TYPE—U.S.A. New Mexico: Doña Ana Co., Organ Mountains, off Baylor Pass Trail, 32.372880 −106.568840, 1832 m, on granite in full sun, 9 Mar 2020, K. Knudsen 19272 and J. Kocourková (PRM, holotype; see photo by J. Kocourková).

Hypothallus endosubstratal, IKI−. Thallus of broadly attached, irregularly shaped areoles, 0.3–0.5(–1.0) mm wide, up to 0.4 mm thick, sometimes with lobulate margins but not forming a stipe, dispersed or contigu­ous, covering areas up to 2 cm, replicating by division. Upper surface light brown, rarely dark brown, epruinose or partially pruinose, once densely pruinose. Lower surface lacking or narrow and white. Epicortex indistinct. Cortex (20–)30(–40) μm thick, upper layer brown, 1–2 cells thick, lower layer hyaline, cells mostly round, mostly 5 μm wide. Algal layer even, uninterrupted, 90–100 μm thick, algal cells 12–15 μm wide, narrowly continuous below apothecia. Medulla 250–300 μm thick, hyphae intricate with much branching, thin-walled, 1.0–2 μm wide, obscure with crystals. Apothe­cia immersed, punctiform, 0.1–0.3 mm wide in full sun or up to 0.5 mm wide in daily half shade, 1 to 3 per areole, immersed, disc darker than thallus, rough, epruinose. Parathecium variable, indistinct to irregularly expanded, sometimes on only 1 side of a disc, 10–40 μm wide and merging into the cortex, hyphae 1 μm wide. Hymenium (100–)120–150 μm high, epi­hymenium 10–20 μm high, paraphyses 1.0–2.0 μm wide, apices barely expanded, hymenial gel IKI+ blue turning red, hemiamyloid. Asci abundant, 80–100 × 18–20 μm, ascospores ellipsoid, narrow or broad, 3.0–5.0 × 1.0–2.0 μm, variable. Subhymenium 30–60 μm high, IKI+ dark blue. Hypothecium indistinct. Pycnidia not observed.

Secondary metabolites: cortex producing gyrophoric acid, C+ red, KC+ red.

Ecology and substrate: rhyolite and granite, in full sun and half shade. World distribution: Organ Mountains, New Mexico, in the Chihuahuan Desert from 1518 to 1832 m.

Notes:  A broad concept of Acarospora fuscata was promulgated by W. Weber in his annotations of specimens from around the world. Specimens identified as A. fuscata from North America need revision. The species does occur in northeastern North America. Sequences are available of the conserved type of A. fuscata and A. agostiniana in GenBank. Specimens of A. agostiniana in herbaria would have been identified as A. fuscata by Weber and other North American lichenologists.