No place among the living: phylogenetic considerations place the Palaeozoic fossilT. protuberansin Fungi but not in Dikarya. A comment on M. Smith (2016)

In this commentary, we discuss evidence for the phylogenetic affiliations of Tortotubus protuberans, the subject of Martin Smith's 2016 paper in the Botanical Journal of the Linnean Society entitled, ‘Cord-forming Palaeozoic fungi in terrestrial assemblages’. We agree that the fossilized, branching, somatic filaments probably represent fungal hyphae. We were not convinced by Smith's proposal that T. protuberans represents Dikarya, the clade of fungi that includes most modern moulds, yeasts and mushrooms. To justify classification, Smith relied on structures that are analogous between T. protuberans and modern fungi, and argued ‘that Dikarya can produce the range of morphologies expressed by T. protuberans’. We review available information about homologies of the characteristics of T. protuberans, including mycelial cords, retrograde growth, septal pores and ornamented hyphae. Retrograde growth in T. protuberans is intriguing from an evolutionary developmental point of view, but it differs sufficiently in fine detail when compared with growth patterns in croziers or clamp connections of Dikarya, so that homologies are unclear. Tortotubus protuberans is an important fossil form, but we suggest taking a step back and relating it to the distribution of character evolution through the fungal phylogeny rather than to derived characters of modern taxa.