Alleged coprolites - Remnants of decayed tissue
deutsche Version
Small dark clots or grains of of various shapes and sizes are often found in fossil plants preserved in a silicified state where they are distinctly seen on cut and polished faces or thin sections. Those found in Palaeozoic plants are usually interpreted as oribatid mite coprolites, mite coprolites, arthropod coprolites, or coprolites of unknown creatures. (For publications and related comments see Misconceptions, Oribatid mite coprolites.)
As a peculiar fact, and a seemingly unbelievable one, a greater number of those interpretations can be declared erronous even without access to the samples, by careful inspection of the published pictures alone. (This is useful when inspection of the samples is refused, as in the present case.)
Doubts arise when the clots come with shapes which one would not expect from coprolites, as in Figs.1, 2.
angular clots in fossil plant, no coprolitesangular clots in fossil plant, no coprolites
Figs.1 and 2: Angular clots in Ankyropteris axis cross section, detail from [1], Plate VII, 2 and 5,
interpreted as coprolites from organisms of uncertain identity [1] and from oribatid mites [2,3], but differently interpreted here.   
Width of Fig.1:  0.5 - 0.57 - 0.63mm. (See text.) Width of   Fig.2: 0.66mm after correction of scales [1,3].

There are different types of tissue with differential cell sizes in the Ankyropteris axis. The clots in Fig.1 are compatible with the cell lumina of the tissue. Two chains of clots on the left look as if they had not been dropped there randomly. They look as if they have kept their positions where they had formed within the cells, a few remaining of which are seen below left. This suggests that the clots are casts of the cell lumina left over after the cell walls of the tissue which had been there where the clots are now had decayed. This idea is supported by several more observations of this kind. The conspicuously angular clots in Fig.2 are most probably also casts of cells although no tissue is seen near them.
Uncertainties of sizes in Figs.1,2 are due to inconsistencies of size data in [1,2,3]. If  6:1 is true for the total view of the Ankyropteris cross-section in [3], Fig.334, the ratio for Fig.336 is 24:1, not 12:1, and for Plate VII,5 in [1] it is 35:1, not 14:1. The captions, too, are erroneous for those two pictures. They do not show a detail from the main axis but from the mirror image of the frond stalk in Fig.334. 

angular clots in Ankyropteris, no coprolites
Fig.3: Angular clots of various sizes in Ankyropteris cross section, detail from [3], Fig.335, (which is the mirror image of Plate VII,4 in [1]). Note the cell above right with a clot of corresponding shape inside. Width of the picture 0.67mm.  

The conducting strand of an aereal root of Psaronius seen in Fig.4 may serve as another example of dark clots representing cell casts of disintegrated tissue, although the clots are interpreted as gnawed-away aerenchyma replaced by coprolites in [4].
First, it must be mentioned that the tissue adjacent to the "star-shaped" xylem strand is no aerenchyma but phloem. Second, the clot sizes vary strongly, same as the cell sizes of the remaining phloem. There is even a small patch of well-preserved phloem, seen above the pocket with the dark clots, with one cell filled with dark matter, surrounded by empty phloem cells. The compact dark areas in the phloem are bounded by straight cell walls nearly everywhere, hence they are no coprolites but patches of tissue filled with dark matter but not yet 
disintegrated. Note also the separate tiny clot of pentagonal outline.
The phloem between the prongs of the xylem of Psaronius roots is very seldom preserved. Hence one may regard the clot formation as a preservation process for structure data of tissues prone to rapid decay, namely for shape and size data of the cell lumina, which otherwise would get lost.

angular clots in Psaronius root, no coprolites
Fig.4: Psaronius root cross-section, conducting strand with big tracheids and alleged coprolites replacing aerenchyma according to [4], here re-interpreted as phloem cell casts. Detail from [4], Fig.8. Width of the picture about 0.75mm.xylem and phloem on Psaronius root cross-section

One of the very rare pictures of the phloem in pockets of the xylem strand of a Psaronius root is the drawing in Fig.5 below. An exceptionally well preserved Psaronius root conducting strand with clearly seen phloem is presented in [7], Fig.12, and [8].

Fig.5 (right): Segment of Psaronius root cross-section, conducting strand with big tracheids and thin-walled phloem cells. Detail from [5], Plate 40, Fig.13.

Now that ample evidence has been presented supporting the assertion that probably most of the dark clots in palaeozoic plant fossils, including coniferous-type wood, and the angular clots in [1-4] in particular, are no coprolites but casts of the cell cavities formed in the coherent tissue and released as the tissue decayed, what remains to be done is to explain the nature of the dark matter. A connection to fungus activity has been proposed in previous treatises, see Misconceptions, Oribatid mite coprolites. The subsequent disintegration of the tissue after invasion of the cells and formation of the dark residue could be due to the ability of fungi to break down the cell walls. (Fungi are the only organisms that can completely break down lignin [6]).

Arguments against the interpretation of angular clots as coprolites in [1,2,3] had been presented to R. Rößler 
since 2007.
The misinterpretations in [4] have first been noticed by Gert Müller in 2010. The only response he got was the advice by one of the experts [4]: "If two professors say the clots are coprolites, you can believe it."
A critical comment on [4] (in German) suitable for publication in the journal "Semana" has been rejected on grounds of pretended copyright violation.

H.-J. Weiss     2011

[1]  R. Rößler: The late palaeozoic tree fern Psaronius  -  an ecosystem unto itself.
      Rev. Palaeobot. Palyn. 108(2000), 55-74.
[2]  R. Rößler: Between precious inheritance and immediate experience.
       in: U. Dernbach, W.D. Tidwell: Secrets of Petrified Plants, D'ORO Publ., 2002, 104-119.
[3]  R. Rößler: Der versteinerte Wald von Chemnitz, 2001, p. 141,169.
[4]  M. Barthel, M. Krings, R. Rößler: Die schwarzen Psaronien von Manebach, ihre Epiphyten,
       Parasiten und Pilze.  Semana* 25(2010), 41-60.   *( recently re-named, former name: Veröff. Naturhist. Mus. Schleusingen)
[5]  C.G. Stenzel**: Über die Staarsteine.               **( not to be confused with Sterzel)
       Breslau, Bonn 1854, p.753-893, Plate 40.
[6]  T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009.
[7]  H. Steur, H. de Kruyk: Psaronius, een boomvaren uit het Laat-Carboon en het Vroeg-Perm.
      Grondboor & Hamer nr. 3/4 2004, 75-82.
[8]  H. Steur:  The tree fern Psaronius, "Star show".
quartz crystal with wood inside
Fossil Wood News  6

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