Description. Thallus of usually dispersed subsquamulose areoles or squamules 0.5–2 mm wide, 0.3–0.6 mm thick, variable in shape, mostly angular, sometimes forming clumps of squamules 5 mm wide, fissured and still attached, in process of replicating by division. Upper surface yellow, usually pruinose, smooth, becoming fissured. Lower surface narrow and usually white when broadly attached and narrow, often brown around the stipe of squamules. Epicortex less than 10 μm thick. Cortex 40–100 μm thick, upper layer yellow, lower layer semi-hyaline, below cortex often a buildup of rhizocarpic acid obscuring upper part of algal layer, cells usually round 2–3 μm wide, often irregular in shape. Algal layer 50–100 μm thick, algal cells variable in size, often 10–12 μm wide, continuous below apothecia. Medulla continuous with attaching hyphae or stipe. 0.2–0.4 mm thick, obscure. with substrate crystals. Apothecia sometimes one per areole or squamule, up to 0.7 mm wide, round to narrow, usually 2–9, punctiform to 0.5 mm wide, usually distinctly yellow especially in locations partially shaded during the day, epruinose, smooth, immersed. Parathecium expanding around disc 10–40 μm wide and sometimes merging with cortex to form a distinct ring around the apothecia the color of the thallus. Hymenium 90–140 μm high, epihymenium 20–40 μm tall, yellow and full of crystals, paraphyses 1.5–2.0 μm thick, apices not expanded, hymenial gel IKI+ red or blue to red (blue in old or weak IKI). Subhymenium 40–50 μm, IKI+ dark blue. Hypothecium 10–20 μm thick, IKI-. Asci 80–110 × 15–23 μm, ascospores variable, thin to ellipsoid, 3–5 × 1.0–2. 0 μm. Pycnidia not observed.
Chemistry. rhizocarpic acid in cortex and epihymenium, sometimes extending in to upper algal layer.
Distribution and Ecology. Described from New Mexico, occurring from the north into the Chihuahuan Desert in the south, 1500–2000 m, on siliceous and volcanic rock, in full sun. It is sympatric with Acarospora chrysops and A. organensis and with A. radicata in northern New Mexico.
Differentiation. Acarospora rimulosa looks similar to the sympatric Acarospora chrysops (Tuck.) H. Magn., which is also usually pruinose. If differs from A. chrysops in usually having more than one apothecium per thallus unit, with as many as nine. The apothecia of A. rimulosa are usually yellowish while A. chrysops usually has a reddish brown apothecia. Acarospora rimulosa is usually heavily fissured and regularly replicates by division while A. chrysops usually lacks fissures and replication by division is not frequent. The cortex of A. rimulosa is 40–100 μm thick while the cortex of A. chrysops is usually less than 40 μm thick. Acarospora subcontigua H. Magn. is sympatric and looks also like A. chrysops or A. rimulosa with pruinose yellow areoles, but it has a contiguous areolate crust. In the Sonoran flora A. rimulosa was treated as a synonym of A. socialis H. Magn. but it is a distinct species differs especially in usually having multiple apothecia instead of usually one apothecium per squamule. They are not sympatric with A. socialis growing along the Pacific coast while A. rimulosa is currently only known from New Mexico.