Conspicuous spheres in the Rhynie chert
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Among the often confusing multitude of decaying plant parts, spheres, especially the larger ones, make conspicuous sights. They had been hollow and filled with silica-rich water before subsequent formation of silica gel and chalcedony.

fungus resting spore half exposedfungus resting spore, chalcedonyfungus resting spore, chalcedony fungus resting spore, chalcedony fungus resting spore, chalcedony
Figs.1-5: Conspicuous spheres in Rhynie chert, possibly fungus resting spores although no connection to hyphae is seen here.
            Frame sizes 0.6mm, sphere sizes 0.39, 0.33, 0.47, 0.44, 0.39mm.

Fig.1 shows a solid sphere half enclosed in chert. The formations in Figs.3,4 indicate that there had been an intermediate stage with homogeneous silica gel in the spheres which enabled spherulite growth. The walls in Figs.3,4,6 are seen in cross-section as very thin dark circles. The walls in Figs.5,7,8 might have been thick originally or become so later by silica deposition. The big sphere in Fig.6 below seems to have been a water tank before silicification, with floating microbial floccules stained with iron oxides:
Rhynie Chert News 85.

The occasionally seen fungus hypha attached to a sphere as in Fig.9 seems to justify the assumption that other spheres around, even in the absence of hyphae, may also be fungus resting spores. Most often this may be true but there are important exceptions. As a highly wondrous fact, the cylindrical cells of the charophyte green alga Palaeonitella are occasionally seen transformed into perfect spheres under the influence of the parasitic fungi Milleromyces rhyniensis and Krispiromyces discoides [1]. Hence, in the absence of other clues, one often cannot tell fungus resting spores apart from transformed alga cells. The bomb-like object in Fig.8 is certainly one of the latter. Spheres arranged in pairs, often interconnected, are typical for bloated alga cells, as in Fig.11. Nearby solitary spheres (Fig.10) are most probably alga cells, too. As it is well known from other cavities in chert, agate-like fills can make fancy pictures. Their complex structures can reveal details of the sequence of silicification steps under changing conditions. One can say that several successive deposition and dissolution processes had been at work in this sphere once filled with silica-rich water, coupled to its surroundings by diffusion.
fungus resting spore (?), chalcedonyfungus resting spore (?), chalcedonysphere: deformed alga cell, chalcedonyfungus resting spores, chalcedony
Figs. 6-9: Spheres of different origin in the Rhynie chert.
Frame sizes 0.8, 0.6mm. Sphere sizes 0.75, 0.6, 0.53, 0.17mm.

sphere: deformed alga cell, fancy fill, chalcedony
spheres: deformed alga cells, chalcedonyFigs.10,11: Spheres paradoxically formed from cylindrical cells of Palaeonitella under the influence of parasitic fungi.
Frame heights 0.9mm. Left sphere size 0.84mm.

All pictures: same scale, own finds.

For the sake of simplicity and beauty, this contribution has been restricted to larger spheres although smaller ones, too, may be highly interesting, like the rare tiny capsules with an opening surrounded by a collar,
recently discovered and ascribed to testate amoebae [2]. Similar bomb-like capsules can be expected from broken dumbbell-like pairs of spheres, as the one in Fig.11.  However, the size difference is large, and the similarity is incidental.

H.-J. Weiss       2020

[1]  T.N. Taylor, M. Krings, E.L. Taylor:
Fossil Fungi. Elsevier 2015, p.64.
[2]  Ch. Strullu-Derrien, P. Kenrick, T. Goral, A.H. Knoll: Testate Amoebae in the 407-Million-Year-Old Rhynie Chert. Current Biology 29 (Feb.2019), 461–467.

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