I recently came across an article I found interesting about a widespread geological mystery I was not previously aware of: the presence of filamentous microfossils found worldwide in sediments from the Permian-Triassic boundary transition. It’s been previously debated that these fossils could be the descendant of either filamentous Ascomycete fungi or freshwater Zygnematateous algae based on their morphology and chemistry. Either choice represents drastically different scenarios for environmental change that occurred 250 million years on a global scale. Could the predominance of this organism be the cause of massive plant destruction or the effect of plant destruction from flooding, which is also characteristically found at the end of the Permian period?
In a paper entitled “Fungal virulence at the time of the end-Permian biosphere crisis?”, published in the journal Geology, a group of researchers push the argument toward identifying these fossils – the morphospecies named Reduviasporonites stoschianus – as ancient relatives of the asexually reproducing fungus Rhizoctonia.
Levels of 13C in the fossils do not exclude them from being either fungi or algae, and nitrogen isotope composition would point to a fungal lifestyle. Cellulosic walls of known filamentous green algae are usually not geologically preserved as well as those identified as Reduviasporonites. Since there has been no conclusive chemical studies of these microfossils the authors hare rely on microscopic morphological comparisons.
Reduviasporonites stoschianus is found in more than 90% of many geological formations at the time of the Permian–Triassic boundary. These organisms formed a characteristic “barrel” shaped filaments anywhere between 10 and 90 μm, which look like monilioid hyphae that are typified by Rhizoctonia. This article states that Rhizoctonia “are mostly Basidiomycota, but some represent Ascomycota” which is incorrect. Rhizoctonia are placed in the family Ceratobasidiaceae, which is in turn placed in the order Cantharellales of the Basidiomycetes.
It’s certainly difficult to tell the extent of pathogenicity on a host from fossilized material as there few real observations of the invasion of plant tissues. Furthermore, by observing fossils you are not certain if the presence of these putative fungi is the cause of plant death or a symptom of decline. While it’s difficult to determine virulence based on fossil evidence, this paper introduces some interesting speculative evidence.