Acarospora glaucocarpa (aka “Rimmed Cobblestone Lichen”)
Description: Thallus consisting of dispersed pale reddish brown to medium reddish brown squamules, each containing a single broad apothecium (up to 3 mm across) with a lightly pruinose, brown disk; thallus tissue usually reduced to a rim around each immersed apothecium, giving the fertile squamule the appearance of a lecanorine apothecium; spores 4 – 8 x 1.3 – 3 um, many per ascus, but often not developing.
Chemistry: All reactions negative (no lichen substances).
Substrate: On exposed calcareous rocks & pebbles.
Lookalikes: This lichen looks like a broad pruinose Lecanora on limestone. Although the multispored asci identify it as an Acarospora, the apothecia– unfortunately– often have no mature asci. Acarospora glaucocarpa should also be compared with Sarcogyne regularis, another pruinose multispored lichen that frequently grows as a neighbor in the same habitat. It has a black (often thin) lecideine apothecial margin, & its thallus is almost entirely endolithic.
Bibliography: Lichens of North America, by Brodo, Sharnoff, & Sharnoff
Database Entry: Distance Everheart 12-26-13