Acarospora fuscata (aka “Brown Cobblestone Lichen”) see also A. glaucocarpa.
Description: Thallus areolate, the areoles often becoming slightly lifted at the edges & becoming squamulose, dispersed or contiguous, 0.5 – 2(-3) mm in diameter, yellowish brown to deep reddish brown, often shiny, sometimes black at the edge, concave, flat, or convex, generally with 1-5 darker brown, irregular disks of immersed apothecia in the center of each areole; spores 4 – 6 x 1 – 1.5 um.
Chemistry: Cortex & sometimes medulla PD-, K-, KC+ red, C+ pink (gyrophoric acid), but the reaction is usually difficult to observe against the brown cortex. (Sections of the thallus should be tested under the stereomicroscope.)
Substrate: On granite rocks in full or partial sun
Lookalikes: This is by far the most common & widespread of the many brown, dispersed areolate species of Acarospora. The C+ pink reaction of the thallus is a good confirming character but cannot always be seen. Acarospora badiofusca has a similar thallus, but the apothecia are more prominent & generally have a complete or partial black margin. Acarospora smaragdula, a widely distributed brown species with about the same amount of thallus variation as A. fuscata, reacts C-, & most specimens are K+ red because of norstictic acid. A few brown species occur on calcareous rock: Acarospora heppii is a widespread but infrrequently collected species with contiguous areoles having several apothecia &– unusual for the genus– large spores (c. 7 x 3 um), only a few dozen per ascus.
Bibliography: Lichens of North America, by Brodo, Sharnoff, & Sharnoff
Database Entry: Distance Everheart 12-26-13