Devonian gloss
deutsche Version

Vegetable cell surfaces keeping glossy for several hundred million years are worth being noticed. In the following, two quite different occasions of large Devonian cells having kept their original gloss are presented. They may have lain protected but unnoticed in chert until a crack revealed their presence, as in the below examples of fungus spheres, or the protecting chert is so transparent that the glossy face is seen without being exposed, as in the below example of a charophyte alga.
Chlamydospores or resting spores are produced by fungus hyphae locally expanding into persistent spheres. They are common fossils in the Lower Devonian Rhynie chert [1] but seldom seen as numerous and beautiful as on the rather smooth 3mm-wide patch of crack face in Fig.1 which is part of the sample surface now. Apparently the thoroughly silicified globules did not act as inhomogeneities so that the crack went right through, randomly cutting them or passing them by so that some are still seen as whole spheres in the depth below the crack face. The conspicuously variable aspect of the globules in this picture has been discussed before in Rhynie Chert News 104,
Uncommon assemblage of fungus resting spores in a decayed early land plant
Fig.1: Uncommon assemblage of persistent fungus resting spores in a decayed early land plant in Rhynie chert, hyhae decayed and vanished. Picture taken on the natural surface of the chert sample. Image width 3mm.

Accumulations of resting spores as in Fig.1 are exceptional. Usually they are seen scattered on the surface or on cut faces of the chert samples. A few of them make beautiful pictures:
Rhynie Chert News 163 .
A rare sight are spheres protruding from a fracture face, as seen in the pictures below. Apparently the crack heading for an inclusion may get deflected around it. As a lucky coincidence, the two options of crack propagation near a globular inclusion, straight through or around, are seen realized by the same crack in the same material: Fig.2. As one may guess from Fig.1, the dark or clear surface aspect in Fig.2 is due to a secondary phenomenon, like the presence or absence of microbial intruders, and therefore irrelevant here. This may also apply to the different aspect of Figs.3,4.
The comparison with Fig.1 may be justified although Figs.2-5 show other samples with larger globules (except Fig.5). Note that the scale of Figs.2-5 is twice that of Fig.1. All these globules partially laid bare by propagating cracks in the disintegrating chert layer were found on the surface of chert samples. No attempt is made here to relate these resting spores to certain fungus species in the Rhynie chert.


Fungus globules cut or laid bare by a crack Fungus resting spore protruding from the sample surfaceFungus resting spore protruding from  the sample surface Fungus resting spore protruding from the sample surface

Fig.2: Two fungus globules in the Rhynie chert laid bare by a crack propagating right through one of them and along the surface of the other one. Image width 1.2mm.
Fig.3: Rather perfect sphere protruding from the sample surface. Diameter 0.39mm.
Fig.4: Dark sphere with small deviations from perfect shape revealed by slightly fuzzy reflection. Diameter 0.44mm.
Fig.5: Sphere comparable to the clear ones in Fig.1. Diameter 0.175mm.
dull Devonian alga glossy Devonian alga

The glossy reflections in Figs.2-5 indicate that the globular faces, after having been laid bare by propagating cracks, were not affected by subsequent mineral or microbial deposition. Doubtless unaffected is the alga enclosed in silica gel, now clear chalcedony, in Fig.6 since it had never been laid open. The incident light entering into the clear chalcedony gets partially reflected at the alga tube (which consists of one cell). This reflected part, after traversing the clear chalcedony towards the observer, is seen as gloss. That part of the incident light which enters into the alga tube traverses the tube content and gets partially reflected at the rear wall of the tube. It reaches the observer after having twice traversed the outer chalcedony, twice the tube wall with interface, twice the tube content, and having been reflected at the back of the tube. The light remaining after these 7 successive losses of intensity is still sufficient for a faint gloss at the inner tube boundary seen beside the main gloss in Fig.6. See also Rhynie Chert News 74 .

Fig.6: Devonian charophyte alga stem filled and embedded with clear chalcedony. Image width 0.6mm.

Fig.7: Devonian charophyte alga stem in a cavity, coated with quartz. Image width 0.3mm. Same scale for Figs.2-7.

Mirror-like reflections indicate that the reflecting face is smooth on a sub-Ám scale. Watery solutions with high supersaturation of silica make silica gel which protects the smoothness: Fig.6. Low supersaturation gives rise to slow formation of tiny quartz crystals and their deposition on the once glossy surface, as seen with another alga stem in a once water-filled but now empty cavity in the same chert sample: Fig.7.

The different aspects of the same alga species surrounded by chalcedony (Fig.6) or by air (Fig.7) are mainly due to light scattering. The chalcedony inside and outside the tube in Fig.6 is essentially transparent, hence it is inconspicuous except for the gloss. The alga coated with tiny quartz crystals in Fig.7 scatters and traps the incident light so that it appears brighter than the one with perfectly preserved surface in Fig.6.
Samples (weights refer to the whole sample):  
Fig.1: Rh12/160.9 (0.54kg) found in 2007;   Fig.2: Rh2/226.2 (32g) found by S.W. in 2014;   Fig.3: Rh2/166.1 (0.36kg) obtained from Shanks in 2011;
Fig.4: Rh12/166.1 (45g) found in 2006;       Fig.5: Rh2/354 (62g) found by S.W. in 2014;    Fig.6,7: Rh5/3.2B,3.2A (1.5kg) found by S.W. in 2001;


H.-J. Weiss    2021

[1] T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi, Elsevier 2015.

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