For over a 150 years now, weird fossils have been turning up in 400 Million-year-old rock. They're up to 9 metes long, and resemble petrified trunks of yew trees, Taxus, hence the name Prototaxites. They're not even that rare, particularly in eastern Holland.
But they certainly weren't conifers. For one thing, they dwarfed anything else on the land at the time.
Prototaxites has turned out to be a fungus. A really big, really really big, fungus.
C. Kevin Boyce and his collaborators relied on isotopic analysis of carbon-12 and carbon-13. In photosynthetic organisms, the composition correlates with the isotopic composition of CO2 in the atmosphere, and don't vary by more than 2 to 4 parts per thousand.
Prototaxites broke the mold. (ho-ho, a pun) "We find a difference of 12 parts per thousand," says Boyce, which means Prototaxites was not photosynthetic, and therefore was not an autotroph - a primary producer. Instead, it was a heterotroph - a consumer of biological material made by other organisms. "If you are a heterotroph, you can be all over the place".
Modern fungi consume many types of dead stuff, and Prototaxites may have eaten whatever it could get its hyphae into. Although terrestrial plants were evolving and spreading fast when Prototaxites was dominating the landscape, some of the Prototaxites fossils have isotopes inconsistent with consuming vascular plants. The most likely alternative is cryptobiotic soil, a crust containing bacteria (including cyanobacteria), lichens, mosses, green algae and fungi - an ecosystem that is now found mostly in deserts.
"Even though most of these organisms are photosynthetic, different species, even in a particular location, have different ratios of carbon isotopes" Boyce says, "which explains the varying C-12/C-13 ratios in Prototaxites." Because cryptobiotic crusts do not fossilize, isotopic analysis of Prototaxites becomes a unique lens into the Devonian landscape. "These fungi show us that well into the period of diversification of vascular plants, there are still large areas of microbial activity."
