COMPARATIVE GEOCHEMISTRY SUGGESTS PROTOTAXITES WAS A GIGANTIC FUNGUS

The enigmatic Devonian fossil Prototaxites produced large, tree-like trunks up to six meters tall that were composed exclusively of three distinct types of interwoven tubes 5 to 50 microns in diameter. Since its original description as a conifer, Prototaxites has been interpreted as an alga, a lichen, and as an extinct lineage not closely related to any extant group. Most recently it has been interpreted as a fungal fruiting body due to its hyphae-like tissue composition -- despite the absence of spores or unambiguous anatomical details that would ally it to any specific fungal group. Carbon isotopic ratios have been measured for Prototaxites and co-occurring fossils from one Upper Devonian and four Lower Devonian localities. Unlike contemporaneous vascular plants, Prototaxites samples have an isotopic range, within and between localities, of up to 13 per mil. If original, such a large range would be difficult to reconcile with autotrophy and would seem to require heterotrophic metabolism on isotopically distinct substrates, consistent with a fungal interpretation. The observed isotopic range comprises discrete light and heavy populations. The light values are close to those of vascular plants from the same localities, while the heavy values may be attributable to algal source carbon, potentially an important part of early terrestrial ecosystems despite poor representation in the macrofossil record. Ongoing organic analyses should provide additional information concerning diagenesis, as well as independent tests of biological interpretations.