Three-dimensional preservation of cellular and subcellular structures reveal 1.6 billion-year-old probable crown-group red algae

The ~1.6 Ga Tirohan Dolomite of the Lower Vindhyan in central India contains phosphatized stromatolitic microbialites. We report from there uniquely well-preserved fossils interpreted as crown-group rhodophytes (red algae). The filamentous form Rafatazmia chitrakootensis n.gen, n.sp. has uniserial rows of large cells and grows through diffusely distributed septation. Each cell has a centrally suspended, conspicuous rhomboidal disk interpreted as a pyrenoid. The septa between the cells have central structures that may represent pit connections and pit plugs. Another filamentous form, Denaricion mendax n.gen., n.sp., has coin-like cells reminiscent of those in large sulfur-oxidizing bacteria, but much more recalcitrant than the liquid-vacuole-filled cells of the latter. Its wider affinities are uncertain. Ramathallus lobatus n.gen., n.sp. is a lobate sessile alga with pseudoparenchymatous thallus and apparent tetraspore formation, suggesting florideophycean affinity. Rafatazmia and Ramathallus are interpreted as crown-group multicellular rhodophytes, antedating the oldest previously accepted red alga in the fossil record by about 400 million years.