A new view on Nematothallus: coralline red algae from the Silurian of Gotland

The thalloid carbonaceous fossil Nematothallus Lang, 1937, has been widely interpreted as an early Palaeozoic land-plant, despite the absence of a convincing modern analogue. Exceptionally well-preserved nematophyte cuticle from the Late Silurian Burgsvik Sandstone Formation, Gotland provides additional insight into the organism's anatomy, phylogenetic affiliations and ecology. Because this material exhibits additional characters not present in the type material we assign it to Nematothallopsis gotlandii gen. et sp. nov. The organism was constituted of a close-packed layer of palisade-like filaments covered by a cuticle that bears a characteristic pseudocellular pattern on its inner surface. Apertures in this cuticle are often encircled by a ring of multicellular filaments, which are sometimes associated with spheroidal, spore-like entities. In the light of the conspicuous similarity of the palisade layers to the pseudoparenchymatous tissue of coralline red algae, and of the filament-fringed apertures to their reproductive conceptacles, we reconstruct the Nematothallopsis organism as an extinct rhodophyte and re-evaluate the putative terrestrial habit of cuticular nematophytes in general.