Diagnosis. Similar to Acarospora radiata but differing in having a higher hymenium 130–170 µm vs. 80–130 µm.
TYPE: U.S.A. NEW MEXICO: Don A˜na Co., Organ Mountains, opposite Soledad Rocks, slope above Sierra Vista trail, 32817049.0 00N 106836041.800W, 1580 m, on rhyolite boulders, 6 Mar. 2020, K. Knudsen 19229.1 & J. Kocourková (holotype, SBBG; isotype, PRM).
Description. Hypothallus endosubstratal, IKI–, no algae observed. Thallus several centimeters in width and capable of expanding to at least 10 cm by replication by division, areoles or squamules 3–5 mm wide, up to 2.0 mm thick, sometimes imbricate. Upper surface yellow, epruinose, smooth, usually with many fissures, eventually replicating by division. Epicortex thin, ca. 10 µm, continuous, or absent. Cortex 50–90 µm tall, upper layer yellow up to 30 µm thick, lower layer hyaline, cells mostly round, 3–5 lm wide. Algal layer 100–130 µm tall, upper and lower surface usually even, dense, but sometimes interrupted by hyphal bundles 10–20 µm wide. algal cells ca. 10–14 µm wide, scattered beneath apothecia. Medulla 200–300 µm thick, narrowing into mycelial base or stipe up to 300–500 µm tall, obscure with crystals and gelatinization, hyphae mostly 2 µm wide, thin walled. Apothecia often rare or absent, but sometimes abundant, one to seven per areole, 0.2–1.0 mm wide, disc yellow to dark blackish red, epruinose, sometimes with fragments or umbo of yellow cortex, sterile hyphae sometimes subdividing an apothecium. Parathecium expanding around the disc up to 100 µm, usually rising slightly above adjoining cortex, same color as thallus, IKI–. Hymenium 135–170 µm high, epihymenium yellow 10–20 µm high, the yellow pigment sometimes extending downward into hymenium 40–50 µm, paraphyses 1–2 µm wide, anastomosed, apices unexpanded, hymenial gel IKI+ red or blue turning red (hemiamyloid). Asci 90–130 x 14–20 µm, clavate, several hundred ascospores per ascus, ascospores ellipsoid, mostly 3–4 x 1.0–1.5 µm, often two oil drops. Subhymenium 50–70 µm tall, persistently IKI+ blue, sometimes with oil drops. Hypothecium narrow, 10–20 µm, IKI–. Pycnidia not observed.
Distribution and ecology. Known from the Chihuahuan Desert in New Mexico growing on granite, lava, sandstone, and rhyolite in full sun at elevations of 1371–1725 m. Shushan’s collection of Acarosporaorganensis from Harding Co., near Las Vegas, was collected north of the Chihuahuan Desert, and identified as A. xanthophana. Magnusson reported A. xanthophana sensu Magnusson from northern New Mexico and from Peach Springs, Arizona, in the Sonoran Desert (Magnusson 1929a,b, 1937, 1956). Acarospora xanthophana is endemic to South America and is currently known as A. congregata K.Knudsen & Flakus (Knudsen & Flakus 2016). Acarospora congregata differs from A. organensis in having an areolate thallus with numerous apothecia and producing epanorin (major or minor) with rhizocarpic acid (major or minor).
Differentiation. Acarospora brouardii is the only other squamulose yellow Acarospora with a high hymenium in North America (Knudsen 2007). Acarosporabrouardii occurs in South America, Mexico and lower Arizona in Chiricahua Mountains (Knudsen 2007; Knudsen et al. 2008). It differs from A. organensis in having large squamules up to 7 mm wide with many punctiform apothecia and a lower melanized surface, black to dark brown. Acarosporaorganensis looks similar to two squamulose species, A. socialis from the Pacific coast and A. radicata from the Mojave and Sonoran Deserts and northern New Mexico (Knudsen & Kocourková 2021). Acarospora organensis has a higher hymenium than either species (135–170 vs. 80–135 µm).
Discussion. In Knudsen (2007) the revision of yellow Acarospora did not use hymenium height as a diagnostic character. Acarosporaorganensis may have been identified as A. socialis or A.radicata because hymenium height was not used for determining specimens. The species concept of A. socialis in Knudsen (2007) was heterogenous and included within its circumscription A. amabilis, A. radicata, and A. rimulosa H.Magn. (Knudsen 2007; Knudsen & Kocourková 2021). Acarospora socialis is now recognized as endemic to California and Baja California in coastal plains and mountains and on the adjoining islands (Knudsen & Kocourková 2021). Many herbarium specimens annotated as A. socialis need to be revised.