Fungus from Rhynie chert helps to refute Permian / Triassic mites hypothesis
deutsche Version
As a common sight on cross-sections of Aglaophyton (former Rhynia major), the most common plant in the Rhynie chert, there are cells with dark content, loosely arranged as a concentric ring. The phenomenon has been described in detail and explained as being due to the fungus Glomites rhyniensis [1]. According to [2], this fungus species seems to be restricted to Aglaophyton. Related species have been found with other Rhynie chert plants, as reported in [3] for Asteroxylon, the biggest and most advanced plant in the chert.
Asteroxylon is easily recognized, even if largely decayed, by its conspicuous central strand (Fig.1).  
angular clots in Asteroxylon resembling alleged mite coprolites
Fig.1: Detail of partially decayed Asteroxylon cross section with ample evidence of fungus activity.
Shape and size of the angular clots below right suggest their origin from individual cells, or clusters of a few cells, filled with fungus matter.

The soft tissue of Asteroxylon is nearly always severely damaged and squeezed. Patches of tissue with preserved cell structure are rare exceptions, and so are the distinctly seen cells with dark content. Most probably the dark matter consists of a dense clot of fungus hyphae like that of Glomites in Aglaophyton cells, known as arbuscular mycorrhiza [1]. (A remarkable image of clot formation by arbuscular mycorrhiza in Aglaophyton, with a hypha seen penetrating the cell wall, is shown in [9], Fig. 19, and [10], Fig. 3.96.)

Similar clots in decaying tissue of Permian and Triassic plants have repeatedly been interpreted as feacal pellets or coprolites of oribatid mites in [4,5,6,7,8] and numerous other publications. (See Misconceptions.) A few related images have been reproduced here: Figs.2-5.

Fig.2,3,4 (below): Angular clots in the tissue of the Permian climbing fern Ankyropteris brongniartii, interpreted as mite coprolites in [4,5,6].
Fig.2: Sample Nr. K4568, Naturkunde-Museum Chemnitz.

Figs.3,4: Sample Nr. K 4569, small clots near tissue with small cells, larger clots near tissue with larger cells.
clotssmall angular clots in Ankyropterisbig and small angular clots in Ankyropteris
angular clots in Triassic fern

Fig.5 (far right): Angular clots of various sizes and shapes in tissue with cells of various sizes and shapes, interpreted as Triassic mite coprolites in [8].

H.-J. Weiss   2009

[1]  T.N. Taylor et al.: Fossil arbuscular mycorrhizae from the Early Devonian,
      Mycologia 87(1995), 560-73.
[2]  T.N. Taylor et al.: Fungi from the Rhynie chert,
      Trans. Roy. Soc. Edinburgh, Earth Sciences 94(2004 for 2003), 457-73.
[3]  R. Kidston, W.H. Lang: On Old Red Sandstone plants showing structure, Part III,
      Trans. Roy. Soc. Edinburgh 52(1920), 643-680.
[4]  R. Rössler: The late palaeozoic tree fern Psaronius - an ecosystem unto itself,
      Rev. Palaeobot. Palyn. 108(2000), 55-74.
[5]  R. Rössler: Der versteinerte Wald von Chemnitz, 2001, p 141,155,169.
[6]  R. Rössler: Between Precious Inheritance and Immediate Experience,
      in: U. Dernbach, W.D. Tidwell: Secrets of Petrified Plants, D’ORO, 2002, p 105.
[7]  R. Rössler: Two remarkable Permian petrified forests,
      Geol. Soc. London Special Publ. 265(2006), 39-63.
[8]  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.
[9]  H. Kerp: De Onder-Devonische Rhynie Chert ... , Grondboor & Hamer 58(2004), 33-50.
[10] T.N. Taylor, E.L. Taylor, M. Krings : Paleobotany, Elsevier 2009.
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