Rhizocarpon petraeum has a dull chalky white to grey thallus with an underdeveloped prothallus. The areoles are small, angular and tightly packed. Apothecia are sometimes arranged in a circle (contrary to the related species R. umbilicatum). They are black, flat and round. Spores are muriform and hyaline, brownish when very old. Crystals, that partly dissolve in KOH (in a microscopic slide), are formed in the proper excipulum.
It is a pioneer species on calcium-enriched rocks all over the northern hemisphere. It can also be found on secondarily enriched silicates and is known from anthropogenic substrates (roof tiles, rock walls etc.) or coastal rocks, too. It is distributed from the lowlands to the high mountains. The lichen is quite rare in the Czech Republic, recently known from xerothermic areas at lower elevations and from calcareous substrates high in the mountains (the Krkonoše and Jeseníky Mts).
When underdeveloped, without the characteristic circular organization of the apothecia and with immature or malformed spores, it can be easily confused with R. umbilicatum, which is even rarer in the Czech Republic.
Literature: Ihlen P. G. (2004): Taxonomy of the non-yellow species of Rhizocarpon (Rhizocarpaceae, lichenized Ascomycota) in the Nordic countries, with hyaline and muriform ascospores. – Mycological Research 108: 533–570. Nimis P. L., Hafellner J., Roux C., Clerc P., Mayrhofer H., Martellos S. & Bilovitz P. O. (2018): The lichens of the Alps – an annotated checklist. – Mycokeys 31: 1–634.taxonomic classification:
Ascomycota → Lecanoromycetes → Rhizocarpales → Rhizocarpaceae → Rhizocarpon
All records: 15, confirmed 9. One click on a selected square displays particular record(s), including their source(s).