Acritarchs are early microfossils, the remains, probably, of eukaryote cells. It is the resistant single-layered cell walls, or perhaps a cover secreted by cell walls, that we see in early rocks of the Proterozoic era. These structures are organic in their chemical composition, not calcium carbonate. Their precise nature is not known, but they look like the resting stages of modern dinoflagellates (the 'red tide' organisms).