Dubious oribatid mite coprolites once more:  Comment on Z. Feng et al. (2010)

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Lately, not much was heard about oribatid mite coprolites, a favourite topic in palaeobotany a few years ago. (See chapter Misconceptions.) So the recent paper by Feng on this subject [1] came unexpected. Unfortunately, it does not fulfill the expectations raised by its title.
A brief look at the photographs of dark clots in silicified wood reveals that most of them are no coprolites. Although the subject had been discussed in detail in connection with previous papers on alleged mite coprolites so that one might expect the matter settled, it has to be considered here anew, with the same old and simple but adequate reasoning applied to the new pictures provided in [1].     
There are several conspicuous details in the pictures which should raise suspicion even with unexperienced observers. One such detail is the presence of clots with distinctly angular shape (Figs.1...4). Furthermore, the shapes and sizes of the clots often agree with the shapes and sizes of nearby cells, as observed before on fossil wood from Antarctica.

Angular clots in Permian wood, misinterpretation as coprolitesAngular clots in Permian wood, misinterpretation as coprolitesAngular clots in Permian wood, misinterpretation as coprolitesAngular clots in Permian wood, misinterpretation as coprolites
Figs. 1...4: Clots in silicified Permian wood, interpreted as mite coprolites in [1] despite of their angular shapes; details from [1], there in Figs. 3I, 4E, 3C, 3I (left to right).


Among the numerous clots in the samples there are often a few ones in positions which seem awkward from a coprolite point of view: They are seen deep inside tracheids where no mite could have crept in. The deceptively plausible explanation offered by the proponents of the coprolite hypothesis that they simply fell into the tracheid at an open end is not a good one: In some cases the clots are seen in several neighbouring tracheid cross-sections (Figs. 5...7), so while falling in they all would have got stuck at the same depth. By the way, things as small as mite droppings would not easily tumble down tubes of about the same diameter since their weight is usually negligible compared to adhesion forces, especially if traces of moisture are present. They would just stick to the wall but not slide down.
The arrangement of the clots in the below pictures suggests quite another explanation: They have been formed inside the cells. The two tiny clots in neighbouring cells in Fig.6 are possibly early stages of clot formation. As a significant feature also observed with samples from elsewhere, the clots are not randomly distributed among the cells but tend to cluster. In other words, a cell with clot is more likely to have got a neighbouring cell with clot than to have not. This supports the previously proposed idea of a fungus infection spreading from one cell to another as the cause of clot formation [2,3]. A phenomenon of this kind, a clot sending a hypha through the cell wall to make another clot in the neighbouring cell, is seen in a remarkable photograph in [4].
Some of the clots eventually keep their position even after the cell walls have decayed, as three clots in a row in Fig.5, and less clearly seen in Fig.6. The most convincing evidence for clot formation in cells is provided by Fig.7, with three and four clots in a row, sitting at the same height in neighbouring tracheids. Note also the two smaller clots in smaller cells below, one above the scale bar and another one on the right. Some of the clots look as if their shapes were tapering towards a site of attachment to the cell wall.

Clots within cells in Permian wood, misinterpreted as coprolitesClots within cells in Permian wood, misinterpreted as coprolitesClots within cells in Permian wood, misinterpreted as coprolitesFigs. 5...7: Clots in silicified Permian wood, interpreted as mite coprolites in [1] despite of their peculiar arrangement suggesting their formation inside cells; details from [1], there in Figs. 4B, 4B, 4F (left to right).


A formation in silicified wood with not really clot-like aspect (Figs.8...10) is also interpreted as oribatid mite coprolites in [1]. There are patches on the petrified wood cross-section where the usual wood structure with cells in radial files (vertical in Figs.8 and 10) is replaced by something resembling an imperfect honeycomb structure.

Light-coloured fills in Permian wood cells, misinterpreted as coprolitesLight-coloured fills in Permian wood cells, misinterpreted as coprolitesLight-coloured fills in Permian wood cells, misinterpreted as coprolitesFigs. 8...10: Silicified Permian wood affected by what appears to be some kind of wood rot, interpreted as mite coprolites in [1] despite of the polygonal shape of the elements and their dense-packed honeycomb-like arrangement; details from [1], there in Fig. 6E.


As seen in Figs. 8 and 10, the light-coloured area adjoins the unaffected wood whose cells appear dark as they were hollow before silicification and are now filled with clear chalcedony so that one can look into the dark depth. The two light-coloured elements below the empty cells in Fig.10 fit into the cell pattern seen above. So they must be cells filled with some substance.
In order to interpret the alleged coprolites in Figs. 8 and 9 as modified cells, it must be explained why they are not arranged in files and seem to be slightly larger than the cells. As a possible explanation, the cells may have been affected by wood rot caused by some fungus or microbe which feeds on the cell wall and produces organic matter inside the cells, be it a dense tangle of tiny hyphae (as in Rhynie chert plants [4],
see also Fossil Wood News 4Rhynie Chert News 28 ) or microbial debris. Judging from Fig.8 where several of the affected cells seem to be not completely filled, the matter forms along the walls first. The fill of the affected cells apparently expands, and since the cell walls are weakened by the fungi or microbes feeding on them, they do not keep the original shape. So by mutual squeezing of the expanding cells, or rather their fills, an imperfect honeycomb structure is brought about.
Such type of decay of the wood structure is not uncommon in Palaeozoic wood. Bloating of affected cells and dissolution of the radial files has been described with wood from the Kyffhäuser mountains, Germany, and elsewhere [5]. Light-coloured cell-size clots beside dark ones are seen in Lower Permian wood from Schallodenbach, Germany.
Most probably, the cell-size clots of various aspect will reveal something interesting in the future but the favourite interpretation as oribatid mite coprolites will become a memorable folly of the past.

H.-J. Weiss     2010


[1] 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.
[2] H.-J. Weiss , 6. Chert Workshop 2007, Naturkunde-Museum Chemnitz.
[3] H.-J. Weiss , 7. Chert Workshop 2008, Naturkunde-Museum Chemnitz.
[4] T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009, Fig. 3.96
[5] H.-J. Weiss : Beobachtungen an Kieselhölzern des Kyffhäuser-Gebirges.
      Veröff. Mus. Naturkunde Chemnitz 21(1998), 37-48.

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