Antarctic shit
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The idea of coprolites as an explanation for tiny dark clots often seen within fossil plant tissue became increasingly popular among palaeontologists in the 1990s. Quite a number of papers related the clots to oribatid mites. However, a large fraction of those papers, or even most of them, will turn out erroneous. No expert knowledge is required to find out, only common sense and a close look at the published pictures. This is demonstrated here with pictures from an often quoted paper on fossil plant tissue from Antarctica [1].
The various sizes and shapes of the dark clots within damaged tissue in Fig.1 agree remarkably well with the sizes and shapes of the nearby cells.
clots of various sizes and shapes fitting to the cells
Fig.1:  Transverse section through a Triassic fern petiole with partially damaged tissue and dubious "coprolites" whose sizes and shapes agree with those of the cells. Picture after [1].
Below: enlarged detail.

tissue with cells of various sizes and shapes
This obvious coincidence, which gives a clue to a quite different explanation, is not noticed by the authors [1] (nor by those of other papers on alleged mite coprolites). Instead, they ponder about the question why most of the cavity is empty in Fig.1 but filled in Fig. 2, which does not lead anywhere.
clots rounded and angular, no coprolites

Fig.2 (right):  Oblique section of Triassic wood with clots whose shapes vary between distinctly angular and elliptical, after [1]. Evidently, the interpretation as coprolites in [1] is not compatible with the shapes.

Apparently the tissue in Fig.2 collapsed after clot formation and before silicification so that the sizes and shapes of the clots cannot be compared with those of the cells in this sample. At first sight it is puzzling that some clots are distinctly angular but others distinctly rounded. However, this feature, too, is compatible with the assumption deduced from Fig.1 that the clots are modelled after the cell lumina, as seen on the enlarged clip from Fig.1, where there is a series of distinctly angular cells (lower left quarter of the picture) next to cells whose inner outline is clearly rounded (above).
So one arrives at the inevitable conclusion that the clots are no coprolites but objects closely related to the cells. As a probable explanation, the cells were invaded by some fungus growing into a dense tangle of tiny hyphae inside, a phenomenon known as arbuscular mycorrhiza, after which the cell walls decayed and the clots remained. See also
Rhynie Chert News 28.
In the light of these findings, some speculations advanced in the coprolite papers get a funny touch: For example, it is thought that the larger cavities in the tissue are so spacious because they were inhabited by several oribatid mites at the same time [1]. The presence of two sizes of clots in one fossil sample is not attributed to the two different tissues seen there but to two oribatid mite species [2]. Other evidence contradicting the interpretation of cell-size clots as mite coprolites has been spread among the palaeobotany community from 2007 on. Nevertheless, more papers on misinterpreted clots appeared since 2010: [3-7].
Carelessness in the scientific literature serves as an encouragement for carefully observing lay palaeontologists to provide surprising revelations.

H.-J. WEISS      2009,   updated 2011, 2013, 2015

[1]  D.W. Kellog, E. L. Taylor Evidence of oribatid mite detrivory in Antarctica during the Late Paleozoic and Mesozoic,
      J. of Paleontology 78(2004), 1146-53.
R. Rößler: The late palaeozoic tree fern Psaronius  -  an ecosystem unto itself,
      Rev. Palaeobot. Palyn. 108(2000), 55-74.
See comment.
[3] Zhuo Feng, Jun Wang, Lu-Yun Liu :
      First report of oribatid mite (arthropod) borings and coprolites in Permian woods from the Helan Mountains of northern China.
      Palaeogeography, Palaeoclimatology, Palaeoecology 288(2010), 54-61.    See comment.
[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.  See comment.
[5]   N.A. Jud, G.W. Rothwell, R.A. Stockey: Paleoecological and phylogenetic implications of Saxicaulis meckertii ... :  
    A bennettitalean stem from the Upper Cretaceous ...
    Int J. Plant Sci. 171(2010), 915-25.   See comment.
[6]  C. Strullu-Derrien, S. McLoughlin, M. Philippe, A. Mørk, D.G. Strullu:
    Arthropod interactions with bennettitalean roots in a Triassic permineralized peat from Hopen, Svalbard.
    Palaeogeography, Palaeoclimatology, Palaeoecology 348–349(2012), 45-58.   See comment.
[7]  Zhuo Feng, J.W. Schneider, C.C. Labandeira, R. Kretzschmar, R. Rößler:  A specialized feeding habit of Early Permian oribatid mites.
       Palaeogeography, Palaeoclimatology, Palaeoecology 417(2015), 121-124.   See comment.
quartz crystal with wood inside
Fossil Wood News  11

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