Common fossils with uncommon aspect in the Rhynie chert
deutsche Version

The famous Rhynie chert fossils representing a Lower Devonian ecosystem have become widely known by publications in monographs and textbooks. Less well known but all the more interesting are the occasionally encountered deviations from the commonly published forms. As a remarkable fact, the following examples of less common structures have been found together on one Rhynie chert slab.
agate in hollow Aglaophyton
From a botanical point of view, the introductory picture of Fig.1 is the most unproblematic one. It is hollow Aglaophyton with everything decayed except for the cuticle on the former epidermis, which had defined the cavity for the agate to be deposited in the same way as agates are usually deposited in silica-rich water.

Fig.1: Agate formed in a cavity left after the decay of Aglaophyton lying prostrate in the swamp water. Image width 3mm.

Fig.2 (right): Aglaophyton with preserved tissue but central strand missing, with uncommon radial fissures and shrivelled surface. Image width 4mm. Same scale for all figures.

Aglaophyton with radial fissures
Radial fissures like these, emerging from a destroyed central strand, have not been seen on thousands of Aglaophyton cross-sections on own samples. They clearly differ from the conspicuous fissures apparently related to fungus infection [1].

Fig.3 (below left): Aglaophyton cross-section with rare type of damage above and a horizontal layer below, resulting from a dark precipitate deposited at an early stage of silicification. Image width 4mm.

Fig.4 (below right):  Stack of horizontal layers deposited successively with different aspect for reasons unknown.
Image width 3mm.

Aglaophyton damagedhorizontal deposits
The horizontal deposits seem to be formed by precipitation of tiny particles making a fluid suspension before everything turns into chalcedony. The thick black coating around the hole in Fig.3 remains unexplained here.
Irregularities in the yellow layer in Fig.4 indicate that there had been hyphae coated with bluish silica gel when the yellow suspension settled.

Fig.5 (below): Aglaophyton, inclined cross-section with remains of tissue (bluish), former inner cavity with fungus hyphae, and microbial formations outside.
Image width 4.2mm.
Aglaophyton with hyphae

The hyphae in Fig.5 had grown in the water-filled cavity and got surrounded by a thick coating of pale silica gel. Remarkable are the wavy hyphae among the perfectly straight ones.
The wavy hyphae might be the mycoparasite Trichoderma.
The irregular or pointed shapes grown on the surface resemble those known from the blue-green alga Croftalania. The combination of wavy hyphae and Croftalania has been observed here for the first time.

microbial sheets around Aglaophyton10Fig.6 (left): Aglaophyton with thin microbial sheets wrapped around, inclined cross-section. Image width 4mm.

Stacks of thin microbial sheets [2] are not rare in the Permian cherts from the Döhlen basin (Saxony) but seem to be rare in the Rhynie chert [3]. Apparently, microbial sheets wrapped around plants like those in Fig.6 have not been seen before. Their branching and merging indicate a way of formation different from that of the Permian sheets in [2].

Asteroxylon tipFig.7 (right): Peculiar structure suggesting an unfolding bud.
Image width 5mm.

The fossil in Fig.7 would readily be recognized as
Asteroxylon if there were not the intriguing fact that none of the several cut faces of this big chert sample shows the  characteristic features, the complex-shaped cross-sections of the central strands, which usually reveal the presence of Asteroxylon even if poorly preserved. The large number of Asteroxylon sections shown in [4], none of them resembling this one, give rise to the suspision that the object in Fig.7 might be something special. It does not appear here as the top of a larger plant but seems to be a peculiar blob of tissue of its own, with a kind of broad bud just unfolding. All of the grown-up plants in this sample, like the ones in the above figures, are lying prostrate along the chert layer, apparently upset by a sudden flow, with only the little lump oriented such that it suggests upward growth. While the tiny leaf-like forms on top strongly resemble the big enations of grown-up Asteroxylon, the distinctly different lateral protrusions on the left are problematic. In view of the fact that the gametophyte of Asteroxylon has not yet been found, enigmatic blobs of tissue like this one deserve attention.  
Sample: Rh2/162.2, obtained from B. Shanks in 2007  

H.-J. Weiss       2020

[1]  H.-J. Weiss: Enigmatic voids in the tissue...  Rhynie Chert News 117.
[2]  H.-J. Weiss: Eerie shapes...  Permian Chert News 28.
[3]  H.-J. Weiss: Aspects of Devonian microbes. Rhynie Chert News 121.
[4]  H. Kerp et al.: Reproductive organs and in situ spores of Asteroxylon ...Int. J. Plant Sci. 174
(2013) Nr.3, 293-308.
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