Black shards in decayed tissue
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edged black clots
Clots of various types and uncertain origin may be found in fossiliferous cherts. Those which are seen in connection with degraded plant tissue can often be interpreted as fungus hyphae having entered into one cell after another [1], having formed dense tangles there, then partially or wholly consumed the cell walls. An interpretation is more difficult in the uncommon case pictured here, where the clots come with a large variety of sizes and shapes, often with edges with acute angles, and without any remains of plant tissue where they could have formed.

Fig.1: Black shards of various sizes and shapes in bluish chalzedony replacing Asteroxylon tissue in Rhynie chert; big chlamydospores of some fungus most probably not related to the shards. Width of the image 1.7mm.

Closer inspection of the sample sections reveals that the rare phenomenon covers an area 8mm across where Asteroxylon tissue had been before but is no more seen after decay, except for the durable xylem strand.
For comparison, a picture of a cross-section with some remains of tissue has been taken from another sample: Fig.2. There, every dark clot apparently had grown within a cell, or in two or three adjacent ones, and became angular by contact with the enclosure. This is the typical growth mode of angular fungus clots.
Such interpretation would not apply to the multitude of shapes and sizes in Fig.1. They could possibly be the result of some microbe entering into the cells and thriving there to produce clot matter spreading across the lumen or as layers or crusts along the cell wall. After consumption of the cell walls by rot, the odd-shaped replicas or deposit fragments were left over and became silicified and enclosed in bluish chalzedony.

Fig.2 (below): Cross-section of Asteroxylon heavily infested with fungi as usual, with cell-size angular clots apparently compatible with the vaguely seen remaining tissue. Width of the image 4mm.
Asteroxylon cross-section with clotsAsteroxylon is never seen with well-preserved soft tissue as it is occasionally seen on  Aglaophyton and Rhynia sections, for example. (See Rhynie Chert News 85.) While the xylem strand is nearly always well preserved and makes a spectacular sight, the soft tissue had largely degraded before silicification, with fungus hyphae running abundantly along the intercellular spaces, and the cellular structure mostly broken down (Fig.2). Cells filled with dark matter and the resulting angular clots are less often seen with Asteroxylon but more often with other fossil plants. They may be the only left evidence of cell sizes and shapes from tissues decayed and vanished shortly after the death of the plant. Their aspect suggests an interpretation as being due to fungus activity. Fungus chlamydospores of about 25Ám are often seen in decayed Asteroxylon in large numbers.  

The quite different aspect of the black shards in Fig.1 compared to the angular clots in Fig.2 suggests a similar origin as that of various black deposits or coatings, then left over when their substrates vanished. (See Rhynie Chert News 83, 87.)
Samples Figs.1,2: found 2001, 2007.

H.-J. Weiss       2016
[1]
 M. Krings, C.J. Harper, J.F. White, M. Barthel, J. Heinrichs, E.L. Taylor, T.N. Taylor:
       Fungi in a Psaronius root mantle from the Rotliegend (Asselian, Lower Permian/Cisuralian) of Thuringia,    
       Rev. Pal. Pal. 239 (2017), 14-30.
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