A crustose species with a rust-coloured thallus that is cracked to regularly angular-areolate, with a sometimes indistinct, black prothallus. The apothecia are persistently flat and not pruinose, with an uneven discs and a proper margin that may become angular or flexuose.
Rhizocarpon oederi is widespread throughout upland Scotland where it is found on iron-rich, siliceous rock. BLS distribution map
Rhizocarpon oederi, Acarospora sinopica and Tremolecia atrata are often found alongside each other on iron-rich, acid rock.
Rhizocarpon oederi is distinguished from Acarospora sinopica on basis of its cracked or angular-areolate rather than squamulose thallus. It is distinguished from Tremolecia atrata on basis of its concave, almost gyrose, rather than smooth, apothecia.
scottishlichens.co.uk 