A microfossil resembling Merismopedia (Cyanobacteria) from the 410-million-yr-old Rhynie and Windyfield cherts - Rhyniococcus uniformis revisited

Rhyniococcus uniformis from the Lower Devonian Rhynie and Windyfield cherts, Scotland, occurs in the form of unistratose, plate-like colonies comprised of (8-)16 -> 250 cell units (3-5 mu m in diameter) situated in rows more or less perpendicular to one another; quadruplets and 16-cell unit clusters are often recognizable as modular units. The fossil resembles extant cyanobacteria in the genus Merismopedia (Synechococcales), but material sufficient for a thorough assessment of R. uniformis has hitherto not been available. Newly discovered specimens provide detailed insights into the morphology and colony organization of R. uniformis, together with specific developmental details, and thus now permit a precise characterization of this fossil that underpins the structural similarity to Merismopedia. Merismopedia-like life forms are exceedingly rare as fossils, likely because the colonies are readily fragmented and destroyed. The new specimens of R. uniformis all occur within small inclusions in silicified substrate, suggesting that the substrate served as a microscopic conservation trap by shielding the enclosed colonies from destructive mechanical forces such as water movement.