A new species of small acritarch with a porous wall structure from the early Cambrian of Estonia and implications for the fossil record of eukaryotic picoplankton

The Proterozoic–Phanerozoic transition recorded a general trend of decrease in phytoplankton cell size, in contrast to the earlier and much larger Ediacaran acritarchs. Particularly minute, unornamented but sculptured organic-walled microfossils have been recovered from the lower Cambrian Lükati Formation in northern Estonia. The lack of any significant thermal alteration in this formation allowed for excellent preservation of fine microstructures on these microfossils. Among the rich palynomorph assemblages in the Lükati Formation, a new species of tiny, spheroidal eukaryotic microfossil is recorded: Reticella corrugata gen. et sp. nov. It is characterised by a corrugated and flexible vesicle wall that is densely perforated by nano-scale pores. Despite its unique morphology, the new species shares diagnostic characters with fossil and extant prasinophyte algae. Reticella corrugata is among the smallest microfossils with typical eukaryotic morphology (conspicuous wall sculpture) and contributes to the diversity of the size class of small acritarchs. Size, abundance, inferred prasinophyte affinity and eukaryotic wall sculpture make this new taxon a likely member of the early eukaryotic picoplankton.