An uncommon fossil fungus

Fungus hyphae and resting spores are abundant in the Lower Devonian Rhynie chert. Most often one can see dark cells arranged as a ring on plant cross-sections. The dark aspect is due to tangles of hyphae inside cells, known as arbuscules, of the symbiotic glomeromycotan fungus Glomites rhyniensis [1]. Other details of fungi in chert have been described in that monograph and elsewhere but many questions had to be left unanswered. For example, the strange clusters of vesicles grown from the surface of Aglaophyton (Fig.1) are not mentioned in [1].
unknown fungus
The arrangement of the clusters on the surface of Aglaophyton resembles that of a blastocladian fungus in the Rhynie chert  [1,2] but this similarity is most probably only superficial.

Fig.1: Clusters of vesicles grown on the surface of largely decayed Aglaophyton. Width of the picture 4.3mm.

Except for the poorly preserved central strand, no trace of plant tissue is left in Fig.1. The contour of the section is marked by the cuticle. When looked at edgewise, the cuticle appears as a narrow black line, otherwise it is seen as transparent light-brown sheet, as in Fig.1 below right. As seen on the left, and enlarged in Fig.2, the vesicles had grown on a mound bulging out from the surface. Obviously the mound had been formed below the cuticle, probably induced by the fungus in the live plant.
The clusters on the right are not detached but thought to be likewise connected to the slightly inclined shoot. The sizes of the vesicles vary from 0.02mm (or smaller) to 0.1mm. Their shapes may deviate from spherical by deformation due to mutual contact.
Vesicles of more or less globular shapes are known from numerous species of fossil chytrids, a group of fungi thought to have separated from the rest as early as during the Proterozoic [1]. The vesicles are interpreted as zoosporangia, with a "discharge pore" occasionally visible as a characteristic feature. Two big vesicles in Fig.2 have got a kind of short funnel, very dimly seen, which might have served as a discharge pore. (Note the pale short extension pointing left on the big vesicle on the left.)
Hence, the assumption may be justified that the clustered vesicles shown here are zoosporangia of a fungus of the chytrid clade. This implies the following statement: The vesicles presented here are bigger than any fossil chytrid zoosporangium described in [1].
The flower-like aspect of the cluster in Fig.3 (right) is an illusion: One sees only the bigger outer vesicles while most of the cluster is unseen in the depth. 

unknown fungus vesiclesunknown fungus vesicles 

Figs.2,3: Details from Fig.1, clusters most probably of chytrid zoosporangia. Note the bulge grown below the cuticle (Fig.2) and the transparent light-brown cuticle in Fig.3. Height of the images 0.5mm.

H.-J. Weiss    2017

[1] T.N.Taylor, M. Krings, E.L. Taylor: Fossil Fungi, Elsevier 2015.
[2] W. Remy, T.N. Taylor, H. Hass: Early Devonian fungi.
      Am. J. Bot. 81(1994), 690-702.



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