Cell-size clots in bennettitalean tissue: No oribatid mite coprolites
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It is surprising to see how some dubious ideas persist once they have entered into the scientific literature. One notorious example from palaeobotany concerns small dark clots in fossil plant tissue for which a plausible explanation had to be found. As the clots were usually seen in damaged parts of the tissue, the idea suggested itself that they were droppings of some small creature feeding on the tissue. This idea, believable as it seemed, was fraught with more than one problem: First, no fossil creature was seen near and far. This was taken as an excuse for introducing guesswork. Since mites are known to feed on plant matter, the mite coprolite idea was brought up. The guesswork became even more specific: The mites, although elusive, were specified as oribatid mites, and "oribatid mite coprolites" became a popular term in the palaeobotany literature since the 1990s. Those palaeobotanists who readily adopted it had apparently not noticed an observational fact: Where there are dark angular clots in damaged tissue, one can be sure that their size and shape variation is compatible with the size and shape variation of the cells of nearby intact tissue, which poses another problem.
cell-size clots in Triassic bennettitalean rootcell-size clots in Triassic bennettitalean root

Figs.1,2: Cell-size clots in damaged tissue of triassic bennettitalean roots, details of Figs.6G,D in [1], interpreted as oribatid mite coprolites there. Note the clot sizes fitting to the largely differing cell sizes. In Fig.2 there are small cells  filled with clot matter in a coherent row, and larger loose clots on the right. Width of the pictures 0.3mm (provided that the scale bars in [1] are correct).

It is hard to believe that it did not occur to those dealing with the subject that such coincidence is strong evidence against the coprolite interpretation. Perhaps it did occur later to those authors who preferred not to propagate that interpretation any more. However, none of them has expressly retracted the coprolite hypothesis. Therefore it is necessary to comment on every such publication, which has been done in the sequence as they have become known to the present writer. (See Google: oribatid mite coprolites, or "Wood rot or coprolites" on this website.)
angular clots in cretaceous bennettitalean stem
Fig.3: Cell-size clots in partially decayed tissue of a cretaceous bennettitalean stem, detail of Fig.4E in [2], interpreted as oribatid mite coprolites there.
angular clots in cretaceous stem
Fig.4 (right): Detail of Fig.3, polygonal outlines of angular clots called "spherical to ovoid" in [2].

The arguments concerning the misinterpretation of the clots are the same as given repeatedly before. In case of good preservation, for virtually every one of the loose clots an intact cell of fitting size and shape can be found in the tissue, which strongly suggests the conclusion that the clots had been some kind of cell casts which came tumbling out when the cell walls broke down. Clot formation inside cells and breakdown of the walls is most probably due to the same cause. Fungi have been observed to form dense tangles of very thin hyphae inside cells, and it is known that they are able to break down cell walls of plants [3].
The clots in Figs.3,4 are described in [2] as "spherical to ovoid" with "smooth to slightly bumpy" surface but it is easily seen that the alleged bumpy spheres are rather angular, often with polygonal outline, and even right and acute angles are seen, as expected from replicas of the cell lumen. This applies also to Fig.5.
The polyhedral clots usually misinterpreted as coprolites may be the only fossil evidence for the original cell sizes and shapes of vanished or compressed tissues. The empty cells have retained their shape in Figs.1,2 but part of them have become deformed or collapsed in Figs.3,5.
Damaged plant tissue with the same aspect as in these images is usually called a frass gallery although there are no specific features of the damage which would justify such interpretation. There are alleged galleries so narrow that no herbivore could have crept there. There are alleged coprolites in intact cells where no herbivore could have deposited them.
angular clots in triassic root

Fig.5: Angular clots in a partially damaged triassic bennettitalean root, detail of Fig.6O in [1],
interpreted as oribatid mite coprolites there.

Finally it can be concluded that the above evidence does not support the hypothesis of oribatid mite activity in cretaceous and triassic bennettitaleans. It remains to be checked whether or not other reports on herbivore coprolites in bennettitalean fossils stand a critical revision or have to be re-interpreted as wood rot as it has been done with numerous cases of alleged oribatid mite coprolite sightings in plant tissue.

H.-J. Weiss 2013

[1]  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.
[2]   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.
[3]  T.N. Taylor et al.: Paleobotany. Elsevier 2009

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