The grey-brown thallus of R. grande is composed of slightly convex verrucose areoles on a black prothallus. The black angular to round apothecia with a disappearing proper margin are formed in between the areoles. Eight green-brown muriform spores are formed in an ascus. The thallus contains gyrophoric acid (KC+ red) with accessory stictic, norstictic or barbatic acids (Timdal & Holtan-Hartwig 1988). A similar R. eupetraeum only contains norstictic acid.
The species is mainly distributed in northern and central Europe. It might be overlooked because it was not distinguished from R. eupetraeum in the past. It grows on exposed siliceous rocks and screes. In the Czech Republic, it has recently been recorded only occasionally, e.g., in the Křivoklát, České středohoří Hills, Middle Vltava and Oslava regions.
Literature: Timdal E. & Holtan-Hartwig J. (1988): A preliminary key to Rhizocarpon in Scandinavia. – Graphis Scripta 2: 41–54.
taxonomic classification:Ascomycota → Lecanoromycetes → Rhizocarpales → Rhizocarpaceae → Rhizocarpon
Red List (Liška & Palice 2010):VU – vulnerable
Red List (Malíček 2023):C2 – strongly endangered
Occurrence in the Czech Republic
All records: 13, confirmed 9. One click on a selected square displays particular record(s), including their source(s).