,
Figs.2-5: Chert sample of 0.3kg, 2009 obtained from  Barron jr., labelled Rh15/6, here surface and cut face of Part1.

H.-J. Weiss      2018


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