Acarospora thamnina is a shiny, black tinged, variously brown squamulose crustose lichen. It has a linear growth pattern, growing along cracks in boulders. It can be found in North America to Alaska and Maine , from the coast to inland locations, also in the Ural Mountains and Novaya Zemlya in Russia. It commonly grows either among, or on other lichens. It grows a longer stipe so its squamules can grow over other lichens when there is competition for space. When it forms thick clumps it is easily identified with its elevated squamules and thick stipes. It grows on acidic rock in full sunlight.