The Early Devonian fungus Mycokidstonia sphaerialoides from the Rhynie chert is a member of the Ambisporaceae (Glomeromycota, Archaeosporales), not an ascomycete

Glomeromycotan propagules (spores) are morphologically diverse in the Early Devonian Rhynie chert; however, only relatively few of these fossils have been documented and critically evaluated. This study re-examines propagules previously described informally as ‘reproductive unit 1’, and identified as glomeromycotan acaulospores borne within the neck of a sporiferous saccule. Size and morphology, spore wall structure, and the close association of specimens in land plant axes with small glomoid spores, are solid arguments for affinities of ‘reproductive unit 1’ to the Ambisporaceae (Archaeosporales). Moreover, the acaulospores correspond morphologically to Mycokidstonia sphaerialoides, a Rhynie chert fossil originally interpreted as an ascomycete perithecium that, consequently, is redefined here as a member of the Glomeromycota conspecific with ‘reproductive unit 1’. The diagnosis for M. sphaerialoides is emended to be compliant with glomeromycotan taxonomy; features of the saccule, acaulospore wall, and associated glomoid spores are included. An epitype is also designated. Mycokidstonia sphaerialoides adds to an increasing body of fossil data depicting Devonian Glomeromycota as a diverse fungal lineage which is likely to have been an ecologically important constituent of early terrestrial ecosystems. •Many fossil Glomeromycota from the Rhynie chert can be placed in modern taxa.•Rhynie chert fossils described previously as ‘reproductive unit 1’ are re-examined.•They belong to the modern family Ambisporaceae (Archaeosporales).•They are conspecific with the Rhynie chert fossil Mycokidstonia sphaerialoides.•Symbiotic fungi in the Glomeromycota played important roles in early terrestrial ecosystems.