At the end of the Permian period, about 250 million years ago and not long before the dinosaurs appeared, life on Earth experienced its greatest catastrophe: a mass extinction that did away with the vast majority of life forms on land and sea. The question arises, who ate the carcasses of the deceased? Surely bacteria and protozoa digested the animal corpses, most likely reaching unusually high population sizes as the result. But who took care of the masses of dead plant material? Fungi are quite good at digesting plants, especially woody plants. So, did the fungi also become prevalent after the catastrophe?
There is evidence for this belief. As the Permian period came to an end, there was a massive spike in the population of Reduviasporonites, a "morphogenus" of microscopic organisms fossilized as single cells or multicellular chains. (The -ites ending is sometimes used to denote organisms known only as fossils. Reduvius, if you care for such things, comes from the Latin reduvia meaning hangnail or remnant. Just why it was called that, I don't know.) Apparently they thrived at that time, as judged by the abundance and the widespread geographic distribution of their fossils. More recently, they have had a checkered taxonomic career. Originally thought of as fungi, they were later relegated to the microscopic algae. It makes a difference. If wood-decaying fungi, they suggest a massive loss of standing biomass on land. If algae, picture widespread swamping. Now, they are back among the fungi. Maybe.
The evidence? A new paper in Geology is one of the latest to deal with this question. They used "high-sensitivity equipment, partly designed to detect interstellar grains in meteorites." Their pyrolysis GC-MS of fossil Reduviasporonites showed that they do not contain the algal biomarkers, such as breakdown products of chlorophyll—a strike against the algal theory. In addition, these researchers found the ratio of N15 to N14 in the material to be characteristic of that seen in fungi, and distinct from that for algae. Another point for the fungal side.
What do these "disaster species" look like? They are usually seen as chains of ovoid cells, each around 25 µm in diameter. Many fungi on today's Earth look quite similar. If Reduviasporonites were indeed fungi, much of the planet must have been covered by a mushy mess of mycelia. For the time-traveler, it would have been the worst of times.
Sephton, M., Visscher, H., Looy, C., Verchovsky, A., & Watson, J. (2009). Chemical constitution of a Permian-Triassic disaster species Geology, 37 (10), 875-878 DOI: 10.1130/G30096A.1