Nearly two years ago, I presented a post on Prototaxites, a mysterious fossil of the late Silurian, the earliest truly large terrestrial organism known from the fossil record. In that post (which I'd recommend reading before this one) I discussed the possibility that Prototaxites might have represented a giant fungus but a recent publication by Graham et al. (2010) presents a new alternative interpretation of Prototaxites. If they are correct, the Silurian may never be the same again.
In Graham et al.'s estimation, Prototaxites should not be classed with the fungi but with the liverworts. Liverworts are small, often mosslike plants of moist habitats. Members of one group of liverworts, the thallose liverworts, lack any distinction between leaves and stem but grow as a flattened thallus anchored to the ground by rhizoids (rootlets) on the lower surface. Liverworts are one of the earliest diverging groups of land plants and they or their ancestors would have certainly been part of the Silurian flora. One group of Silurian plant fossils, the nematophytes, possess a microstructure of criscrossing tubular filaments; Graham et al. (2004) demonstrated that this structure was also found in the decaying remains of modern thallose liverworts, as the upper tissue of the thallus rotted away to leave the more resistant rhizoids and connective tissue. The microstructure of Prototaxites is also similar to that of nematophytes, to the extent that some palaeontologists have regarded nematophytes as Prototaxites leaves (this interpretation is not currently supported as nematophytes have never been found actually attached to Prototaxites). But modern liverworts lack strong supporting tissue and would be pushing to reach an inch in height - how could they have produced the eight-metre columns recorded for Prototaxites?
A transverse section of Prototaxites shows a ring structure like that found in a tree trunk. Hueber (2001), who interpreted Prototaxites as a perennial fungal fruiting body, felt that this ring structure also resembled tree rings in indicating discontinuous growth by the organism. Graham et al. (2010) interpret the ring structure differently. They suggest that large mats of thallose liverworts covered the Silurian landscape. These mats could become detached from their substrate by agents such as wind and rain, and start to roll up as they decayed. As they rolled, they would form the large columns that, after being compressed by burial and fossilised, would eventually be identified as Prototaxites.
Under this interpretation of Prototaxites, the fungal hyphal structures identified by Hueber (2001) within Prototaxites sections would be those of fungi growing among the liverwort mats. Boyce et al. (2007) identified significant variations in carbon isotope ratios between Prototaxites individuals as supportive of fungal identification because they suggested heterotrophy (nutrients being obtained from the surrounding environment rather than being produced by the organism itself); however, Graham et al. (2010) establish that thallose liverworts may grow heterotrophically when conditions encourage it. The liverwort interpretation is also more consistent with the size of most Prototaxites filaments (much larger than found in modern fungi) and also explains the occasional discovery of other land plants embedded in Prototaxites columns - these would have been growing among the mats and become swept up when the mats became rolled, like Silurian Cleopatras.
I find this new interpretation intriguing, if a little difficult to accept outright. Prototaxites is represented by a reasonable number of specimens (I don't know the actual number, but thirteen species have been named from numerous localities around the world) - were the conditions that would have lead to mat-rolling common enough to have produced that number of fossils? I wonder if it would be worth investigating how Prototaxites specimens compare in abundance to nematophyte specimens and what that might tell us about the likelihood of 'Prototaxites' formation from liverwort mats. Certainly, the only thing that could be more intriguing than the existence of these giant pillars from so early in the earth's history would be if it turned out that they never existed at all.
Boyce, C. K., C. L. Hotton, M. L. Fogel, G. D. Cody, R. M. Hazen, A. H. Knoll & F. M. Hueber. 2007. Devonian landscape heterogeneity recorded by a giant fungus. Geology 35: 399–402.
Graham, L. E., M. E. Cook, D. T. Hanson, K. B. Pigg & J. M. Graham. 2010. Structural, physiological, and stable carbon isotopic evidence that the enigmatic Paleozoic fossil Prototaxites formed from rolled liverwort mats. American Journal of Botany 97 (2): 268-275.
Graham, L. E., L. W. Wilcox, M. E. Cook & P. G. Gensel. 2004. Resistant tissues of modern marchantioid liverworts resemble enigmatic Early Paleozoic microfossils. Proceedings of the National Academy of Sciences of the USA 101 (30): 11025-11029.
Hueber, F. M. 2001. Rotted wood–alga–fungus: the history and life of Prototaxites Dawson 1859. Review of Palaeobotany and Palynology 116 (1-2): 123-158.