Carbon sources for the Palaeozoic giant fungusPrototaxitesinferred from modern analogues

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Abstract
A wide range of carbon isotope values in the Devonian fossilPrototaxiteshas been interpreted to support heterotrophy and the classification ofPrototaxitesas a giant fungus. This inference remains controversial because of the huge size ofPrototaxitesrelative to co-occurring terrestrial vegetation and the lack of existing fungal analogues that display equally broad isotopic ranges. Here, we show wide isotopic variability in the modern saprotrophic fungusArrhenia obscuratacollected adjacent to shallow meltwater pools of a sparsely vegetated glacial succession in the Washington Cascades, USA. Soils collected specifically around the edges of these pools were up to 5‰ higher inδ13C than adjacent soils consistent with C3origin. Microbial sources of primary production appear to cause these highδ13C values, and the environment may be analogous to that of the Early Devonian landscapes, wherePrototaxitesindividuals with extreme isotopic variance were found. Carbon isotopes are also compared inPrototaxites, Devonian terrestrial vascular plants, and Devonian algal-derived lake sediments.Prototaxitesisotopic values show little correspondence with those of contemporaneous tracheophytes, providing further evidence that non-vascular land plants or aquatic microbes were important contributors to its carbon sources. Thus, a saprotrophic fungal identity is supported forPrototaxites, which may have relied on deposits of algal-derived organic matter in floodplain environments that were less dominated by vascular plants than a straight reading of the macrofossil record might suggest.

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