What to Do with Prototaxites?

Prototaxites (Nematophyta) is among the most enigmatic of terrestrial fossils. This Late Silurian–Late Devonian taxon formed multiple types of tubular filaments of varying size interwoven to produce massive treelike or columnar structures, up to more than 8 m tall and 1 m wide, making it the largest terrestrial organism of its time, towering over the surrounding vegetation. The affinities of this organism—even at the kingdom level—have been the subject of debate for over 150 years; however, the seminal work of Francis Hueber turned the tide toward a fungal interpretation that continues to remain the most widely accepted alternative. However, this characterization was made during a period in which fungal relationships and their evolutionary time lines were poorly understood. Advances in systematics and paleontology have both resulted in a substantial revision of our understanding of fungal evolutionary history. Here we reconsider Hueber's interpretation of Prototaxites as a fungal sporocarp in the context of more recent research postdating his publication. In addition, we consider alternative suggestions by others that Prototaxites represents a lichen thallus. We argue that the weight of the evidence argues against interpretations as an agaricomycete sporocarp or ascomycete lichen association and that while the taxonomic identity remains unclear, we suggest that if Prototaxites was indeed of fungal origin, it may represent part of an extinct lineage lacking extant descendants.