TYPE: U.S.A. ARIZONA: Tucson, in the vicinity of the Desert Botanical Laboratory, 1908, J.O. Blumer s.n. (holotype, W!).
Description. Squamules (0.5–)1–3 mm wide, 0.4–1 mm tall, dispersed or congregated. Upper surface light brown, usually epruinose, shiny, with numerous foveoles (small pits). Lower surface around stipe dark, brown or black. Epicortex even, ca. 15 µm thick. Cortex 30–40 µm thick, thin upper layer brown, lower layer hyaline, cells mostly round 2–4 µm wide. Algal thick, even, uninterrupted 130–180 µm thick, algal cells 8–16 µm wide, continuous beneath hymenium. Medulla 100–150 µm thick, with substrate crystals and hyphae 2–3 µm wide, branching and intricate, merging into the stipe.
Apothecia one per squamule, usually white pruinose, 0.2–1.5 mm wide, rough, immersed. Parathecium 10–40 lm wide, sometimes varying around same apothecium, apparently pushing up the cortex and merging with it to sometimes form a ring around the apothecia the color of the thallus. Hymenium 125–160 µm high, epihymenium 10 µm tall, reddish-brown, paraphyses mostly 2 µm wide, apices barely expanded, filled with oil drops, hymenial gel IKI+ blue to red (hemiamyloid). Asci cylindrical to clavate, asci 80–110 x 10–19 µm, ascospores 3–5 x 1.5–2.5 µm. Subhymenium 30–60 µm tall, IKI+ dark blue, stain spreading up into lower part of the hymenium. Hypothecium 10–20 µm tall, IKI–. Pycnidia not observed.
Chemistry. Producing gyrophoric acid (major) and lecanoric acid (minor) in cortex. Spot tests: C+/KC+ red, P–.
Ecology and distribution. In full sun on siliceous rock, growing alone or among other lichens including Acarospora chrysops (Tuck.) H.Magn, apparently not parasitic, competitive for space. Like A. thamnina (Tuck.) Herre, the stipe can elongate and the squamules overshadow lichens around it. It occurs at least in Arizona and New Mexico in the desert mountains. Our collections from New Mexico occur at the upper elevational border of the Chihuahuan ecozone at 1742–1762 m on the east side of the Organ Mountains on granite.
Differentiation. The holotype of Acarospora obpallens was collected growing in a soil crust at edge of Santa Monica Mountains by H. E. Hasse (Hasse 1897; Knudsen 2007; Magnusson 1929a; Zahlbruckner 1902). During an extensive inventory of the Santa Monica Mountains in the beginning of the 21st century, A. obpallens was common and collected on soil and sandstone (CNALH 2021). It has been reported from China and South America (Abbas & Nurtai 2018; Knudsen et al. 2008). On the thallus surface foveoles (small pits) are observed and this is a diagnostic character (Knudsen 2007). In Knudsen (2007), Acarospora carnegiei, A. tucsonensis, and A. instrata were treated as synonyms of A. obpallens based on the occurrence of small pits on the thallus. In our phylogenetic analyses, Acarospora obpallens and A. carnegiei are not closely related. Acarospora carnegiei does not differ significantly from A. obpallens in appearance and both have distinct foveoles (Magnusson 1929a; Zahlbruckner 1908). They do differ in thallus type and hymenium height. Acarospora obpallens has broadly attached areoles with a hymenium 100–130 lm high. Acarospora carnegiei is squamulose with a usually short stipe and distinct underside (Zahlbruckner 1908). It has a hymenium 125–160 µm high. Magnusson described A. tucsonensis as having broadly attached areoles, but he later recognized that New Mexican specimens and the holotype had a dark underside with a stipe but did not synonymize it with A. carnegiei (Magnusson 1930). We treat A. tucsonensis and A. instrata as heterotypic synonyms of A. carnegiei. Acarospora carnegiei and its synonyms are not recognized as synonyms of A. obpallens (Esslinger 2019; Knudsen 2007). Acarospora instrata is only known from a single New Mexican specimen and is A. carnegiei growing in pits of a siliceous red rock and has the diagnostic foveoles around the apothecia (Magnusson 1929a).
Discussion. For revisions of herbarium specimens, it is mostly likely that specimens from at least Sonoran and Chihuahuan Desert mountains are Acarospora carnegiei. In Joshua Tree in Mojave Desert specimens of A. obpallens grow on soil and rock and are broadly attached and this is common throughout at least southern California. The sequenced specimen of A. obpallens in our phylogeny was growing on sandstone in Santa Ana Mountains in southern California. Interestingly, Magnusson did not describe the foveoles on Acarospora obpallens and A. carnegiei though it is on specimens he examined and rarely does an areole or squamule lack foveoles (Magnusson 1929).