For already as long as 150 years the Devonian fossil Prototaxites has been collected and studied, but still there is no certainty about its identity. The American J.W. Dawson, who was the first to describe it (1859), thought it was rotted wood, related to Taxus. That’s why he gave it that name. At the end of the nineteenth century scientists began to think it was an alga, in fact a brown alga, and this opinion has been established and is now mentioned in nearly every book.
Until an American paleobotanist, Francis Hueber, after 20 years of research published an elaborated paper (2001) in which he defended that Prototaxites was the fruiting body of an enormous fungus.
Boyce et al. (2007) measured the ratio between different carbon isotopes to determine the nature of the fossils. The argument runs that a photosynthetic primary producer will have a relatively constant ratio of carbon isotopes between individuals because they are all using atmospheric carbon. On the other hand organisms (like fungi) that are classified as consumers will take on the isotope ratios of whatever they are digesting locally, and therefore individual specimens end up with widely differing isotope ratios. Boyce et al. (2007) found too much isotopic variance between individual Prototaxites fossils for them to be photosynthetic primary producers. Instead, Prototaxites was a consumer, and taken together with direct microscopic observation of their anatomy (Hueber, 2001) this demonstrates that these enormous fossils, the largest land organisms to have lived up to their point in time, were actually giant fungi.