A crustose species with a cracked-areolate thallus that is bright rust-coloured, but almost black in well-lit situations. It is limited by a grey-black prothallus. The apothecia are immersed with black discs that are shiny, smooth and slightly concave with prominent margins.
Thallus and medulla: C-, K-, UV-
Tremolecia atrata is common is upland Scotland where it is found on hard, iron-rich, siliceous rocks. BLS distribution map
Tremolecia atra, Acarospora sinopica and Rhizocarpon oederi are often found alongside each other on iron-rich, acid rock.
Tremolecia atrata is distinguished from Acarospora sinopica on basis of its cracked-areolate rather than squamulose thallus. It is distinguished from Rhizocarpon oederi on basis of its smooth rather than uneven apothecia.
scottishlichens.co.uk 