Asteroxylon tissue affected by fungi
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Asteroxylon cross-sectionOne may not expect this neat Asteroxylon specimen in Fig.1, with the central xylem strand and two circles of small strands branching thereof, to have been in some state of degradation while becoming silicified. Really, proceeding degradation is indicated by the absence of a clearly visible tissue structure. While other plants in the Rhynie chert are usually seen in various states of preservation, not seldom with very well preserved tissue, Asteroxylon is never seen well preserved, except for the roots. Its early decay is most probably due to fungus activity. A few remains of the tissue are very faintly seen here. In idealised drawings, the cross-section is occasionally shown with large fungus-induced voids  arranged in a concentric ring as if this were an intrinsic property of this plant, which it is not.

Fig.1 (right): Asteroxylon cross-section with clear outline and xylem but faint remains of tissue. Image width 11mm.

Asteroxylon tracheids with dark fills
Fig.2 (left): Cross-section of disintegrated Asteroxylon xylem, tracheids filled with dark fungus matter keeping their original shapes, others shrivelled; fungus globules on the left.
Image width 1.4mm.

A fungus effect quite different from degradation is seen in Fig.2, where a xylem strand has disintegrated into a bunch of separate tracheids for reasons unknown. Here, the tracheids are clearly subdivided into two conspicuously different types: Those with black fills have kept their polygonal outline representing their original shape but the empty ones have shrivelled. This phenomenon is seldom met with Asteroxylon.  A similar and likewise rare phenomenon is seen in Fig.3, where cells of the soft tissue are still visible because they had become filled with fungus matter. The tissue between this uncommonly preserved area and the central strand is seen to be degraded as usual, possibly due to another fungus species. Notably there are no large voids in Figs.1,3.
The dark fills are most probably made up of dense tangles of tiny fungus hyphae which are not seen here. A hypha penetrating the wall of a cell and growing a tangle inside has been found and documented elsewhere [1].  
Asteroxylon cells with dark fills

Fig.3: Asteroxylon cross-section with central strand well preserved, adjacent tissue heavily fungus-affected and degraded, outward area with some cells filled with fungus matter,
others hardly visible. Image width 4mm.

Cells filled with fungus matter, rarely seen with Asteroxylon, are a common sight with Aglaophyton, where the fungus is known to have thrived in the live plant [2] and has got the name Glomites rhyniensis [3,9]. It is not known whether this fungus had produced the cell fills in Figs.2,3. Asteroxylon had probably been affected by more than one fungus, judging from the phenomena and from the often abundant big resting spores like those in Fig.2.

It is hard to believe that cell-size dark clots in more or less decayed fossil plants have repeatedly been misinterpreted as mite coprolites, despite of glaring contrary evidence: angular shapes fitting to the cell cross-sections [4], sizes fitting to the tissues of bigger or smaller cells [5],
often arranged in rows fitting to the rows of cells [2,6], and finally the absence of any fossil mites in the samples with the alleged coprolites. Apparently the idea of mite coprolites had become self-accelerating, with feedback provided by mutual encouragement within the community of coprolite fans. Persistent warning against the obviously nonsensical coprolite idea [7] has hopefully ended that craze in 2016. The fungus clots in Asteroxylon (Fig.3) had been part of this effort since 2009 [8], and the recently found clots in Fig.2 may add to the abundantly available evidence against angular clots being coprolites.
The proponents of the once favourite mite coprolite hypothesis apparently do not mention it any more but do not retract it. Hence, it is still lingering in the palaeobotany literature, spreading confusion. Therefore it should be contradicted anew with every new evidence, which has been done here.

H.-J. Weiss       2018   
[1]  H. Kerp: De Onder-Devonische Rhynie Chert ... . Grondboor& Hamer 58(2004), 33-50.        See image here: Fossil Wood News 4, Fig,3.
[2]  Rhynie Chert News 85.
[3]  T.N. Taylor et al.: Fossil arbuscular mycorrhizae from the Early Devonian.  Mycologia 87(1995), 560-73.
[4]  Fossil Wood News 5 , Figs.1-5; 8, Figs.4,5; 18, Figs.1-5.
[5]  Fossil Wood News 18, Figs.1,2.
[6]  Fossil wood News 8, Figs. 2,3,9;
[7]  Google: coprolites Roessler
[8]  Rhynie Chert News 28,
[9] T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi, Elsevier 2015, p121.

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