Further observations on stalked microfossils from the Lower Devonian Rhynie cherts that resemble the algae Characiopsis (Eustigmatophyceae) and Characium (Chlorophyceae)

The Lower Devonian Rhynie cherts yield abundant fossils of many different microorganisms. One of them is an oblong cell (up to 15 μm long) on a slender stipe; a basal attachment disc or cushion is visible in some specimens. This fossil was originally described (but not named) based on specimens associated with fungal spores in an aquatic environment, and its striking resemblance to present-day epibiotic freshwater algae of the genera Characiopsis (Eustigmatophyceae) and Characium (Chlorophyceae) was noted. However, none of the original specimens had been preserved attached to a substrate. Here, I revisit the fossil using new, better-preserved specimens from the same setting, most of which are attached to land plant cuticles. Although the affinities of the fossil remain unresolved, the new material reinforces its status as a discrete aquatic life form, most likely belonging to the algae. A new fossil-genus, Characrhynium gen. nov., and species, C. amoenum sp. nov., are therefore formally proposed for it. Characrhynium amoenum is evidence that submerged surfaces in the Rhynie ecosystem could be overgrown with epibiotic autotrophs.