Rock Hairball Lichen

The thallus of Spilonema revertens forms dense cushions composed of minute, dark filaments. These cushions feature a distinctive structure: a dense central mass of rhizine-like filaments supports an outer layer of actively growing lichenized branches, allowing continuous regeneration of the lichen's outer layer. Its growth form varies from somewhat shrub-like (subfruticose) to tree-like (dendroid), consisting of tiny, upright lobules that occasionally spread out along the substrate. These lobules bear numerous minute side branches, forming rounded to angular, convex cushion-like structures. In some cases, the thallus may fragment into small, crust-like (areolate) patches, giving it a more encrusting appearance. The lichen attaches to the rock surface via a hypothallus, a network of interwoven bluish-black fungal filaments, which becomes gelatinous when wet. The upper surface of the thallus is typically dark olive to blackish in colour, often appearing rough or granular, though it is rarely shiny. Internally, S. revertens has a uniform structure (homoiomerous) composed of densely packed thick-walled fungal filaments and lacks a distinct outer protective layer (ecorticate). Near the base, the tissue may adopt a more organized, paraplectenchymatous (brick-like) arrangement. …

Spilonema revertens has a broad Holarctic distribution, occurring across Europe, Asia, North America, and Greenland. It has also been recorded from the Canary Islands. In Russia, it has been documented in the northern European part of the country, the Northern and Southern Urals, the North Caucasus, Southern and Eastern Siberia, and both the southern and northern regions of the Russian Far East, as well as in Mongolia. In Greenland, it typically occurs in association with Euopsis pulvinata and Lecanora chloroleprosa, especially on moist charnockitic rocks and siliceous gravel. The species generally grows on exposed siliceous rock that undergoes alternating dry and wet periods. It is often encountered in lower mountain zones, particularly in light coniferous forests and stony meadow-steppes, but has also been documented on serpentine rocks and soils derived from peridotite. Such substrates feature low calcium-to-magnesium ratios, elevated concentrations of heavy metals, and nutrient deficiencies; however, S. revertens is not considered serpentine-specific because it is recorded on a variety of rock types. Wave exposure influences its distribution in coastal environments. …