Spilonema revertens

Spilonema revertens, the rock hairball lichen, is a species of fruticose lichen in the family Coccocarpiaceae. This dark-coloured cyanolichen forms small, dense cushions on rocks across the Northern Hemisphere. Found in Europe, Asia, North America, Macaronesia, and Greenland, it typically grows on exposed rocks that experience alternating wet and dry periods. The species is most commonly found in lower mountain zones, particularly in open coniferous forests and rocky steppes, though it also occurs in coastal areas. First described in 1865 from specimens collected in Finland, it is distinctive for its minute, branching filaments that create cushion-like structures and for its role as the exclusive host of the parasitic lichen Psorula rufonigra. Unlike many other rock-dwelling lichens, it shows a preference for sheltered locations on rock faces and tends to grow in scattered, small patches rather than forming extensive coverage.