Fungi from the phylum Glomeromycota form arbuscular mycorrhizas with plants and represent an ancient lineage that is morphologically slow-changing. Well-preserved fossils of these organisms have contributed greatly to our understanding of their evolutionary history. Scotland’s Rhynie chert, which originated from an early Devonian (circa 410 million years ago) palaeoecosystem, has been an important source of these fossils.
One of the earliest-diverging lineages of glomeromycotan fungi is the Archaeosporaceae, a family that cannot be determined to species without molecular evidence, but which is identified morphologically by small, colourless, spherical propagules that form within the stalk of a colourless, thin-walled, balloon-like structure. These are referred to as spore-saccule complexes.
In a new article published in Annals of Botany, Carla J. Harper and colleagues went looking for Archaeosporaceae-like propagules in samples from the Rhynie chert to determine if the lineage occurred there. The researchers studied thin sections of chert from the Rhynie beds using light microscopy. Finding fossils resembling those of the genus Archaeospora there, they compared them to the modern-day examples of Archaeosporaceae and related families.
The spore-saccule complexes discovered differ morphologically from any previously described in the Rhynie chert. The general type of spore-saccule development seen here occurs in three extant glomeromycotan families, the Acaulosporaceae, the Ambisporaceae, and the Archaeosporaceae, but the dimensions of the propagules exclude the former two families. The spore wall architecture, which is diagnostic based on the number and nature of its layers, corresponds well to modern-day Archaeosporaceae.
The new find cannot be assigned to a modern-day genus without molecular evidence, so the authors have described a new genus and species for it, Archaeosporites rhyniensis (Archaeosporaceae). The diversity of morphology seen in the samples suggests an earlier divergence for this type of fungus than had been realized. “This fossil is so morphologically similar to some extant Glomeromycota that, were it not for the need for molecular evidence in modern day taxonomy, it may well be placed in a modern genus, and thus suggests that the genus Archaeospora perpetuated for considerably more than 400 million years,” write the authors.
“There is considerable interest in the mycorrhizal component of modern ecosystems and the roles these symbioses play in ecosystem functioning, while we are only beginning to assess such interactions in the fossil record.”