Acarospora thamnina is a shiny, black tinged, variously brown squamulose crustose lichen.

[1] 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.

[1] It may be many different kinds of brown, with reddish-brown to yellowish-brown specimens growing on the same rock.

[1] There are 0-1 or more round to irregular apothecia, up to 0.5 mm wide, on each squamule, with rough and black discs that are sometimes surrounded by elevated margins of thallus tissue.

Apothecia variably grow from large and obvious lecanorine discs, to being immersed in the squamule.